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Critical ReviewsTM in Biomedical Engineering, 44 (1-2): 73–89 (2016)
Opportunities and Challenges of 7 Tesla Magnetic
Resonance Imaging: A Review
Muhammad Irfan Karamat,1,∗Sahar Darvish-Molla,1& Alejandro Santos-Diaz2,3
1Department of Medical Physics and Applied Radiation Sciences, McMaster University, Hamilton, ON, Canada;
2McMaster School of Biomedical Engineering, McMaster University, Hamilton, ON, Canada; 3Imaging Research Centre,
St. Joseph’s Healthcare, Hamilton, ON, Canada
†All authors contributed equally to this paper.
∗Address all correspondence to: Muhammad Irfan Karamat, Department of Medical Physics and Applied Radiation Sciences, McMaster University,
1280 Main Street W, Hamilton, ON, Canada L8S 4K1; Tel.: +1 905 521 2100 ext. 73389; karamami@mcmaster.ca
ABSTRACT: The desire to achieve clinical ultra-high magnetic resonance imaging (MRI) systems stems from the fact that higher
field strength leads to higher signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and spatial resolution. During last few years
7T MRI systems have become a quasi standard for ultra-high field MRI (UhFMRI) systems. This review presents a detailed account
of opportunities and challenges associated with a clinical 7T MRI system for cranial and extracranial imaging. As with all of the
previous transitions to higher field strengths, the switch from high to UhFMRI is not easy. The engineering and scientific community
have to overcome challenges like magnetic field inhomogeneity, patient safety and comfort issues, and cost and related problems
in order to achieve a clinically viable UhFMRI system. In addition, a large number of clinical studies are still required to show the
improvements in quality of diagnostics that would come with 7T MRI, in order to bring such a research tool to the clinic.
KEY WORDS: ultra high field magnetic resonance imaging, 7T MRI system, B0inhomogeneity, B1inhomogeneity, specific ab-
sorption rate, brain MRI, extracranial MRI, magnetic resonance spectroscopy
ABBREVIATIONS: AD, Alzheimer’s disease; CE-MRI, contrast-enhanced MRI; CNR, contrast-to-noise; CSLE, chemical shift
localization error; DIR, double inversion recovery; DWI, diffusion-weighted imaging; FDA, Federal Drug Agency; FLAIR, fluid at-
tenuation inversion recovery; FLASH, fast low-angle shot; FSE, fast spin-echo; FSPGR, fast spoiled gradient echo; GRAPPA, gen-
eralized autocalibrating partially parallel acquisition; MPRAGE, magnetization prepared rapid gradient echo; MRS, magnetic reso-
nance spectroscopy; MS, multiple Sclerosis; PD, Parkinson’s disease; RF, radiofrequency; SAR, specific absorption rate; SENSE,
sensitivity encoding; SMASH, simultaneous acquisition of spatial harmonics; SNR, signal-to-noise; SSFP, steady state free pre-
cession; TOF-MRA, time of flight MR angiography; TSE, turbo spin-echo; UhFMRI, ultra-high field MRI; VIBE, volumetric
interpolated breath-hold examination; VOI, volume of interest
I. INTRODUCTION
Since the beginning of the magnetic resonance imag-
ing (MRI) era in the 20th latter part of the century, an
urge to achieve the highest possible magnetic field in
clinical settings has been a major motivation for sci-
entists and engineers.1,2The efforts towards achiev-
ing higher magnetic field stem from the fact that
higher field strength implies higher signal-to-noise
(SNR), contrast-to-noise (CNR), and spatial resolu-
tion. However, transition from the first generation
MRI scanners (≤0.5T) to the conventional (1.0–1.5T)
and then to high field 3T scanners was gradual with
each transition presenting a different set of challenges
for the scientific and engineering community.3The
transition from conventional to high magnetic field
(3.0T) clinical MRI scanners was not straightforward
either. It took almost 15 years before being accepted
widely by the clinical community. Over the years,
many ultra-high field MRI (UhFMRI) systems rang-
ing from 7 to 11.7T have been designed, but a 7T sys-
tem has evolved as a quasi standard currently.2There-
fore, in this review UhFMRI will refer to a 7T scanner
unless stated otherwise. In order to reach the goal of
clinical 7T MRI scanner and to gain the above men-
tioned advantages, a considerable amount of work has
been done and efforts are still continued by the re-
search and scientific community to solve problems
like safety, susceptibility related artifacts, B0and B1
inhomogeneities and other technical issues. The cur-
0278–940X/16/$35.00 c
°2016 by Begell House, Inc. www.begellhouse.com 73
74 Karamat, Darvish-Molla, & Santos-Diaz
rent review focuses on the efforts and developments
towards transformation of a 7T MR imaging system
from a topic of research to a routinely used clinical
MR imaging system. In this regard, a detailed account
of opportunities, challenges and the ways proposed
to overcome these challenges, and clinical studies to
identify possible advantages offered by UhFMRI in
cranial and extracranial imaging applications with an
intent to achieve a clinical 7T system, is given in the
review.
II. 7T MRI: OPPORTUNITIES
A. Enhancement of Signal-to-Noise Ratio
(SNR)
There is a lack of consensus among MRI scientists
about the definition of SNR. However, it is generally
accepted that SNR is directly proportional to the field
strength B0.1,3There are still some who think SNR
increases more steeply with B0:4
SNR ∝B7/4
0.(1)
To be on the safe side, one assumes a linear relation-
ship between SNR and B0. Under such an assump-
tion, the difference between parallel and antiparal-
lel spins, calculated using the Boltzmann probabil-
ity distribution, is as small as 10 spins in one mil-
lion at B0=3T that implies around 2.3 times (i.e.,
≈23 spins in one million) more spins per million at
7T.1,3Such an increase in SNR with B0implies more
refined spin manipulation and image reconstruction
with fewer spins. In addition, greater SNR provides a
signal enhancement to nuclei other than protons, such
as sodium-23 (23Na) and phosphorus-31 (31 P).5The
signal boost given to these nuclei by UhFMRI pro-
vides a mean to study novel relaxation mechanisms,
different metabolic pathways, and cellular phenom-
ena.
