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The fat halo sign

Authors:
Nasreen Mahomed at University of the Witwatersrand
Nasreen Mahomed
Tanusha Sewchuran at greys
Tanusha Sewchuran
  • greys
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96 SA JOURNAL OF RADIOLOGY • September 2011
SIGNS
The fat halo sign
T Sewchuran, MB BCh
N Mahomed, MB BCh, FCRad (D)
Department of Radiology, University of the Witwatersrand, Johannesburg
Corresponding author: T Sewchuran (tanusha.sewchuran@yahoo.co.uk)
Intramural stratification with deposition of fat in the submucosal layer
of the bowel wall, visualised on computed tomographic (CT) scans of
the abdomen, is known as the fat halo sign.1 Owing to infiltration of the
submucosa by fat, the inner layer mucosa are separated from the outer
layer of muscularis propria/serosa (both being of soft tissue density)
by a layer of fat (of low attenuation) measuring between -18 to -64
Hounsfield units.1-3
Historically, the fat halo sign has been associated with patients
suffering from chronic inflammatory bowel disease.1-3 Less commonly,
it has also been associated with cytoreductive therapy, graft-v.-host
disease and renal calculi.1-3 When seen in both the small and large
bowel, the fat halo sign has been considered pathognomonic of Crohn’s
disease.1,2
Harisinghani et al. conducted a study in 2003 evaluating the
presence and frequency of the fat halo sign in patients undergoing
abdominal CT for clinical indications unrelated to the gastrointestinal
tract.1 They concluded that 21% of the study population were positive
for the fat halo sign, of whom 6 (28%) had renal stone disease and 15
(72%) did not have renal stone disease.1 The intestinal distribution
of the fat halo sign in these patients was the terminal ileum (4%),
ascending colon (28%), transverse colon (34%), descending colon
(36%), sigmoid colon (14%) and rectum (10%).1 None of the patients
in whom the fat halo sign was demonstrated had previous or current
gastro-intestinal symptoms suggestive of inflammatory bowel disease
or a history of gastro-intestinal disease.1-3 This argues the fat halo sign
as being a normal variant seen in a certain portion of the population.1,2
A statistical relationship between the fat halo sign and obesity has also
been established.1
Fig. 1a. Axial post contrast CT abdomen at the level of the rectum
demonstrating the fat halo sign, i.e. the central fatty submucosal layer of low
attenuation surrounded by higher attenuation inner and outer layers grossly
corresponding to the mucosa and muscularis propria/serosa of the rectum
respectively.
Fig. 1c. Axial post contrast CT abdomen of the same patient with oral contrast
demonstrates a low-attenuation fat layer in the wall of the caecum and
terminal ileum.
Fig. 1b. Coronal post contrast CT abdomen of the same patient with oral
contrast demonstrates a low-attenuation fat layer in the wall of the caecum,
ascending colon and terminal ileum. The patient did not have clinical or
radiological features of inflammatory bowel disease, and the fat halo sign
was a normal variant.
SA JOURNAL OF RADIOLOGY • September 2011 97
SIGNS
Pitfalls associated with interpretation of the fat halo sign specifically
involve intestinal distension. The sign has a tendency to disappear or
become less apparent when the bowel lumen is more distended.1 It is
thought that distension of the bowel lumen causes obscuring of the
thin fat layer.1 Therefore, the fat halo sign is best appreciated when the
lumen is partially collapsed.1 Other factors advocating a positive fat
halo sign are prone position of the patient, and not using a contrast
agent.1 The constellation of 4 signs that strongly suggests the fat halo
sign representing a normal variant, as opposed to inflammatory bowel
disease, includes increased prevalence in the collapsed state, decreased
prevalence with distension of the bowel lumen either by gravity or gas,
disappearance of the fat halo sign with additional distension, and a thin
calibre of the fatty layer.1 The presence of a normal haustral pattern also
supports the sign being a normal variant.1 More commonly, a normal
intramural fat layer is seen in the terminal ileum and descending colon.3
It is generally much thinner than the fat layer seen in inflammatory
bowel diseases.3
In conclusion: the presence of the fat halo sign may in the absence
of clinical and radiological features of inflammatory bowel disease
represent a normal finding that may also be related to obesity.
1. Harisinghani MG, Wittenberg J, Lee W, Chen S, Gutierrez AL, Mueller PR. Bowel wall fat halo sign in
patients without intestinal disease. AJR 2003;181:781-784.
2. Ahualli J. The fat halo sign. Radiology 2007;242:945-946.
3. Wittenberg J, Harisinghani MG, Jhaveri K, Varghese J, Mueller PR. Algorithmic approach to CT diagnosis
of the abnormal bowel wall. Radiographics 2002;22:1093-1109.
The hyperdense MCA sign and the MCA dot sign
N Mahomed, MB BCh, FCRad (D)
Department of Radiology, University of the Witwatersrand, Johannesburg
Corresponding author: N Mahomed (nasreen.mahomed@wits.ac.za)
The hyperdense middle cerebral artery sign (HMCA), first described
in 1983, refers to the hyperattenuation of the middle cerebral artery
(MCA) M1 segment on non-enhanced computed tomography (CT).1,2
The sign is due to a thromboembolus of the M1 segment of the MCA.
