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Everything you need to know about Computed Tomography (CT) & CT Scanning


Small Bowel: The Current Status of Multidetector-Row CT and 3D Imaging of the Small Bowel

Karen M. Horton, MD and Elliot K. Fishman, MD

Introduction

Since its introduction into clinical use in the late 1970’s, computed Tomography (CT) has made spectacular technological advancements. These advancement have dramatically improved the detection and characterization of a wide variety of small bowel pathology and today, CT is considered to play crucial role in the evaluation of the small intestine .

Early scanners acquired scan data relatively slowly, requiring 15 seconds to obtain a single 10mm slice with at least a 60 second reconstruction time. Although oral and intravenous CT contrast agents were available, long acquisition and reconstruction times resulted in significant respiratory and motion artifact and limited resolution of the small intestine and bowel wall. At that that time, the small bowel series still played the dominant role in radiological imaging of the small intestine.

The introduction of spiral (helical) CT in the late 1980’s revolutionized CT imaging of the gastrointestinal tract by allowing faster scanning, thinner collimation (3-5mm) and faster reconstruction times as well as volume acquisition. For the first time, CT did not consist of merely a series of slices, but as a complete volume of information. This paradigm shift led to the introduction of 3D imaging which greatly expanded the clinical role of CT. It was at this time in which CT began to play a more serious role in evaluation of gastrointestinal tract pathology, although barium studies were still the first line of imaging for most indications.

Today, multidetector-row CT (MDCT) represents the latest technical advancement in CT scanning and has completed the evolution from a slice based technique to a volume based technique. 8 detector -row scanners are now widely available and allow four slices to be obtained in less than 500ms with thinner collimation (0.5mm-1.0mm). 32-detector row CT scanners are now being introduced and will offer at least a three-fold increase in speed and a doubling of resolution compared with current systems. The availability of MDCT along with advancements in 3D CT imaging systems has greatly expanded the role of CT in evaluation of suspected small bowel pathology. Applications which were once routinely performed with barium studies (e.g. evaluate suspected small bowel obstruction) or angiography (e.g. evaluate for mesenteric ischemia) have now been replaced with CT scans. CT is now considered a first –line modality for the evaluation of a wide variety of small bowel diseases.

The chapter will discuss the current role of MDCT and 3D imaging in the detection and characterization of small bowel pathology, including mesenteric ischemia, Crohn disease, small bowel obstruction and neoplasms.

Technique

Oral Contrast

Adequate opacification of the small intestine is essential. Collapsed loops can mask disease or simulate pathology. Three categories of oral contrast agents are available: positive agents, neutral agents, and negative agents.

Positive oral contrast agents, such as iodinated or dilute barium solutions, are still the most widely used today. These appear white on CT usually result in good opacification of the small intestine. However, these agents can be problematic when performing 3D imaging and 3D angiography in particular since the high-density bowel contents can obscure the opacified blood vessels and therefore need to be edited.

Neutral agents such as water are gaining widespread acceptance as an oral contrast agent for CT. Water has distinct advantages over tradition positive agents. Water is inexpensive and well tolerated. When performing 3D imaging, extensive editing of the bowel contents is not necessary. Also, the use of water allows better visualization of the enhancing bowel wall. After the administration of intravenous contrast, the normal small bowel wall enhancing brightly, up to 120HU. If positive intraluminal contrast agents are utilized, subtle changing in bowel wall enhancement may not be appreciated. Many investigators are now recognizing the advantage of neutral agents such as water. However, water is not the perfect oral contrast agent. Because it is emptied rapidly from the stomach and is transported quickly through the small intestine, it does not always result in optimal distention of the distal small bowel. The administration of agents such as Glucagon may improve distension but is not routinely done. Even if the distal small bowel is not optimally distended, it is often possible to adequately visualize the bowel wall due to the density differences between the intraluminal water and enhanced bowel wall. Milk has been used by some groups in order to distend the small bowel in patients undergoing CT angiography. Whole milk has a CT density similar to water, but has a slower small bowel transit time and should therefore result in better distention of the bowel. Other agents such as methylcellulose have also been investigated as potential CT small bowel contrast agents.