B. Spatial Resolution
The UhFMRI provides SNR that can be spent on res-
olution. It has been theorized that the spatial resolu-
tion is related to a factor Nby which field B0is in-
creased as:5
Spatial Resolution ∝N1/3.(2)
The above relationship implies about 30% improve-
ment in resolution when B0is changed from 3T to
7T. It must be emphasized here that resolution is
not dependent only on B0and can further be en-
hanced in practice by improvements in receiver and
gradient design and by the use of imaging sequence
that involves improvement in contrast-to-noise ratio.5
This increase in spatial resolution promises to reveal
anatomical and pathological details that were never
resolved before by other lower-field MR imaging sys-
tems. The examples of such revolutionary detail de-
piction by UhMRI are given in almost all the follow-
ing sections of this review.
C. Enhancement of T1 Relaxation
The prolongation of T1relaxation time can be gener-
alized as a heuristic relation based on human brain
studies or extrapolated from animal brain studies
as:5,6
T1 = 1
a1+a2Ba3
0
.(3)
In order to give the reader an idea about the T1pro-
longation, constants a1,a2, and a3for cortical gray
matter are found approximately to be 0.35, 0.64, and
–0.7, respectively, in SI units.7−9This suggests an
overall prolongation of about 26% in the case of 7T
MRI compared to 3T MRI. This prolongation of T1
relaxation time with field strength may be regarded
as a disadvantage, as longer T1leads to lesser SNR
recovery per unit time. However, the increase in T1
in applications that involve labeling of magnetiza-
tion before imaging can be an advantage as it implies
less decay of the label. Arterial spin labeling that in-
volves labeling of blood to obtain perfusion weighted
images10 and cardiac tagging, in which saturation of
magnetization lines enable visualization of cardiac
muscle contraction over the cardiac cycle,11 are the
examples of imaging applications in which prolonga-
tion of T1is an advantage. Another such application
where increase in T1value could be advantageous is
the time-of-flight angiography where the background
signal is suppressed with the help of a relatively high
flip angle and repetition times much shorter than T1
are used to prevent signal recovery.12−14 Such back-
ground suppression enables improved signal from the
fresh blood entering the tissue that has not previously
experienced RF pulse.
Critical ReviewsTM in Biomedical Engineering
MRI at 7T from Research to Clinic 75
D. Reduction of T2 ∗Relaxation
A heuristic expression for T2∗similar to that de-
scribed for T1[Eq. (3)] in Sec. II.C. can be written
as:5,6
T2∗=1
a4+a5B0
.(4)
In the case of cortical gray matter a4and a5are
7 and 3.5, respectively, in SI units.7−9This rela-
tion implies a reduction in T2∗by a factor of about
0.56 at 7T compared to 3T for cortical gray matter.
The shortening of the T2∗time constant at UhFMRI
makes it more sensitive to the presence of calcium
and iron in microbleeds and hemorrhages. This leads
to an improved depiction rate of microbleeds in
patients,15−16 detection of small venules17,18 and
envisioning of deep brain structures.19,20 Also, in-
creased blood oxygen level dependent (BOLD) con-
trast (due to enhanced T2∗dephasing caused by de-
oxygenated blood)21 and better visualization of cap-
illary beds (due to higher SNR)22,23 enables high-
resolution functional MRI (fMRI) at the highest field
strength possible.
E. Enhancement of the Chemical Shift
The difference in molecular distribution of different
compounds results in different resonance frequen-
cies for the compounds. This molecular distribution
based variation of resonance frequencies is known
as chemical shift. The extent of resonance frequency
spread, 4ν, is directly proportional to field strength
B0i.e.,4,5
4ν∝ 4B0.(5)
According to Eq. (5), molecular distribution based
spectral changes become more pronounced at higher
field and provide higher spectral resolution in ap-
plications involving MR spectroscopy in UhFMRI.
Higher spectral resolution and increased SNR would
result in a greater number of metabolites detected at
7T compared to lower field strengths.5Such an in-
crease in chemical shift at UhFMRI is also beneficial
in achieving better suppression due to considerable
enhancement in frequency differences between water
and fat at higher field strength.24
F. Magnetic Susceptibility Effects
The magnetic susceptibility (χ) is the inherent prop-
erty of the tissue or material that describes the re-
sponse of that tissue to external magnetic field.3The
sensitivity of χto magnetic field strength, B0, is de-
picted in the following expression (6).5
χ∝B0×T E (6)
The expression (6) shows that at higher B0, shorter
TEs can be used to achieve the same phase effect,
which in conjunction with higher SNR can be used
to achieve susceptibility weighted images (SWI) at
higher resolution and contrast.3,5This increase in χ-
effects with increase in B0allows more effective de-
piction of microvasculatures, microbleeds, iron and
calcium deposits25 and better differentiation between
blood containing iron and calcification.26 Also, it is
worth mentioning here that SNR in phase images in-
creases quadratically with field strength that can help
to achieve an order of magnitude better CNR in phase
images compared to magnitude images in brain.27
III. CHALLENGES OF 7T MRI
In order to take full advantage of the opportunities de-
scribed in Sec. II and to develop MRI at 7T in clini-
cal practice, a number of technical, patient safety, and
practical issues need to be addressed. The technical
issues include B0inhomogeneity, B1inhomogene-
ity, susceptibility related artifacts and chemical shift
error. The quadratic increase in radiofrequency (RF)
specific absorption rate (SAR) with B0is the main
patient safety concern at UhFMRI.3The location of
7T MRI and its cost are practical considerations in-
volved in clinical 7T MRI system. Some additional
patient comfort and safety requirements of UhFMRI
include noise level, peripheral nerve stimulation, and
other ultra-high field related physiological effects that
will also be addressed below.
