British Journal of Radiology (2004) 77, S64-S73
© 2004 British Institute of Radiology
doi: 10.1259/bjr/31892755
Multislice CT of the abdomen
J G Cahir, FRCR,
A H Freeman, MB, FRCR and
H M Courtney, PgCert CT, DCR(R), BSc
Department of Radiology, Addenbrookes Hospital, Hills Road, Cambridge CB2 2QQ, UK
Correspondence: Dr A H Freeman
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Abstract
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Major advances have been made in CT technology since its introduction 30 years ago. Examination time has been markedly reduced and it is possible to cover considerably larger scan ranges. Multidetector technology now allows near isotopic imaging with the ability to perform high quality multiplanar reconstructions. It is also now possible to image in well defined vascular phases. In this article we share our experience in the use of multislice CT and detail protocols used in imaging of the abdomen.
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Introduction
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Since the introduction of CT into radiology 30 years ago [1], major technical developments have been achieved with regard to speed of examination and image reconstruction. These include the introduction of slip ring technology with continuous rotation of the tube and detectors, and development of the spiral technique [2]. Dual slice CT progressed to multislice CT (MSCT) in 1998, when arrays of between 8 and 32 detector rows became available [3]. MSCT has resulted in a transformation from a transaxial cross-sectional technique into a true three-dimensional (3D) imaging modality that allows for viewing examinations in arbitrary planes as well as 3D rendering techniques that provide excellent image quality for CT angiography and bone work. MSCT has gained rapid acceptance by radiologists and there has been concurrent growth in the number of machines. In 1998 there were ten machines installed, in 1999 it was 100 by the middle the year, and by the end of the year 2000 over 1000 machines were in use worldwide [4]. MSCT provides a dramatic gain in performance that can reduce examination times, reduce section collimation and increase examination ranges substantially. The remit of this article is to detail the protocols used in imaging of the abdomen and to highlight cases that show the added diagnostic abilities of MSCT.
MSCT provides considerable benefits compared with single slice CT systems and these advantages are summarized in Table 1
. The main disadvantages are that large amounts of data and images are created [5] and there are potential increases in patient dose [6, 7], partly owing to the ability to image in multiple vascular phases.
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Examination protocols
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Table 2 outlines suggested protocols for CT examination of the abdomen and gives details of technical parameters including slice reconstruction and detector use [8–12].
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Filming and reporting
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MSCT produces a large number of images (e.g. 500 axial images in an examination of the abdomen and pelvis). In the absence of a picture archiving and communication system (PACS), this produces a problem and a decision has to be made regarding which images need to be printed, as it is obviously not practical or cost effective to hard-copy all images. A representative set of images, including images of an area of abnormality, should be hard-copied. Unfortunately, it is not uncommon to find at the time of reporting that certain images which are important have not been filmed and it is time consuming to re-image a study. This situation will be remedied with the wider introduction of PACS.
Multiple workstations are desirable in the reporting of MSCT. There is general agreement that efficient reporting is only feasible on dedicated workstations [13–15]. It is more difficult to report accurately from a selection of hard-copy images [16] and this should be kept in mind in the purchase of a new CT machine. Mathie and Strickland [17] demonstrated the differences in reporting efficacy depending on how images are displayed to the user. This study showed that radiologists were able to perform interpretation tasks quicker and more easily when images were displayed in stack mode with the ability to scroll through the examination. 3D reconstruction is not essential for the reporting of every examination. It is, however, useful in the further evaluation of an area of abnormality, especially when the axial images do not enable easy interpretation [18].
The raw data sets provide the capabilities for slice reconstruction. It is common for additional clinical questions to arise several days after the examination and therefore temporary storage of raw data is important [19]. It is this department's practice to store raw data for several days after the examination. CT raw data are not DICOM compatible and cannot be stored in external PACS archives.
Pending the arrival of PACS, studies are currently archived on optical disc. This format allows a large amount of data to be stored on a single disc (4.1 GB). On average an examination produces 250–300 MB of data for storage, and 8000 images can be stored on a single disc. This currently results in the use of three optical discs per week for image storage.