The third category of oral contrast agents are those with negative density, such as oil based agents. Although they can result in good distention of the small intestine and good visualization of the enhancing wall they are not used routinely. Air or carbon dioxide distention of the small bowel would also be considered a negative oral contrast agent and would potentially work well, but is not practical at his time. There is currently no easy way to noninvasively distend the small intestine with air.

In general, the clinical indication will dictate the type of oral contrast agent which should be used. For example, in routine cases (e.g. abdominal pain) traditional positive contrast agents are adequate. If 3D imaging of the small bowel or mesenteric vessels is anticipated, we routinely administer water.

Intravenous Contrast

The use of intravenous contrast is essential for the evaluation of inflammatory and neoplastic disease of the small bowel. For routine studies, we administer 120 cc of non-ionic contrast (Omnipaque 350, Amersham, Princeton, NJ) and inject at a rate of 2-3 cc per second. For detailed studies of the mesenteric vasculature, higher injection rates (3-5cc per second) are helpful.

When water is used as oral contrast along with rapid IV injection of iodinated agents, the normal small bowel wall will enhance brightly, and should not be mistaken for pathology.

MDCT

MDCT offers two distinct advantages over traditional spiral CT. First, scanning is faster which typically minimizes or eliminates respiratory motion and allows more accurate timing of the bolus during the arterial and venous phases. This results in better CT angiography images of the mesenteric arteries and veins. Currently available 8 detector scanners allow up to 4 slices to be created per 500ms rotation. Therefore 8 slices can be obtained per second! Depending of the collimator settings, the entire abdomen and pelvis can be scanned in 10 seconds. In addition, this faster scanning allows us to better visualize the enhancing bowel wall. Little has been published about this technique, but the ability to measure the enhancement of the bowel overtime, could be helpful in the diagnosis of certain diseases such as ischemia or Crohn disease.

Second, thinner collimation is possible with MDCT, 0.5-1.0 slices can be obtained if necessary which decreases partial volume effects and greatly improves the quality of the CT angiograms. In the past we would routinely obtain 5mm slices through the abdomen and pelvis when evaluating all small bowel pathology. However, now in select cases we can obtain a more detailed examination using 1.25 or 3mm slices, if necessary. MDCT offers a unique capability over traditional spiral scanners by allowing the slice thickness to be selected after the study has been completed. For example, if the 4 X 1mm beam collimation is selected, then the following slice widths are possible: 1.0mm,1.25mm 2.0mm, 3.0mm, 4.0mm, 5.0mm, 6.0mm, 7.0mm, 8.0mm and 10.0mm. The resultant slice width can be varied depending on the beam collimation selected. This choice can be altered after the data acquisition. Therefore when 3D studies are desired, we would routinely reconstruct and film 3mm slices while reconstructing and sending 1.25 mm slices to the 3D workstation. The thinner 1.25mm slices will improve the 3D images but would appear grainy on review of the axial images. Hence MDCT is flexible and allows different slices thickness to be selected after the initial acquisition. Another example of when this is useful is in routine studies when an abnormality is detected. For example, in our routine studies of the abdomen we utilize the 4 X 2.5mm-collimator setting to create 5mm slices. However, if an abnormality is detected, we can then go back and create 3mm slices through the abdomen from the initial data set.

Overall, MDCT imaging has resulted in improvement in CT of the abdomen and newer 32 detector-row scanners offer even faster scanning with thinner collimation, which can only improve our ability to visualize small bowel pathology.

3D Imaging

3D reconstruction of CT data has been possible for almost 20 years. However, early systems were crude and offered only simplistic renderings of the surface of structures such as the bone. They offered little applications for imaging of the gastrointestinal tract. Fortunately, major advancements in both CT scanner technology and computer hardware/ software have now made powerful and affordable 3D imaging systems available. Current systems offer real-time volume- rendering software which is easy to use and simply to incorporate into existing practices. We currently utilize the Siemens 3D-Virtuoso (Siemens Medical Solutions, Iselin, NJ).