A. Technical Challenges
B0Inhomogeneity. The magnetic susceptibility, χ,
scales linearly with B0, as does the inhomogeneity in
B0.3,5The inhomogeneities in B0in UhFMRI lead
to geometrical and intensity related distortions in the
Volume 44, Issues 1-2, 2016
76 Karamat, Darvish-Molla, & Santos-Diaz
MR images. Especially, rapid single- or few-shot ac-
quisition schemes are more prone to susceptibility re-
lated artifacts that mainly lead to geometric distortion
caused by the B0inhomogeneity. The B0variations
in UhFMRI also introduce not only shifts, but also
cause broadening of metabolite peaks in MR spec-
troscopic applications. It also makes it extremely dif-
ficult to use selective frequency pulses designed for
spectral band specific data sampling. The intensity
distortions caused by B0inhomogeneity also make
fat and water suppression relatively more problem-
atic in UhFMRI. More powerful referencing schemes,
dynamic B0shimming and smaller voxel size are re-
quired to overcome the B0inhomogeneity issue.5
B1Inhomogeneity. Is the most difficult prob-
lem to solve at ultra-high B0.3,5The wavelength
of RF transmit field in UhFMRI becomes so small
(≈11.2 cm at 7T) that the phase effect of the hu-
man body no longer remains negligible at these
wavelengths.3,28 These material dependent phase
changes caused by the reduction in wavelength (due
to higher RF frequency required at UhFMRI) result
in nonuniform flip angle distribution and unexpected
changes in SNR and CNR over the volume of in-
terest (VOI). The central brightening within brain
commonly manifests due to B1inhomogeneity in
UhFMRI.
One possible solution proposed to overcome B1
inhomogeneity is to use specially designed adiabatic
pulses to obtain uniform flip angle distribution, in-
dependent of B1inhomogeneity, above a certain am-
plitude known as “adiabatic threshold”.29 There are
pulse sequence design methods available to tailor adi-
abatic pulses that are behaviorally similar to conven-
tional RF pulses.30
Another solution offered to overcome B1inhomo-
geneity is to use specifically designed RF pulses tai-
lored using rapidly acquired accurate B1maps. Even
though these 2D pulses are used to compensate for
B1inhomogeneity at reduced RF power consumption
compared to adiabatic pulses, they may get affected
by B0inhomogeneity.31,32
Use of multichannel parallel transmit arrays also
offers not only a method to mitigate B1inhomogene-
ity, they also help to overcome SAR related issues.
These arrays divide VOI into partially overlapping
spatial regions and overcome B1inhomogeneity ei-
ther by (1). changing RF amplitude or phase (B1
shimming) at each transmit channel to compensate
for B1inhomogeneity, or (2) using customized RF
pulse for each of the transmit channels to overcome
B1variations.28,33
Chemical Shift Localization Error (CSLE). Is
the measure of spatial offset in precise location
with RF frequency and resonance frequency of the
metabolite in MR spectroscopy within VOI, specified
at the scanner. The CSLE varies as:5
CSLE ∝B0×Slice thickness
(Transmit RF bandwidth).(7)
As depicted in Eq. (7), the CSLE is proportional to
field strength and slice thickness and inversely pro-
portional to transmit RF bandwidth. CSLE scales lin-
early with frequency shift which leads to a reduction
of usable volume for MR spectroscopy, due to spatial
offset introduced by multiple metabolites involved in
spectroscopic data acquisition at 7T.5
B. Patient Safety Challenge
The SAR increases with the square of B0which puts
a more stringent limit on the number, duration, and
amplitude of applied RF pulse at 7T compared to 3T
for a given time period. The use of MRI pulse se-
quences like turbo spin-echo (TSE) or fast spin-echo
(FSE) that involve dense high flip-angle RF pulses
also limits the number of image slices that can be ob-
tained at 7T.3,5
Parallel imaging, described earlier in this section,
with segmented readout to accelerate image data ac-
quisition used to mitigate field inhomogeneity, can
also be used to overcome some limitations posed
by SAR.34 In order to reduce RF energy deposition
lower-flip angle sequences have also been designed
with an intent to achieve contrast similar to more SAR
intensive pulse sequences like TSE or FSE.35
C. Practical Considerations
Siting of whole body 7T clinical MR system. Larger
7T magnet compared to 3T and a requirement of 200-
400 metric tons of passive iron shielding to reduce
stray magnetic field2are no longer the limiting factors
in siting options for a 7T MRI system. The use of
active shielding has greatly improved the feasibility
to use these UhFMRI systems in clinical settings.5
Critical ReviewsTM in Biomedical Engineering
MRI at 7T from Research to Clinic 77
Cost. The two factors that govern the cost of clin-
ical 7T MRI systems are (1) helium boil-off and (2)
Economy of scale. In newer 7T systems, the helium
boil-off problem has been addressed and these sys-
tems are called ”Zero helium boil-off” systems with
very little helium leak. Since the 7T systems are not
being used in common research and clinical practice,
these systems are not produced in large numbers. The
cost of these systems will definitely go down with
widespread adoption of these systems by institutions
and clinics. The cost may vary, currently (in mid-
2015) it costs about $10 million to install a 7T system
in the USA.5
FDA Approval. Although, federal drug agency
(FDA) has declared MRI scanners operating at 8T
and below as non-significant risk to adults and chil-
dren, but clinical use of 7T has not yet been approved
by the FDA.5
D. Patient Comfort and Safety
Risks associated with a 7T scanner are similar to
those for lower-field MRI systems. However, there
are some additional patient comfort and safety con-
siderations related to a 7T MRI system which are dis-
cussed in this section. One of the major patient safety
issues related to higher SAR related to 7T systems has
already been discussed in Sec. III. Continuous SAR
monitoring systems are installed on these systems to
ensure that the SAR limits set by the FDA they do not
exceed.5
Noise level and peripheral nerve stimulation.
