google ads
Search

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


Small Bowel: Normal Enhancement of the Small Bowel: Evaluation with Spiral CT

Abstract

Purpose: To determine normal contrast enhancement of the small bowel with biphasic spiral CT, using water as oral contrast.

Materials and Methods: Biphasic spiral CT was performed in 50 healthy patients undergoing evaluation as potential renal donors. All patients received 500 cc of water as oral contrast and 150 cc of Omnipaque 350 administered by mechanical injector at a rate of 3 cc/sec. Dual phase CT of the abdomen was performed in each patient. Acquisition of early phase images began 30 seconds after the start of the intravenous injection, and portal phase images were obtained 60 seconds after initiation of the contrast injection. Attenuation measurements (in Hounsfield units) were obtained from the wall of the small bowel (duodenum, jejunum, ileum) in both the arterial and portal phases.

Results: During the arterial phase, the mean (95% confidence interval) attenuation of the duodenum, jejunum, and ileum was 120 (± 5) HU, 119 (± 5) HU, and 118 (± 5) HU, respectively. During the portal phase, the average attenuation of the duodenum, jejunum, and ileum was 111 (± 4) HU, 111 (± 3) HU and 107 (± 3) HU, respectively. There was no statistically significant difference between the attenuation of the duodenum, jejunum or ileum within either the arterial or portal venous phases. There was a statistically significant difference in small bowel enhancement between the arterial and portal venous phases.

Conclusions: There is no important variation in small bowel attenuation during the 30sec and 60 sec scanning phases. This study serves as a normal reference which may be helpful when spiral CT is used to evaluate ischemic bowel or inflammatory small bowel diseases.

Introduction

Spiral CT couples fast data acquisition with rapid intravenous contrast infusion, allowing imaging in different phases of contrast enhancement. This has been well described in the liver1,2 and kidney3,4 but has not been previously reported in small bowel imaging. These techniques can be adapted for evaluation of the small bowel to obtain better visualization of the mesenteric vessels and to evaluate and quantify enhancement patterns of the small bowel. This may aid in the early diagnosis of a variety of small bowel diseases and may allow assessment of disease activity.

In order to optimize visualization of wall enhancement with CT after intravenous contrast, it is necessary to distend the small bowel. Collapsed bowel segments can simulate disease or obscure pathology. Currently, oral contrast agents such as dilute barium or iodinated solutions are used for abdominal CT and help to distend the gastrointestinal tract. These solutions are positive contrast agents. Since the bowel wall enhances significantly, subtle mural abnormalities or alterations in wall enhancement could be obscured by the adjacent high density oral contrast within the bowel lumen (Fig. 1). Positive contrast agents also interfere with 3D manipulation of CT data when performing CT angiography. This can be avoided by administering a neutral contrast agent such as water.

Water is being used more frequently for imaging of the gastrointestinal tract, especially the stomach5-8. Water allows excellent visualization of the enhancing wall of the gastrointestinal tract and is safe and inexpensive. In a study of 102 patients by Winter et al, water was shown to be efficacious in evaluating normal anatomy of the upper gastrointestinal tract as well as a variety of pathologic conditions. In that study, water was well tolerated and was preferred over positive contrast agents by 89% of patients9.

Our goal in this study was to determine the normal enhancement of small bowel using water as oral contrast and biphasic spiral CT after the administration of intravenous contrast.

Materials and Methods

Study Population

During a 10 month period, from 2/97 to 1/98, 51 consecutive patients underwent biphasic spiral CT for evaluation as potential renal donors. One patient was discovered to have malrotation of the small bowel and was thus excluded. The remaining 50 patients were healthy.

CT Protocol

All patients were asked to fast for four hours prior to the study and were given 500 cc of water 15-20 min before the exam. Spiral CT was performed in all patients on a Siemens Somatom Plus 4 scanner using 120 kVp and 280 mAs. The scan collimation was 3 mm, with a pitch of 1.7.

 

Patients received 150 cc of contrast (Omnipaque 350, Nycomed Inc., Princeton, NJ) administered intravenously using a mechanical injector at a rate of 3 cc/sec. Dual phase CT of the abdomen was performed in each patient. Acquisition of early phase images began 30 seconds after the start of the intravenous injection, and the portal phase images were obtained at around 60 seconds after the initiation of the injection.