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Post-processing techniques
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There are four main post-processing techniques available — multiplanar reformation (MPR), maximum intensity projection (MIP), shaded surface display (SSD) and volume rendering (VR). MPR is a useful technique as it allows a study to be viewed in multiple arbitrary planes that can be determined by the viewer. CPR (curved planar reformation) is similar to MPR. It allows creation of a single-voxel thick tomogram that enables visualization of a curved structure by fixing points manually along the structure of interest. CPR is useful for displaying tubular structures. The disadvantage is that it is highly dependent on accurate selection of the curve [15]. MIP is a useful technique for visualization of vascular structures. A MIP is obtained by projecting onto an image plane the highest attenuation voxels encountered through a scan volume. This results in the entire volume being collapsed with only the highest attenuation structures being visible, which allows visualization of structures that do not lie in a single plane. The disadvantages are that vessels adjacent to bony structures may be obscured and calcification in vessels may not be visualized, which poses a problem with calcifications in the wall of an artery. SSD provides 3D representations of anatomy using grey scale. A voxel threshold is selected allowing the exclusion of irrelevant structures. Disadvantages of this technique are that it can be affected by noise and artefacts, and vascular calcification may be obscured. VR is a technique that allows visualization of the vasculature as well as the maintenance of 3D relationships. It allows some of the advantages of SSD and MIP [20].
With the arrival of MSCT, radiology departments have become faster and more efficient [21] and clinical questions that have not previously been posed to radiologists can now be answered. The result of this is that clinicians have higher expectations of a radiology department, which results in an inevitable increased number of examinations and thus the need for more efficient reporting.
The plain film is still the mainstay in initial investigation of the acute abdomen. The plain film obviously has a lower sensitivity and specificity than CT and sometimes may be misleading. The patient in Figure 1 presented with abdominal pain and, initially, a partially obstructing lesion in the proximal ascending colon was suspected as evidenced by the abdominal radiograph (Figure 1a). However, CT revealed extensive inflammatory change due to diverticulitis of the sigmoid colon (Figure 1b).
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Figure 1. Patient presenting with abdominal pain, initially suspected to have a partially obstructing lesion of the proximal ascending colon as evidenced by the abdominal radiograph (a). CT revealed extensive inflammation due to diverticulitis of the sigmoid colon (b).
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Another advantage of CT over the plain abdominal radiograph is the ability to show multiple pathologies in the abdomen, for example small bowel obstruction is demonstrated in an elderly male patient (Figure 2). The colon, however, is markedly thick walled owing to pseudomembranous colitis (arrows).
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Figure 2. Small bowel obstruction in an elderly male patient, but note the grossly thickened colon wall (arrows).
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The following cases demonstrate the use of MSCT and the benefits of post-processing techniques available with multislice technology.
Case 1
A 60-year-old male presented with recurrent abdominal pain and an intermittently raised amylase. The axial image (Figure 3a) shows a 6 mm area of calcification in the region of the head of the pancreas. The pancreatic duct is prominent in the body and tail of the pancreas and there is stranding of the peripancreatic fat. Using axial images alone it was not possible to accurately determine whether the area of calcification was intraductal (Figure 3a). The coronal oblique reconstructed image clearly shows the intraductal position of the calcification (Figure 3b).
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Figure 3. (a) Axial CT image showing a 6 mm area of calcification in the region of the head of the pancreas. (b) Coronal oblique reconstructed image clearly shows the intraductal position of the calcification.
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The clinical severity and outcome of patients with pancreatitis has a good correlation with the findings on CT [22, 23]. CT findings may be normal in up to 28% of patients, but CT finding are always abnormal in patients with necrotising and haemorrhagic pancreatitis [24]. Common CT findings are diffuse enlargement and irregularity of the contour of the pancreas, abnormal increased density of the surrounding peripancreatic fat, fluid collections and the presence of gas [25, 26]. The presence of pancreatic necrosis correlates well with areas of unenhancing pancreatic parenchyma after intravenous contrast medium has been given [21].
Case 2
A 33-year-old female was recently diagnosed with hypertension. The axial images of contrast-enhanced CT show a possible abnormality of the right renal artery (Figure 4a). Reconstructed images show the stenosis and reduced renal size on the right (Figure 4b). A subsequent angiogram showed a stenosis due to fibromuscular dysplasia.
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Figure 4. (a) Axial contrast-enhanced CT image showing a possible abnormality of the right renal artery. (b) Maximum intensity projection image showing a small right kidney and renal artery stenosis.
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Case 3
The axial contrast-enhanced CT image suggests a splenic artery aneurysm in this patient with an enlarged spleen (Figure 5a). The CPR image shows this to be a tortuous splenic vein (Figure 5b).
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Figure 5. (a) Axial contrast-enhanced image suggests splenic artery aneurysm. (b) Curved planar reformation image shows it to be a tortuous splenic vein, and the artery is normal.