Acoustic noise is produced by bulk vibrations in gra-
dient coils. These vibration are caused by Lorentz
force that scales directly with the magnetic field for
a given gradient coil orientation.1A 7T system gen-
erates acoustic noise that is about 7.4 dB higher than a
3T system for a given field and gradient orientation.2
In order to achieve a safe noise level below 99 dB,
modern systems are tested for this specified comfort
level.2,5
The peripheral nerve stimulation, which is related
to the switching speed of the gradients, is also moni-
tored and limited on all 7T systems.2,5
Ultra-high field related physiological effects.
Among the most commonly reported transitory phys-
iological effects caused by ultra-high magnetic field
brain imaging are dizziness and vertigo. These ef-
fects are caused by magnetohydrodynamic forces ex-
erted on the ionic fluids when a person moves through
the fringe field. In order to minimize these effects
the subjects are instructed to avoid quick head mo-
tion during the scan and also the scan table is moved
slowly to ensure patient speed reduction through the
fringe field.5In extracranial UhFMRI, specific ab-
sorption rate, especially when imaging deeply lying
structures, is the most important limiting factor that
needs careful monitoring.3
Metallic implants. To date, metallic implants of
almost any sort in the body are considered as a con-
traindication for scanning at 7T systems. So far only
two contrast injectors and one radiofrequency identi-
fication chip are approved for 7T.5However, studies
to test a wide range of implantable devices, to allow
a scanning of wider range of patients at 7T, are being
performed.36−38
A summary of major challenges of a clinical 7T
MRI system is provided in Table 1.
TABLE 1: Major challenges of clinical 7T MRI system
Challenges Proposed solution
B0inhomogeneity More powerful referencing schemes, dynamic B0shimming, and smaller
voxel size5
B1inhomogeneity Custom designed adiabatic pulses,29 multichannel parallel transmit array28,33
Chemical shift —
localization error
SAR Multichannel parallel transmit array28,33 ; lower flip angle sequences35
Metallic implants More studies to overcome contraindication are being performed36−38
Volume 44, Issues 1-2, 2016
78 Karamat, Darvish-Molla, & Santos-Diaz
IV. 7T MRI IN NEUROIMAGING
Most of the research geared towards developing the
clinical applicability for 7T MRI systems has focused
on examining anatomical aspects of the brain due
to the very high resolution achievable at this field
strength in T1-weighted imaging, high CNR in fluid
attenuation inversion recovery (FLAIR) imaging, and
time-of-flight MR angiography (TOF-MRA), as well
as the ability of the system to detect microbleeds with
T2∗-weighted images due to the higher susceptibility
effect. On the functional side of MR applications, ad-
vantages offered by 7T rely on two main points, first
the increased SNR to measure the BOLD effect for
functional MRI (fMRI) making more accurate the lo-
calization of brain activity due to a specific task and,
second, the increased spectral separation in metabo-
lite peaks that improve the resolution and quantifica-
tion for magnetic resonance spectroscopy MRS. In
this section we describe major contributions of 7T
MRI in brain imaging with a view to its validation
as a clinical tool.
A. Anatomical Brain Imaging
Ultra-high field MR creates a complete new challenge
in terms of understanding and rediscovering the hu-
man brain. Both normal and disease conditions need
to be fully understood to address the real benefits on
7T MR in a clinical environment involving studies on
the brain. Because of this caveat, most brain imag-
ing at this field strength has found its way in study-
ing multiple sclerosis (MS), cerebrovascular diseases
(CVS), degenerative brain diseases, brain tumors, and
epilepsy.5,39
1. Multiple Sclerosis (MS)
MS is a demyelinating disease with prevalence
among relatively young patients (20s–30s) and its
pathogenesis is unknown. There are very few measur-
able biomarkers that can predict treatment response
and disease progression in patients with MS. The aim
of MRI to assess MS focuses on imaging white mat-
ter lesions, as they might be a key point in the eval-
uation of disease. In particular, cortical lesions are a
challenge because of the relatively low resolution at
clinical field strengths (1.5T and 3T) and some stud-
ies have shown its relevance in getting a better insight
into the pathogenesis.40 Moving to ultra-high field
MR could provide the resolution and CNR needed to
visualize these lesions with better accuracy. Graff et
al. showed improved identification of cortical lesions
caused by MS using 7T FLAIR imaging comparing it
with 3T.4Other high-resolution sequences like T2∗-,
double inversion recovery (DIR), and MPRAGE
imaging have also been used to examine MS cortical
lesions.40,42−44 Among these, some lesions showed
characteristics not seen before at lower fields; for ex-
ample, the appearance of peripheral distortion rings
caused by susceptibility effect.40,42
Another characteristic that has been related with
the development of MS is the relationship between
white matter cortical lesions and veins; this has been
explored throughout T2∗-weighted images. Tallan-
tyre et al. found that 80% of lesions in patients with
MS were perivenous compared with 20% in normal
aging white matter lesions.45,46 This relationship was
clearer at 7T when compared to 3T, with a higher
percentage of lesions showing veins. Thus, such en-
hanced recognition may be helpful to definitely diag-
nose MS in uncertain cases; however, no studies have
yet showed superiority of 7T to diagnose MS, when
compared to 3T MRI.
2. Cerebrovascular Diseases
The assessment of cerebrovascular diseases using
MRI is mainly through high resolution MR angiog-
raphy (MRA) and T2∗-weighted sequences able to
show microbleeds. The optimization of MRA for 7T
has allowed the visualization of lenticulostriate arte-
rial branches,47 the small vessel that feed deep brain
structures like the basal ganglia. Thus, small vessels
disease like infarcts in such structures can be studied
now, increasing the interest for the field. Kang et al.48
found a reduction in the number and configuration of
lenticulostriate arteries in patients with chronic stroke
compared with controls. They also found reduced
visible arteries in hypertensive patients compared to
the control group.49 It should be noted that despite
the capabilities of high-resolution MRA at 7T, even
smaller vessels such as resistance arterioles are still
out of reach with inflow-based methods. Therefore,
improvement in these techniques is still needed to as-
sess small artery disease that results in white matter
lesions.