CT Attenuation

Attenuation measurements (in Hounsfield units) were obtained from the wall of the small bowel (duodenum, jejunum, ileum) in both the arterial and portal phases. The duodenum was sampled as it crossed the midline, the jejunum was sampled in the upper left abdomen, near the ligament of Treitz and the ileum was sampled at the level of the iliac crests . A region of interest of measuring 2-3 mm2 was used for each measurement. Care was taken to exclude intraluminal water or extraluminal fat from the regions of interest. Attenuation measurements of the aorta were also recorded as a reference in both phases of each study at the same level as the proximal small bowel measurements. All measurements were made by one board certified radiologist (K.M.H.).

Data analysis

The main outcome variables were the 8 CT attenuation measurements for each subject (aorta, duodenum, jejunum, and ileum each during the arterial and venous phases). Student t tests were used to analyze any differences in the CT attenuation between the groups: the t tests were unpaired for comparing gender groups and paired for comparing the two imaging phases. Simple linear regression was used to analyze any potential confounding correlations between the attenuation measurements and age. Data analysis was performed with standard statistical software (Stata 4.0 for Macintosh, Stata Corporation, College Station, TX). A two tailed p value of less than 0.05 was considered statistically significant.

Results

The study population consisted of 29 women and 21 men with a mean age of 44 +/- 12 (S.D.) years. There were no statistically significant correlations between age and any of the 8 variables.

 

During the arterial phase, the mean attenuation (± 95% confidence intervals) of the duodenum, jejunum, and ileum was 120 (± 5) HU, 119 (± 5) HU, and 118 (± 5) HU, respectively. During the portal phase, the mean attenuation (± 95% confidence intervals) of the duodenum, jejunum, and ileum was 111 (± 4) HU, 111 (± 3) HU and 107 (± 3) HU, respectively. CT density values of the small bowel are plotted in Fig 3 for each of the two imaging phases. Within each phase, all pairwise comparisons between the CT attenuation of the three small bowel locations revealed no statistically significant differences, except for the ileum in the venous phase. The mean CT attenuation of ileum in the venous phase was 4 HU less and either the duodenum or ileum, which is not thought to be practically significant.

 

Statistically significant, though small differences were observed between the two imaging phases for all three small bowel regions .In the arterial phase the mean CT attenuation of the duodenum was 8 HU higher than that in the venous phase. In the jejunum, this difference was also 8, and the ileum this difference was 11 HU.

Discussion

Computed Tomography (CT) of the gastrointestinal tract relies on static imaging of luminal distention, bowel wall thickening and mesenteric inflammation for evaluation of many small bowel diseases. These parameters may be inadequate for early detection of certain diseases and are limited in their ability to assess activity of disease. The development of subsecond spiral CT with the ability to obtain volume data sets at defined enhancement points (i.e. arterial phase or venous phase) makes it possible to quantify small bowel enhancement and therefore allows acquisition of functional as well as anatomic information.

 

In this study, the enhancement of the small bowel in the arterial and venous phase was assessed in 50 healthy patients. The analysis revealed two important points. First, there was no clinically significant difference in normals between the enhancement values of the duodenum, jejunum, and ileum within either the arterial or portal phases. Thus, a patient could serve as his/her own internal control when assessing the attenuation values of a segment or region of diseased bowel.

 

Second, although the data demonstrates a statistically significant difference between small bowel enhancement in the arterial and portal phases, the difference was small and is probably not practically important. This suggests that the exact timing of the scans after injection may not be critical as long as the attenuation of the bowel segments is constant in the individual patient. Although the portal phase did suggest a statistically significant difference between the duodenum/jejunum and the ileum, (4HU) this is not an important practical difference.

This data serves as a useful normal reference of small bowel enhancement with spiral CT. Assessing small bowel perfusion patterns may aid in the early diagnosis of a variety of diseases and may allow assessment of disease activity. For example, in a study of 20 patients with pancreatic cancer, Sheiman et al measured small bowel attenuation before and after the administration of intravenous contrast and found a detectable increase in small bowel enhancement in patients with splanchnic venous invasion10. This was thought to be due to splanchnic congestion causing a delay in contrast wash-out from the bowel wall. In a study by Harvey, at al, kinetic curves of small bowel enhancement were obtained in a group a patients with Crohn's disease who underwent CT enteroclysis or CT pneumocolon. Actively inflamed segments of affected bowel demonstrated elevated perfusion rates11. Finally, obtaining accurate functional information of small bowel perfusion may improve evaluation of patients with mesenteric ischemia, as small bowel ischemia can appear as segments with decreased wall enhancement or delayed wall enhancement12,13.