3D-volume rendering has proven to be useful for many applications in the abdomen such as: staging pancreatic cancer, evaluating renal donors, surgical planning for partial nephrectomy, virtual colonoscopy etc. In addition, when coupled with the improved resolution and speed on MDCT scanners, investigators are now starting to realize the potential value of 3D CT imaging for the evaluation of many small bowel conditions such as small bowel ischemia.

In the remainder of this chapter we will describe and illustrate the current status of MDCT and 3D imaging for evaluation of the small intestine. The normal appearance of the small bowel and mesenteric vessels will be reviewed followed by a discussion several specific clinical indications in which we feel that the use of MDCT and 3D imaging is valuable.

Normal Anatomy

Small Bowel

When water is used as oral contrast along with a rapid injection of intravenous contrast, the normal small bowel wall enhances brightly and should not be mistaken for pathology. With the thin collimation available on MDCT scanners, individual small bowel folds can be visualized . Even if the distal small bowel is not optimally distended, it is often possible to visualize the enhancing small bowel wall and to detect pathology. The use of multiplanar reformations or coronal 3D projections are helpful in visualizing the entire small bowel and are well received by referring physicians who are comfortable with that orientation which is similar to a small bowel series. In addition to standard soft tissue window, current 3D CT imaging systems allow adjustment of window opacity and brightness in addition to level and window center. Using these tools it is possible to create other image displays to accentuate certain anatomy or pathology. For example, a simulated "small bowel series" can be created . 3D imaging has a distinct advantage over traditional small bowel series but allowing the use of cut planes thus eliminating the problem with overlapping loops. Each small bowel loop can be visualized, but simply rotating the volume and using clip planes to delete any overlying loops.

3D imaging of the colon (Virtual Colonoscopy) is gaining acceptance as a potential screen tool for the detection of colon polyps. The same technology, which is utilized to create the 3D endoluminal views of the colon, can be applied to imaging of the small bowel. However, the small bowel presents unique obstacles due to its tortuosity and because there is no easy system to noninvasively distend the entire small intestine, further investigation in to the technique is necessary to determine if it will be feasible.

Mesenteric Vessels

MDCT coupled with significant improvements in 3D imaging software and hardware have greatly improved out ability to noninvasively image the abdominal vessels including the mesenteric vasculature. In the past, angiography was considered to the only acceptable radiological imaging study to evaluate the complex mesenteric arteries and veins, since it was difficult to visualize these vessels adequately on traditional axial CT scans. Although early 3D imaging systems, using spiral CT data, could generate vascular maps, these systems were time consuming, often requiring hours of editing and resulted in limited resolution due to slower scanning times and relatively thick (5mm) collimation. However, newer 3D imaging systems along with the thinner collimation and after scanning possible with MDCT, have made it possible to generate detailed 3D CT angiographic maps of the mesenteric arteries and veins (Figures 4,5,6). CT angiography can now routinely imagine these complex vessels and there branches and identify anatomic variations. This has eliminated the need for conventional angiography in certain applications, such as pancreatic cancer staging. In the past, referring clinicians would routinely order angiography in addition to a CT scan in patient with pancreatic cancer. However, today the same information can be obtained from a dual phase CT with 3D imaging which can adequately demonstrate vascular invasion of the celiac axis, superior mesenteric artery, portal or mesenteric veins. In addition, the ability to visualize the mesenteric vessels with CT Angiography has allowed CT to play a more important role in certain small bowel conditions such as mesenteric ischemia, where in the past angiography would be necessary.

Small Bowel Pathology

Mesenteric Ischemia

Mesenteric ischemia is a complex disorder which can be divided into two distinct categories: acute and chronic. MDCT with 3D imaging can play a role in the diagnosis of both.