Critical ReviewsTM in Biomedical Engineering
MRI at 7T from Research to Clinic 79
The accurate detection of aneurysms is another ca-
pability of high-resolution 7T MRA, where the clear
depiction of characteristics in the aneurysm, like the
presence of small perforating branches, can be made.
These branches could result in an infarct if not no-
ticed in treatment planning. Only a few studies have
compared the capabilities of 7T time-of-flight-MRA
versus clinical field strength in its capability to detect
aneurysms,50,51 showing a better-rated image qual-
ity at 7T. However, it is yet to be determined if 7T
MRA is better than 1.5T and 3T in detecting clin-
ically relevant aneurysms. In terms of follow-up of
treated aneurysms 7T is contraindicated for clips and
coils.
The study of microbleeds using MRI has gained
attention especially at high field strengths. Due to
the paramagnetic nature of microbleeds, it is pos-
sible to visualize them using T2∗-weighted images.
The increased sensitivity of higher fields to detect
microbleeds is because of the increased susceptibil-
ity effect and the higher resolution available. Fur-
thermore, due to the increased susceptibility at 7T,
microbleeds can be detected even using very short
echo times (TE = 2.5 ms).52 Using lower resolu-
tion (and lower field) the intensity difference be-
tween microbleeds can be lost in the noise. Conijn
et al. showed that 7T is better than 1.5T for detecting
microbleeds.16
The study of intracranial atherosclerosis is another
area that may benefit from the high SNR at 7T. In or-
der to get information regarding the nature of steno-
sis, it is necessary to visualize the intracranial vessel
wall. At clinical field strengths this wall was depicted
only in the presence of large atherosclerotic lesions or
vasculitis.53−55 The increased resolution at 7T may
be exploited due to its capacity to depict the intracra-
nial vessel wall even in healthy subjects.56
3. Degenerative Brain Diseases
Dementia [vascular dementia and Alzheimer’s dis-
ease (AD)] and Parkinson’s disease are three major
pathologies where 7T MRI has found applications.
Theysohn et al. assessed the presence of white mat-
ter lesions using T2∗-weighted sequences at 7T and
found an increased sensitivity for finding these le-
sions when comparing with 1.5T in patients with vas-
cular dementia.57
One of the biggest objectives to be achieved study-
ing 7T in degenerative brain disease is the direct
detection of amyloid plaques. Sequences to acquire
T2∗-weighted images used at very high resolution
may show changes in tissue due to the presence of
amyloid, these changes have been elucidated during
post-mortem.58 However, it is not clear if such a goal
can be achieved at shorter scan times.
The evaluation of detailed substructures of the hip-
pocampus also plays a main role in the assessment of
dementia. In this regard, 7T MRI has shown an in-
creased depiction of substructures of the hippocam-
pus when compared with clinical field strengths,59,60
due to a high contrast between tissues. The differenti-
ation of these substructures may be important due to
its possible relation with memory loss. Currently, it
is not known which sequences are better to depict the
hippocampus substructures. In this regard, high res-
olution T2∗-, FLAIR, T1- and T2-weighted images
have shown promising results.59−61
In regard Parkinson’s disease (PD), the use of 7T
MRI has focused on imaging the substantia nigra,
thought to play a meaningful role in PD’s pathogene-
sis. Cho et al. showed the superiority of 7T MRI de-
lineating substructures in the substantia nigra com-
pared with 1.5 and 3T.19 Changes in these substruc-
tures, like increased iron content62 and differences in
anatomical patterns,63 have also been found in pa-
tients with PD versus normal subjects.
4. Brain Tumors
The most important clinical marker to assess brain tu-
mors using MRI is the enhancement due to a MR-
gadolinium-based test. It allows differentiation and
a rough grading of tumors. Such an enhancement
is due to the disruption of the blood-brain barrier
causing contrast agent leakage. In this regard, 7T
MRI has not shown differences in the enhancement
area compared with lower field strengths.64 Other
methods at 7T like T2∗-weighted imaging may show
added value when examining the arterial microvas-
culature of the tumor. Tumor metabolism could also
probably be assessed using this technique, as the in-
creased metabolism of the tumor may lead to an
increase in deoxyhemoglobin signal in veins. Cur-
rently, only small samples of brain tumor patients
have been studied at 7T64 and there is no evidence
Volume 44, Issues 1-2, 2016
80 Karamat, Darvish-Molla, & Santos-Diaz
that shows an added value of 7T MRI compared with
3T MRI.
5. Epilepsy
The increased SNR and high resolution at 7T MRI
plus its novel contrast mechanisms can provide delin-
eation of epileptic foci not often visible at lower field
strengths, thus improving treatment planning and pa-
tient outcome. So far, 7T MRI has shown value when
characterizing features related with epilepsy like hip-
pocampal sclerosis,65 cortical dysplasia66 and vascu-
lar malformations.67
B. Functional Brain imaging
The increased SNR/CNR available at 7T MRI needs
to be exploited on every front in order to become a
clinical tool that can be used on a daily basis. Func-
tional magnetic resonance imaging (fMRI) is not the
exception of it as this technique has gained power for
diagnostic and treatment planning purposes in lower
field strengths. The improved sensitivity in BOLD
signal at 7T seems promising and its benefits have
already been demonstrated when comparing versus
1.5T and 3T for healthy subjects.68 Furthermore, a
study from Beisteiner et al. has shown evidence of
the benefit at 7T over 3T in a patient population.69
However, issues related with high field systems like
increased susceptibility artifacts, B1inhomogeneity,
SAR limitations, and patient motion (especially in pa-
tients with brain pathology) still remain as a challenge
and clear advantages are yet to be determined for the
jump of field strength in this regard. In addition to
acquisition methodology, post processing techniques
need to be studied in greater detail. Yang et al. showed
statistical distribution differences in the data when
comparing 7T and 3T.70
Magnetic resonance spectroscopy (MRS) is an-
other technique useful in the assessment of brain
function due to its capability to measure the concen-
tration of different metabolites in the tissue. The in-
creased magnetic field at 7T results in a separation
of metabolite peaks leading to a higher resolution
and better quantification. In applications related to the
brain, MRS imaging at 7T has been shown to pro-
vide a better measuring of metabolic markers in tu-
mors when compared with 3T.71 There is an increase
of 35% in SNR of 7T over 3T leading to a clear su-
periority of the higher field in measuring glutamate,
glutamine, and GABA.72 Thus an understanding of
metabolic processes in healthy and disease conditions
can be improved at higher field strength.