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These cases highlight the use of post-processing techniques in the evaluation of vascular structures [9].
Case 4
A 65-year-old female presented with gastrointestinal bleeding and abdominal pain. Images obtained following intravenous contrast medium injection show thickening of the small bowel wall, poor perfusion of the right kidney and a filling defect in the superior mesenteric artery (Figure 6a). Coronal images show a segment of thrombus occluding the superior mesenteric artery (Figure 6b).
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Figure 6. (a) Contrast-enhanced images showing thickening of the small bowel wall, poor perfusion of the right kidney and a filling defect in the superior mesenteric artery (SMA). (b) Coronal images show a segment of thrombus occluding the SMA.
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Bowel ischaemia and infarction result in a high morbidity and mortality. The mortality of patients with acute mesenteric ischaemia exceeds 60%. It is common for the diagnosis to be made only after bowel infarction has already occurred [28]. CT can reliably detect bowel ischaemia in patients presenting with an acute abdomen. The recognized associated findings are bowel lumen dilation, bowel wall thickening, abnormal bowel wall enhancement, visible occlusion of the superior mesenteric artery or vein, intramural portal venous or intraperitoneal gas, the presence of ascites and infarct of other abdominal organs (seen in nearly half of cases) [29].
Case 5
A patient presented with abdominal pain and abnormal liver function tests. The common bile duct stone may be misinterpreted as gastrografin in a small bowel loop on the axial image (Figure 7a). The coronal MPR clearly shows the stone in the distal common bile duct (Figure 7b).
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Figure 7. (a) Common bile duct (CBD) stone misinterpreted as gastrografin in a small bowel loop on the axial image. (b) Coronal multiplanar reformation clearly shows the stone in the CBD.
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Case 6
In another patient, emphysematous cholecystitis is best appreciated on the MPR (Figure 8); this was difficult to visualize using ultrasound.
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Figure 8. Emphysematous cholecystitis on coronal multiplanar reformation image, with free fluid in the subhepatic area.
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Ultrasound is the preferred initial modality for investigation of the biliary tract. CT may, however, be the initial investigation if the diagnosis is unclear. A sensitive CT sign of acute cholecystitis is thickening of the gall bladder wall (>3 mm) and enhancement of the inflamed wall. Other signs are pericholecystic fluid, stranding in the pericholecystic fat and increased attenuation of the bile [30]. CT has a high sensitivity (88%) and specificity (97%) in the detection of common bile duct stones [31]. MSCT with narrow collimation and small reconstruction intervals is now able to detect small stones, which cause distal common bile duct obstruction [32].
Case 7
An elderly man presented with small bowel obstruction. The abdominal radiograph revealed dilation of proximal small bowel loops. The axial image shows a thickened gall bladder and duodenal wall that superficially resembles tumour (Figure 9a). However, the coronal image clearly shows a gallstone in the proximal jejunum causing gallstone ileus, and therefore the appearance on the axial image was entirely inflammatory in nature (Figure 9b). An incidental duodenal diverticulum is noted.
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Figure 9. (a) Axial image showing thickened gall bladder and duodenal wall that superficially resembles tumour. (b) Coronal image clearly shows a gallstone in the proximal duodenum.
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Intestinal obstruction is common, accounting for approximately 20% of surgical admissions for an acute abdomen. Adhesive disease counts for 60% of obstruction, with 80% of adhesions arising following surgery. The remaining 40% of obstructions are accounted for by hernias, inflammation, or congenital or unexplained causes [32]. An important role of CT is to differentiate patients that require early surgical intervention from those that can be safely managed conservatively, for example by the demonstration of occult tumour or hernia. CT has the additional ability of demonstrating signs of small bowel ischaemia by the following: poor or no enhancement of bowel wall, a serrated beak sign, a large volume of ascites, an unusual course of the mesenteric vasculature, and diffuse engorgement of the mesenteric vasculature [33]. Using these criteria, the detection of strangulated obstructions was found to be 83–85% [34, 35].
Summary
The arrival of MSCT into clinical practice has been a major and exciting development. It is now possible to obtain near isotropic imaging with high z-axis resolution with high quality two- and three-dimensional reformations. Imaging ranges have been dramatically increased and examination times markedly reduced, which allows imaging of organs in well defined perfusion phases. There is a disadvantage however, as there are an increasing number of CT examinations being performed with the same amount of time and number of radiologists available for reporting.
Received for publication November 7, 2003.
Revision received March 15, 2004.
Accepted for publication March 22, 2004.
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Abstract
Introduction