Acute mesenteric ischemia occurs as a result of a sudden compromise of blood flow to the intestines. Thromboembolic arterial occlusion is the most common cause of acute mesenteric ischemia, with thrombus usually arising from the heart and most commonly involves the proximal portion of the superior mesenteric artery. This represents approximately 50% of cases. Thrombosis of an existing atherosclerotic plaque in the superior mesenteric artery accounts for 25% of cases of acute mesenteric ischemia. Non occlusive ischemia, occurring in patients with low flow states, and mesenteric venous thrombosis or occlusion of the mesenteric vessels by tumor are less common, together accounting for 25% of cases.

Since most cases of acute mesenteric ischemia involve emboli or thrombus occurring at the proximal portion of the superior mesenteric artery, CT angiography is well suited as a first line imaging modality . Dual phase imaging of the abdomen is necessary during the arterial and venous phases of enhancement in order to adequately opacify both the mesenteric arteries and veins. In patients with suspected mesenteric ischemia, we routine scan 25s (arterial) and 50s(venous) after the administration of 120 cc on nonionic contrast given at a rate of 3-5cc/second. Using MDCT, thin collimation (1.25mm) is performed during both phases and reconstructed at 1mm intervals. 3D imaging is performed in all cases and is crucial to visual more distal branches, which can be involved, resulting in segmental involvement of the small intestine. Axial images alone are not adequate, as the branching pattern of the mesenteric vasculature is complex and branches often measure only 2-3mm in caliber. Tiny branches are very difficult to appreciate on axial images, but are easily identifies with 3D CT maps.

In addition to evaluating the mesenteric arteries, the mesenteric veins are easily visualized on the venous phase images and again are better appreciated using 3D imaging. Although mesenteric vein thrombosis is relatively uncommon, we have diagnosed several cases in the past year in post-operative patients or in patients with hypercoagulable states . The most difficult etiology of mesenteric ischemia to detect is those cases resulting from low flow states. The CT angiogram findings are similar to the findings on conventional angiography, but are often subtle. The mesenteric vessels may appear pruned and small in caliber, with less side branches visible.

In addition to the thrombus or atherosclerotic plaque in the mesenteric vasculature, CT can detect changes in the bowel wall in patients with acute mesenteric ischemia.

The most common reported CT finding in patients with ischemic bowel is circumferential wall thickening which usually does not exceed 1.5cm in thickness. The bowel wall thickening can be low in attenuation due to submucosal edema and inflammation or the bowel wall may demonstrate increased attenuation as a result of submucosal hemorrhage . Intramural hemorrhage, however, is not specific for ischemia, as it can occur as a result of condition including trauma, anticoagulation therapy or radiation. Although the bowel wall thickening is usually homogeneous, a halo appearance to the bowel wall has also been described in patients with ischemia. Associated stranding and fluid in the mesentery is often present. Although bowel wall thickening is a common findings in patients with ischemic bowel, it is very nonspecific, as it occurs in many inflammatory, infectious, or neoplastic conditions. However, it is often the distribution of the bowel wall thickening which may suggest the diagnosis. For example, if the jejunum and ileum are affected, along with the right colon, ischemia should be a consideration, as this represents the vascular supply of the superior mesenteric artery.

Thickened small bowel loops may demonstrate absence of enhancement or in some cases, delay in enhancement when compared to unaffected loops. Visualization of the bowel wall and its enhancement is easier to appreciate when low denisty oral contrast is utilized. In addition, the thinner collimation possible with MDCT allows better visualization of the enhancing bowel wall and can be utilized to obtain functional as well as anatomic information. The flexibility and real-time capabilities of 3D imaging is useful to visualize the entire small bowel and colon .