V. 7T MRI BEYOND THE BRAIN
A. Musculoskeletal Imaging
Despite the issues discussed earlier in Sec. III, such
as homogeneous radiofrequency (RF) coil design, in-
creased chemical shift artifacts, susceptibility arti-
facts, RF energy deposition, and changes in relax-
ation times, MRI at 7T likely will provide some ex-
cellent opportunities for high-resolution morphologic
imaging and functional imaging of musculoskele-
tal systems.73−81 Regatte and Schweitzer in 200776
studied the feasibility of acquiring high-resolution
in vivo images of the musculoskeletal system in 15
healthy human volunteers on the knee, ankle, and sev-
eral muscle groups at 7T using high-resolution ax-
ial/sagittal morphological imaging [3D fast low-angle
shot (FLASH)]. They reported 70%–80% higher car-
tilage SNR at 7T compared to corresponding regions
at 3T. Generally, there is not any signal intensity from
menisci on clinical scanners (1.5T and 3T) at the
usual imaging TE values, due to low SNR and res-
olution. However at 7T, the contrast and high SNR
can enable better detection of subtle morphological
changes by improving image resolution. It is con-
cluded that MRI at 7T provides a powerful, rapid,
high-resolution functional imaging tool of muscu-
loskeletal systems (1H, 23Na, and 31 P).
In 2009 the same group77 demonstrated, for the
first time, the feasibility of acquiring high-resolution,
isotropic 3D-sodium knee images of healthy and os-
teoarthritis patients at 7T in less than 15 minutes.
Their preliminary results suggest that for osteoarthri-
tis imaging, lesion localization, and clinical diagno-
sis, sodium imaging at 7T may be a better alterna-
tive. Sodium MRI has been shown to correlate with
glycosaminoglycan (GAG) concentration in many
studies.82−85 Compared to proton MRI, the sodium
MRI has a low SNR, poor spatial resolution, and long
acquisition times due to the low natural abundance of
sodium, and rapid biexponential T2signal decay.77
Hence, in vivo sodium and phosphorous imaging at
clinical magnetic field strengths poses major chal-
Critical ReviewsTM in Biomedical Engineering
MRI at 7T from Research to Clinic 81
lenges and is rather limited. However ultra-high field
systems with improved gradient hardware and pulse
sequences have potential for improving the spatial-
temporal resolution of sodium MRI of the knee in
vivo, in clinically acceptable scan times.77
Jordan et al.80 carried out a study to compare the
relaxation times of musculoskeletal tissues at 3T and
7T in order to use these measurements to select ap-
propriate parameters for musculoskeletal protocols at
7T. T1and T2relaxation times of cartilage, mus-
cle, synovial fluid, bone marrow, and subcutaneous
fat were measured at both 3T and 7T, in the knees
of five healthy volunteers. It was shown that at 7T,
the T1relaxation times increased by an average of
51%, whereas the T2relaxation times decreased by
an average of 21%. Further parameter optimization
for clinical musculoskeletal imaging protocols at 7T
will be viable with the knowledge of these relaxation
parameters.
Recently, it was shown that there is a substantial
benefit from UhFMRI of ankles with routine clinical
sequences at 7T compared to 3T.78 Three clinical se-
quences: (1) 3D gradient-echo, T1-weighted; (2) 2D
fast spin-echo, PD-weighted; and (3) 2D spin-echo,
T1-weighted were used on both systems to examine
10 healthy volunteers. SNR was calculated for car-
tilage, bone, muscle, synovial fluid, Achilles tendon,
and Kager’s fat-pad. It was shown that the mean SNR
significantly increased by 60.9% and 86.7% at 7T
compared to 3T for 3D GRE, and 2D TSE, respec-
tively. However, an average SNR decrease by 25%
in the 2D SE sequence was also observed. The CNR
was obtained for cartilage/bone, cartilage/fluid, carti-
lage/muscle, and muscle/fat-pad, and showed an in-
crease for 2D TSE images, and in most 3D GRE im-
ages.
The first clinically comprehensive hip MRI pro-
tocol at 7T was developed by Chang et al. (2014),
to demonstrate the feasibility of performing bone
microarchitecture, high-resolution cartilage.81 AT1-
weighted 3D fast low-angle shot (3D FLASH) se-
quence (0.23 ×0.23 ×1–1.5 mm3) was used for
bone microarchitecture imaging, and T1-weighted
3D FLASH and volumetric interpolated breath-hold
examination (VIBE) sequences (0.23 ×0.23 ×
1.5 mm3) with saturation or inversion recovery-
based fat suppression for cartilage imaging and
2D intermediate-weighted fast spin-echo (FSE) se-
quences without and with fat saturation (0.27 ×0.27
×2 mm3) for clinical imaging. It was found that us-
ing 7T MRI (1) depicting individual trabeculae within
the proximal femur in vivo; (2) performing high-
resolution cartilage MRI of the hip joint with T1-
weighted 3D gradient echo sequences, and (3) per-
forming fast spin-echo imaging both without and with
fat suppression would be possible. As a future per-
spective, unique insight into the pathogenesis of dif-
ferent hip disorders or the ability to detect disease
changes in the hip joint in its earliest stages will be
achievable using an amalgamation of high-resolution
morphologic proton imaging methods and physio-
logic imaging methods at 7T.81
B. Breast
Currently imaging of breast using MRI is mainly per-
formed by 1.5T and 3T MRI systems86−88 in which
the diagnostic performance yields a sensitivity of 0.90
and a specificity of 0.72.89 Recently, ultra-high field
7T MRI has become available in a clinical research
setting. At 7T, substantial improvements in image
quality can be provided and there is a potential to
improve diagnosis and staging of breast cancer pa-
tients if technical challenges can be overcome with
7T breast MRI.90−93
Stehouwer et al.91 carried out the first clinical ex-
perience with contrast-enhanced MRI (CE-MRI) of
breast cancer at 7T, compared to 3T and histopathol-
ogy. T1weighted CE-MRI with unilateral breast
coil results obtained at 7T and 3T showed en-
hanced SNR as well as the enhanced contrast be-
tween the tumor and glandular tissue at 7T. A sin-
gle dose of gadolinium-based contrast agent was used
in both cases. Contrast enhancement-to-noise ratio
was found to be 4.6 at 7T and 2.8 at 3T when
comparing contrast to noise of the mass before and
after contrast administration. It was concluded that
clinical contrast-enhanced MRI of the breast at 7T
is technically feasible. Ultra-high spatial and tem-
poral resolution contrast-enhanced breast MR imag-
ing was reported using parallel imaging at 7T.94,95
High potential of breast MRI at 7T with sensitiv-
ity of 100% and a specificity of 92% were reported
by Pinker et al. in 2014.96 Images without signif-
icant artifacts and satisfactory fat suppression were
obtained using an isotropic spatial resolution of 0.7
Volume 44, Issues 1-2, 2016
82 Karamat, Darvish-Molla, & Santos-Diaz
×0.7 ×0.7 mm3with a temporal resolution of
14 s.