Because the sign is a marker of vascular occlusion rather than a direct
image of the resulting parenchymal changes, the HMCA sign can be
considered an indirect indicator of subsequent infarction and is one of
the earliest signs of ischaemic stroke.3
The MCA dot sign is a punctate focus of hyperattenuation located
in the sylvian fissure on non-enhanced CT, and is a recently described
variant of the HMCA sign.3 The sign represents a thromboembolus
within a segmental branch of the MCA, M2 or M3 segment, located
within the sylvian fissure.3 As the M2 and M3 segmental vessels tend
not to course in the transverse plane of imaging, the occluded vessel is
seen in cross section, appearing as a hyperattenuating dot within the
sylvian fissure.3
While the HMCA sign and MCA dot sign are similar in that they
both depict thromboembolus at different levels of the MCA, there
are important clinical and prognostic distinctions. The HMCA sign
suggests that a major cerebral vessel is occluded, suggesting a larger
territory at risk for hypoperfusion compared with the more distal vessel
occlusion of the MCA dot sign. Therefore, the MCA dot sign in the
absence of the HMCA sign is associated with improved short-term
clinical outcome.3
These signs have a high specificity of almost 100 % with a high
positive predictive value but a low sensitivity of approximately 38 - 40%
for thromboembolic occlusion of the MCA.1-3
Mimics of the HMCA sign, the pseudo hyperdense MCA sign,
include vascular calcification, raised haematocrit, intravenous contrast
and partial volume averaging.1 Another important cause of a pseudo
Fig. 1. HMCA sign. Non-enhanced axial CT brain (at the level of the suprasellar
cistern) in a patient with sudden onset left hemiparesis demonstrates
hyperattenuation (measuring 82 HU) of the right middle cerebral artery,
which is suggestive of occlusion of this artery and is an indirect sign. There is
also diffuse hypodensity of the right temporal lobe, a direct sign of ischaemic
stroke.
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Algorithmic Approach to CT Diagnosis of the Abnormal Bowel Wall1
Article
  • Sep 2002
Computed tomography demonstrates intestinal wall abnormalities that can be analyzed by categorizing attenuation changes in the intestinal wall and transposing morphologic characteristics learned from barium studies. These attenuation patterns include white, gray, water halo sign, fat halo sign, and black. The white pattern represents avid contrast material enhancement that uniformly affects most of the thickened bowel wall. If the bowel wall is enhanced to a degree equal to or greater than that of venous opacification in the same scan, it should be classified in the white attenuation pattern. Common diagnoses with this pattern include idiopathic inflammatory bowel diseases and vascular disorders. The gray pattern is defined as a thickened bowel wall with limited enhancement whose homogeneous attenuation is comparable with that of enhanced muscle. This pattern is used to differentiate between benign and malignant disease, but it is the least specific of the patterns and should be combined with morphologic observations. The water halo sign indicates stratification within a thickened bowel wall that consists of either two or three continuous, symmetrically thickened layers. Common diagnoses with this sign include idiopathic inflammatory bowel diseases, vascular disorders, infectious diseases, and radiation damage. The fat halo sign refers to a three-layered target sign of thickened bowel in which the middle or "submucosal" layer has a fatty attenuation. Common diagnoses with this sign include Crohn disease in the small intestine and idiopathic inflammatory bowel diseases in the colon. Black attenuation is the equivalent of pneumatosis, and this pattern is commonly seen in ischemia, infection, and trauma.
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Bowel Wall Fat Halo Sign in Patients Without Intestinal Disease
Article
  • Sep 2003
Stratification with a fat layer in the intestinal wall is thought to be a reliable marker for inflammatory bowel disease. We evaluated the presence and frequency of the bowel wall fat halo sign in patients undergoing abdominal CT for clinical indications unrelated to the gastrointestinal tract. We performed a retrospective review of 100 consecutive abdominal and pelvic CT examinations in 61 men and 39 women (mean age, 56 years) with clinical suspicion of renal stone disease. Two radiologists experienced in abdominal imaging performed qualitative and quantifiable assessment of the images. Five segments of the colon (ascending colon, transverse colon, descending colon, sigmoid colon, and rectum) and the terminal ileum (for approximately 1 ft [30 cm]) were evaluated for the presence of the fat halo sign. If the fat halo sign was present, fat density and total wall-thickness assessments were made. Presence or absence of clinical and radiologic signs of inflammatory bowel disease was determined. The Student's t test was used to evaluate the statistical significance, correlating body weight and presence of the halo sign. The fat halo sign was seen in 21 (21%) of 100 patients. Of the 21 patients with the fat halo sign, six (29%) had renal stone disease and 15 (71%) had no stone disease. The density value of the halo sign ranged from -18 to -64 H (mean, -41 H). The distribution of the fat halo sign was as follows: the terminal ileum, 4%; the ascending colon, 28%; the transverse colon, 34%; the descending colon, 36%; the sigmoid colon, 14%; and rectum, 10%. No patient with this sign had any remote, recent, or subsequently recorded history of inflammatory bowel disease. A statistically significant relationship (p < 0.001) was seen between the presence of the fat halo sign and body weight distribution, with 16 of 21 patients weighing over 200 lb (90 kg). In the absence of clinical or radiologic evidence of inflammatory bowel disease, the presence of the fat halo sign may represent a normal finding that is possibly related to obesity.
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The Fat Halo Sign 1
Article
  • Apr 2007
The fat halo sign is seen on CT scans of the abdomen and appears as a thickened bowel wall demonstrating three layers: an inner and an outer layer of soft-tissue attenuation, between which lies a third layer of fatty attenuation.
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