Dilatation of the affected small bowel loops may be present probably as result of disruption of the normal peristaltic activity The dilated bowel loops are often filled with fluid which is most likely due to fluid and blood which has seeped from the ischemic bowel wall. In severe cases of ischemia, intraluminal gas can dissect in to the friable and inflamed small bowel wall resulting in pneumotosis. Although pneumatosis has been reported in benign condition such as collagen vascular disease, steroid use, pulmonary disease, etc, the clinical presentation and history usually will allow differentiation. On CT pneumatosis appears as air within both the bowel wall (Figure10). In some patients the intramural air may the dissect from the bowel wall into the mesenteric veins or portal vein, which can be easily detected with CTAlso, free intraperitoneal air has been reported, and as with pneumatosis is an ominous sign, usually signifying transmural infarction of the bowel.

In patients with symptoms suggesting chronic mesenteric ischemia, atherosclerotic plaque may be identified in the mesenteric arteries. This is especially easy to detect if the plaque is calcified. However, atherosclerotic plaque affecting mesenteric arteries is a common finding in older patients and does not necessarily signify significant disease. In addition to the presence of atherosclerotic plaque, patients with chronic mesenteric ischemia may also demonstrate collaterals, which have developed between the celiac, SMA, and IMA, in an effort to maintain adequate perfusion to bowel. MDCT with 3D imaging also allows visualization of these anastomotic pathways and can also be used to evaluate patients after bypass surgery. Typically, in the CT evaluation of patients with chronic mesenteric ischemia, the small bowel appears normal, as this condition usually develops slowly with the development of sufficient collateral vessels to maintain adequate blood flow to the bowel.

Some patient with chronic mesenteric ischemia can present with acute symptoms due to thrombus forming of in a region of existing atherosclerotic plaque. These patients may demonstrate a combination of CT findings including, atherosclerotic plaque, thrombus, collaterals and small bowel wall thickening representing acute on chronic ischemia.

MDCT with 3D imaging can also be utilized in the follow-up of these patients after surgery, which typically includes intestinal bypass with grafts. The patency of the grafts can be easily be documented, as can the adequacy of perfusion to the intestine. In the past, angiography was necessary to evaluate these patients post-op.

Crohn Disease

CT has come to play an increasingly important role in the evaluation of patients with Crohn’s disease because of its ability to accurately demonstrate the bowel wall as well as adjacent structures and extraluminal extension of disease. For over twenty years CT has been used along with barium studies such as small bowel series or enteroclysis in order to manage patients with Crohn disease. The barium studies were necessary to optimally evaluate the bowel wall , mucosa and enteroenteric fistulae, while CT was essential to detect extramural disease such as abscess.

The role of CT in the evaluation and management of patients with Crohn disease has expanded as CT technology has improved. The improved resolution possible with new MDCT allows better evaluation of the bowel wall. In many patients with active disease, mucosal and submucosal ulcerations can now be visualized . In addition, the faster scanning and faster IV contrast bolus injections also improves visualization of the enhancing bowel wall layers especially if water is used as oral contrast (Figure12). This may help in distinguishing active from inactive disease. In turn, this increased resolution, faster scanning, and faster contrast bolus, improved the quality of the 3D images. Because of the nature of the process, patients with Crohn disease often have complex disease, involving multiple bowel loops and or adjacent organs. The ability to visualize the scan in more than the axial plane is essential to complete understanding of the extent of the disease. In 1997, Raptopoulos et al studies 22 patients with Crohn disease to determine the usefulness of multiplanar reconstructions in revealing complications. Although in this study, the multiplanar reconstructions did not detect additional abnormalities over axial images alone, the multiplanar reconstructions significantly improved the observer’s confidence in their interpretation of the imaging and in their ability to detect the extent of the bowel involvement. In that study, which included high volume oral contrast (1600cc), the CT examination was found to be comparable with the barium studies in 9/14 patients and superior to the barium studies in 4/14 patients. The CT study was thought to be inferior to the barium study in one patient. Since 1997 the quality of multiplanar reconstructions have significantly improved due to the improvements in scanner technology and due to the ability to obtain thinner collimation. 3D imaging has also improved and allows greater flexibility than multiplanar reformations to display complex anatomy. CT angiography can also be performed to evaluate changes in vessels supplying the diseased loops . . Although no large studies have yet been performed to determine the exact role of 3D imaging in patients with Crohn disease, it has been our experience that 3D imaging adds definite value when evaluating patients with Crohn disease. The speed and flexibility of current systems makes 3D review of the data quick and easy. Complex disease can be better understood and in addition to improving the radiologists understanding of the process, 3D imaging of patients with Crohn disease is well received by the clinicians and surgeons who are more comfortable reviewing the disease in multiple planes vs. axial images alone.