 

A potential limitation of this study is the lack of noncontrast scans. The absolute change in attenuation of the small intestine after contrast administration may be an important variable, especially in the evaluation of certain conditions such as small bowel ischemia. The patients in our study were undergoing evaluation as potential renal donors with dual phase spiral CT, and an additional noncontrast CT would have added unnecessary radiation exposure without contributing to their evaluation.

Another limitation of using water as oral contrast is that it is quickly absorbed and may not result in adequate distention of the distal small bowel, although this did not significantly hamper our ability to obtain density measurements. In our patients, the water resulted in some distention of bowel loops, which allowed densimetric measurement of the enhancing wall to be made. The use of glucagon would probably have increased small bowel distention. Complete distention of the small bowel in normal individuals would render the bowel wall almost imperceptible, and would not allow acquisition of accurate density measurements. However, in patients with diseased small bowel, wall thickening would persist even with maximal distention, thus allowing attenuation measurements to be obtained.

 

In summary, improvement in spiral CT technology now allows functional data to be obtained in addition to anatomic information. This data serves as a useful normal reference of small bowel enhancement with spiral CT. Potential applications include improved assessment and diagnosis of conditions such as mesenteric ischemia, and Crohn's disease by providing more physiologic evaluation of small bowel perfusion.

References

1. Platt JF, Francis IR, Ellis JH, Reige KA. Liver metastases: early detection based on abnormal contrast material enhancement at dual-phase helical CT. Radiology 1997;205:49-53.

2. Dual-phase helical CT of the liver: value of an early-phase acquisition in the differential diagnosis of noncystic focal lesions. AJR 1997;168:1185-1192.

3. Goldman SM. Dual phase helical CT of the kidney: value of the corticomedullary and nephrogenic phase for evaluation of renal lesions and preoperative staging of renal cell carcinoma. J Urol 1998;160:1586-1587.

4. Szolar DH, Kammerhuber F, Altziebler S, et al. Multiphasic CT of the kidney: increased conspicuity for detection and characterization of small (<3-cm) renal masses. Radiology 1997;202:211-217.

5. Baert AL, Roex L, Marchal G, Hermans P, Dewilde D, Wilms G. Computed tomography of the stomach with water as an oral contrast agent: technique and preliminary results. J Comput Assist Tomogr 1989; 13:633-636.

6. Gossios KJ, Tsianos EV, Demou LL, et al. Use of water or air as oral contrast media for computed tomographic study of the gastric wall: comparison of two techniques. Gastrointestinal Radiology 1991; 16:293-297.

7. Hori S, Tsuda K, Murayama S, Matsushita M, Yukawa K, Kozuka T. CT of gastric carcinoma: preliminary results with a new scanning technique. RadioGraphics 1992; 12:257-268.

8. Rossi M, Broglia L, Maccioni F, et al. Hydro-CT in patients with gastric cancer: preoperative radiologic staging. Eur Radiol 1997; 7:659-664.

9. Winter TC, Ager JD, Nghiem HV, Hill RS, Harrison SD, Freeny PC. Upper gastrointestinal tract and abdomen: water as an orally administered contrast agent for helical CT. Radiology 1996; 201:365-370.

10. Sheiman RG, Raptopoulos V. Delayed intravenous contrast medium washout from the small bowel in patients with pancreatic carcinoma and splanchnic venous invasion. Jour Comput Assist Tomogr 1996;20:924-929.

11. Harvey, CJ, Fidler H, Gillams AR, Bonsi D, Bloom S, Lees WR. Spiral CT derived perfusion measurements of the bowel wall in intestinal malignancy and inflammatory bowel disease. Abstract at Radiological Society of North America, 1998 in Chicago, IL.

12. Taourel PG, Deneuville M, Pradel JA, Regent D, Bruel JM. Acute mesenteric ischemia: diagnosis with contrast enhanced CT. Radiology 1996;199:632-636.

13. Zalcman M, Van Gansbeke D, Lalmand B, Braude P, Closset J, and Struyven J. Delayed enhancement of the bowel wall: a new CT sign of small bowel strangulation. Jour Comput Assist Tomogr. 1997;20:379-381.

© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.