Diffusion-weighted imaging (DWI) has a high po-
tential for characterizing breast tumors and monitor-
ing or predicting treatment response.97 Compared to
dynamic contrast-enhanced MRI (DCE-MRI), DWI
has lower spatial resolution. If the spatial resolu-
tion of 1 ×1×2.5 mm3, which is the recom-
mended resolution for CE-MRI,98 could be achiev-
able for DWI sequences as well, both the detection
of smaller lesions and the morphological evaluation
of the breast may be possible. Hence, there is a need
of measure at higher magnetic field strength, imag-
ing technique and hardware improvement.92 Just re-
cently, Pinker et al.93 carried out a feasibility study of
multiparametric MR imaging of the breast in com-
bination with DCE imaging and DWI at 7T and
to investigate any improvement in diagnostic accu-
racy. Twenty-five patients with BI-RADS category
4 and 15 patients with BI-RADS category 5, to-
tal of 40 patients, age 18 years or older, not preg-
nant and not breastfeeding underwent bilateral mul-
tiparametric MR imaging of the breast at 7T and a
dedicated four-channel double-tuned 31P/1H breast
coil. They concluded the feasibility of multiparamet-
ric MR imaging of the breast with DCE MR imag-
ing and DWI at 7T in clinical practice and diag-
nosis of breast cancer with a high diagnostic accu-
racy of 94.1% (44 of 46 lesions) and excellent inter-
rater agreement (k = 0.833 to 1) would be achiev-
able.
C. Prostate
MRI has been widely used for a variety of clinical
purposes in the prostate, including tumor detection
and localization, treatment planning, and assessment
of aggressiveness.99 There is a need of biopsy in the
case of suspected aggressive prostate cancer, in which
small malignancies could be easily missed.3To iden-
tify tumor tissue magnetic resonance spectroscopy is
a better option to image metabolites such as spermine,
choline, creatine, and citrate.100 This is where ultra-
high field strength has advantages of increased sep-
aration of spectral peaks compared to lower strength
fields.101 However, as it was mentioned imaging the
human body at 7T is very challenging. There have
been studies dedicated to the investigating and over-
coming these challenges in prostate imaging by de-
signing an external transmit-receive coil array,102 im-
proving SNR and B1transmit field for an endorec-
tal coil103,104 and assessing image quality and cancer
visibility of prostate MRI at 7T.105
Rosenkrantz et al. in 2013102 developed a more
simply designed system for 7T prostate MRI, with
two transmit-receive elements and six receive-only
elements, avoiding parallel transmission and RF
shimming, for the goal of optimization of coil ar-
rangement for high-resolution T2-weighted prostate
MRI at 7T and substantial improvements in SNR
compared with 3T MRI. It was shown that prostate
tumors diagnosed or confirmed with MR guided
biopsy or prostatectomy were well visualized with 7T
MRI.106
D. Heart
Despite the major challenges in ultra-high field
MRI, a number of research groups already have
demonstrated the feasibility of cardiac imaging at
7T and have compared it to 3T.107−112 Due to a
higher SNR in a higher strength field, 7T MRI is
practically advantageous in the areas that are crit-
ical to diagnosis of cardiac disease, such as coro-
nary artery angiography.113,114 Also cardiac MR
at 7T offers the potential for imaging of cardiac
physiology such as perfusion, coronary lumen and
wall, tissue oxygenation, and cardiac metabolism
which are substandard at 1.5T or even at 3T at
present.112
Temporally-resolved and high spatial resolution
myocardial T2∗mapping is another application of
ultra-high field cardiac MRI which was recently re-
ported by Hezel et al. to assess myocardial iron depo-
sition or myocardial perfusion deficits to character-
ize myocardial tissue.115 In another study,116 a direct
quantitative and qualitative comparison between 7T
and 3T for right coronary MR angiography in young
healthy volunteers was undertaken. It was shown that
the SNR of the blood pool measured on the 7T im-
ages was 60% higher compared to 3T. Also compared
to 3T, improved CNR between the blood pool and
the epicardial fat, and increased vessel sharpness at
7T were observed. Suttie et al.112 carried out a com-
parison of cardiovascular magnetic resonance imag-
ing (CMR) at 1.5T, 3T, and 7T field strengths using
Critical ReviewsTM in Biomedical Engineering
MRI at 7T from Research to Clinic 83
steady state free precession (SSFP) and fast low-angle
shot (FLASH) cine sequences to assess cardiac func-
tion. No significant differences in cardiac volume or
mass were found using SSFP at 3T or 7T when com-
paring with the clinical gold standard of 1.5T SSFP
imaging. There were also no significant differences
between cardiac volume and mass determined using
FLASH imaging at 1.5T, 3T, and 7T field strengths.