Small Bowel Obstruction

Small bowel obstruction is one of the leading causes of admissions for acute abdominal pain. Over the years, there has been an increasing tendency for initial conservative management of these patients instead of immediate surgical intervention. This has resulted in a growing reliance on radiological imaging to diagnose the presence of an obstruction in addition to determining the site, level and cause of obstruction as well as the presence of life threatening complications such as ischemia or perforation. For many years the evaluation of possible small bowel obstruction relied on plain films of the abdomen and small bowel series which demonstrated as sensitivity or 69% and specificity of 57%. With the development of CT, radiologist soon realized that CT could be utilized in this clinical setting. Many studies have evaluated the role of CT in the diagnosis and management of patients with suspected small bowel obstruction, with a sensitivity ranging between 94-100% and an accuracy of 95%. CT has proven to be more sensitive and specific than barium studies and is much better that plain films and barium studies in its ability to determine the level and cause of the obstruction as well as to detect complications especially in cases of long standing or high grade obstruction. Enteroclysis may still be necessary in patients with low grade obstruction.

Improvements in CT technology have expanded the role of CT for this clinical indication. New MDCT scanners allow thinner collimation and faster scanning which increases resolution and decreases respiratory and motion artifact. These patients who are often in severe pain can be scanned in less than a minute. Often oral contract in not necessary, since obstructed patients have dilated fluid filled bowel. However, IV contrast is essential to be able to determine the cause of obstruction and identify any associated strangulation.

The ability to view the CT data in more than one plane can be useful in the evaluation of patients with small bowel obstruction. In some patients the exact level of transition may be difficult to identify on axial images alone are reviewed. Multiplanar reconstruction is now widely available and allows a time efficient method to view the data simultaneously in the axial, sagittal and coronal planes. This ability is especially useful in complicated cases and is often increasing the radiologist confidence. In addition the clinicians and surgeons appreciate the availability of additional planes, which helps to better understand complex cases. The usefulness of 3D imaging in the setting of small bowel obstruction has not yet been determined. However, 3D imaging is more flexible than simple multiplanar reconstructions and allows display in unlimited planes. 3D imaging systems allow adjustment of window levels, center, opacity, and brightness levels to best display the anatomy and pathology. We routinely perform 3D imaging in complicated cases of small bowel obstruction. It only takes a few minutes and allows a much more comprehensive understanding the case.

Small Bowel Neoplasms

Neoplasms of the small bowel continue to pose a significant diagnostic challenge to both the clinician and radiologist. Despite advancements in diagnostic examinations, the survival of patients with small bowel malignancies had not changed significantly in over 40 years. These tumors are uncommon with an annual incidence of only 0.5 to 1.0 per 100,000 people in the western hemisphere and therefore the diagnosis of small intestinal neoplasms may be overlooked. Also these tumors usually produce nonspecific symptoms such as abdominal pain, gastrointestinal bleeding, nausea, vomiting, or weight loss which may contribute to a delay in diagnosis. In addition most small bowel neoplasms are small, especially in the early stages which makes diagnosis by conventional radiological examinations difficult. Although small bowel enteroscopy may come to play a larger role in the diagnosis of small bowel tumors, this technology is still evolving and has not yet replaced more traditional radiological examinations such as barium small bowel series, enteroclysis or Computed Tomography.

CT is starting to play a more active role in the detection and staging of small bowel neoplasms. The thinner collimation possible with MDCT, coupled with the use of water as oral contrast allows the diagnosis of even small, non obstructing tumors. Some investigators advocate the use of CT enterocylsis which involves high volume distention of the small intestine in order to detect subtle masses. This technique can be helpful in evaluation of patients with unexplained gastrointestinal blood loss. However, in our experience, adequate CT examination of the small intestine can be achieved when water is administered as oral contrast and 3D imaging is performed to ensure optimal visualization of each small bowel loop.

Even subtle adnenocarcinomas, which are more common in the duodenum, can be visualized when the duodenum is well distended with water and a good IV contrast bolus is administered . The tumor most frequently appears as eccentric or circumferential wall thickening involving a short segment of the small bowel. This may result in an "apple core" appearance, similar to that seen with barium contrast studies.

The radiological appearance of carcinoid tumors varies depending on the size and location. The ileum is the most common location for small bowel carcinoid, followed by the jejunum. Carcinoid tumors arise from the Kulchitsky cells in the crypts of Lieberkuhn and therefore grow as submucosal nodules. Traditionally, CT examinations were not very successful at detecting the carcinoid tumor when it was confined to the bowel wall. However, these small submucosal masses are vascular and now can sometimes be visualized when water is given as oral contrast, a good IV contrast bolus is administered and thin collimation MDCT is performed (Figure 17). As the small intestinal carcinoid grows, there may be extension outside the bowel loop, with infiltration of the mesentery. Carcinoids which have infiltrated the mesentery demonstrate a characteristic CT appearance . On CT, this appears as an infiltrating mesenteric mass, containing calcification in up to 70% of cases. MDCT scans with intravenous contrast along with CT angiography nicely demonstrates the relationship of the mass to the mesenteric vessels which is crucial for surgical planning. Thickening and ischemia of the involved small bowel loops may also be demonstrated as a result of mesenteric vessel encasement. Although the CT appearance of a mesenteric mass with calcifications and desmoplastic reaction is suggestive of carcinoid tumor, other conditions such as treated lymphoma or sclerosing mesenteritis can have a similar CT appearance.

CT has always played an important role in evaluation and follow-up of patients with lymphoma. Most small bowel lymphomas are of the non-Hodgkin’s cell type involving the mesenteric small bowel and rarely the duodenun. The primary tumor can often be detected with small bowel contrast studies and CT. However, CT offers the advantage of simultaneously detecting adenopathy as well as the extraluminal extent of disease. Primary small bowel lymphoma often appears as a focally thickened loop, which usually does not result in obstruction (Figure 19). Its appearance can simulate an adenocarcinoma.

Gastrointestinal stromal tumors (GIST) which arise from smooth muscles cells in the muscularis propria are estimated to comprise between 10 - 16% of all intestinal neoplasms. These tumors are characterized pathologically by their differentiation into smooth muscle or neural elements and are then categorized as benign, borderline, low malignant potential, or malignant. CT cannot reliably distinguish benign from malignant GIST unless there is obvious metastases or local extension. They can ulcerate or calcify and usually are not associated with significant adenopathy . Gastointestinal stromal tumors can be very large. These tumors can be so large, that it becomes difficult to determine their site of origin. 3D CT imaging may be helpful in these instances to better define the site of origin and to help the surgeon planned for resection.

Conclusion

Radiologists have always played an important role in evaluation of patients with small bowel pathology. Early on, the small bowel series and later the enteroclysis were the mainstay in radiological diagnosis of many small bowel diseases, since the resolution and speed of CT was limited. Continued improvements in CT technology over the last two decades have resulted in an expanding role of CT for evaluation of the gastrointestinal tract, including the small intestine. Many conditions, such as small bowel obstruction and ischemia, which would traditionally be imaged with other modalities (small bowel series or angiography), are now routinely imaged with CT. The development of MDCT along with improvements in 3D imaging systems has greatly improved our ability to examine the small bowel and mesenteric vasculature. With the introduction of new CT oral contrast agents and faster 32 detector row CT scanners, the diagnosis and evaluation of patients with small bowel disease will continue to improve.

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