A detailed review on human cardiac magnetic reso-
nance at ultrahigh fields was published by Niendorf et
al. in 2013.117 The authors addressed many channel
radiofrequency (RF) technology concepts, and basic
principles of mapping and shimming of transmission
fields including RF power deposition considerations
for the ultra-high field. Clinical application of car-
diac MR at 7T, including assessment of cardiac func-
tion, myocardial tissue characterization, and MR an-
giography of large and small vessels as well as het-
eronuclear MR of the heart and the skin were dis-
cussed.
E. Angiography
MR angiography at ultra-high field has been often
used for cerebrovascular imaging and has yielded
measurable benefits in assessing cerebrovascular
diseases as described before.118−120 Whole body
MR angiography images have also been recently
recorded. As the SNR decreases due to accelera-
tion techniques such as parallel acceleration121 and
compressed sensing,122,123 the increase in SNR us-
ing ultra-high strength MR is favorable for MR an-
giography in which high spatiotemporal resolution
is essential.124 The feasibility of aortic 4D flow at
7.0T for both contrast enhanced and non-contrast en-
hanced was investigated by Hess et al. in 2015.125
They compared and quantified SNR as a function of
field strength and contrast enhancement for 1.5T, 3T,
and 7T. The non-enhanced acquisition at 7T SNR was
found to be 2.2 times that of 3T. It is interesting to
note that the SNR of 3T MRI is 1.7 times that of
1.5T.
Metzger et al.126 investigated the feasibility of
non-contrast enhanced renal MR angiography at 7T
using a respiratory-gated turbo-FLASH sequence
consisting of a slab-selective inversion and chemi-
cal shift selective fat suppression followed by signal
readout. Vessel conspicuity, which would be bene-
ficial for several clinical applications including pre-
operative evaluation of renal vascular anatomy, un-
controlled hypertension, and the evaluation of pa-
tients with renal insufficiency or failure, is the motiva-
tion behind performing these studies at 7T. They ac-
quired high quality images of the renal arteries with-
out venous and background signal artifacts by using
more efficient B1shimming solutions for the inver-
sion preparation and more homogeneous solutions for
the excitation. Likewise, in another study by Umutlu
et al.,127 the feasibility of 7T non-enhanced high-
field MR imaging of the renal vasculature was stud-
ied using non-enhanced T1-weighted FLASH imag-
ing. It was concluded that both methods are promis-
ing techniques for good quality non-enhanced renal
artery assessment at 7T. Contrast-enhanced MR an-
giography is currently a valuable tool in the diagno-
sis of patients with peripheral arterial disease. How-
ever, there are patients who are at risk of nephro-
genic systemic fibrosis due to the gadolinium contrast
agent.128 Hence, feasibility of non-enhanced MR an-
giography technique at 7T might be an important
development.129−131
VI. CONCLUSIONS
SNR increases linearly with field strength, B0. There-
fore, achieving a higher B0is a motivation to im-
prove capabilities of the MRI. Such a scanner im-
proves MRI capabilities as (1) the SNR starved tech-
niques, like fMRI and DTI, benefited greatly with
higher field strength; (2) faster imaging sequences,
such as SENSE, SMASH, and GRAPPA can also
get a sensitivity boost due to the increase in SNR at
higher field strength; and (3) higher spectral disper-
sion and sensitivity achievable at higher field strength
greatly improve MRS outcomes.
In order to take advantage of the research poten-
tial offered by 7T MRI systems, a significant amount
of progress has been made in the development of
software and hardware solutions to address major is-
sues such as B0and B1nonuniformities, reduction of
SAR, and susceptibility related artifacts.
UhFMR imaging has shown significant potential
in revealing the anatomical abnormalities that would
remain unresolved at lower field strengths. A large
number of studies in brain imaging have revealed
the ability of higher field strength to visualize fine
Volume 44, Issues 1-2, 2016
84 Karamat, Darvish-Molla, & Santos-Diaz
structural abnormalities associated with different dis-
eases; depiction of different metabolites and their
distribution present in smaller structures; detection
of microbleeds, microvasculature, and blood prod-
ucts; better spectral dispersion and sensitivity for
the signals related to underlying neuronal activity
and tapping into the signal for nuclei other than
1H.
Although in extracranial imaging applications,
UhFMRI has shown promise in terms of improved
SNR and resolution, it is still a challenge to de-
velop more robust and safe protocols to acquire uni-
form images, especially for deep seated structures.
Another important question that needs attention is,
“Does higher SNR and spatial resolution offered by
UhFMRI improves diagnosis of disease compared to
MRI machines with lower fields, in extracranial ap-
plications?” Introduction of dedicated coils for organs
like breast and knee has greatly improved the situa-
tion in terms of safety and robustness for these or-
gans.
Based on the studies performed in neuroimaging,
it can be concluded that the combination of high-
resolution anatomic, spectroscopic, and functional
imaging at 7T has the ability to become a strong, non-
invasive tool for diagnosis and treatment of different
neurological diseases and disorders. However, more
technological developments and several clinical stud-
ies are required to show diagnostic improvements, if
any, provided by a 7T MRI system in extracranial
imaging applications.
In short, continued technical developments to
achieve better B0and B1homogeneity and lower
SAR, reduction in number of MRI contraindications
related to implantable devices at 7T, more clinical
studies to prove added diagnostic value with 7T, and
making the cost of a 7T system comparable to current
clinical MR imaging systems are the key factors that
would lead the way for ushering the 7T scanner from
research laboratories to the clinic.
ACKNOWLEDGMENT
The authors would like to thank Dr. Mike Nosewor-
thy of Department of Biomedical Engineering, Mc-
Master University, for all his help, guidance and for
the precious time he spent in reviewing this arti-
cle.
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Volume 44, Issues 1-2, 2016
