With increasing concerns about patient radiation exposure from medical imaging, the development of low-radiation CT protocols is of value. This has been employed for renal stone evaluation, diagnosing suspected diverticulitis and for lung nodule follow-up [7, 8, 9]. The good spatial resolution, high contrast resolution, fast image acquisition and multi-planar reconstruction capabilities of computed tomography make this imaging modality very attractive for imaging the bowel as well. MRI and sonography are other imaging techniques that do not expose patients to ionizing radiation that can also be used to evaluate the bowel. Although MRI does not use radiation and has excellent contrast resolution, it is not as easily accessible, it requires the use of intravenous contrast, and the fairly long scan times can affect image quality because of intrinsic peristalsis of the small bowel [10].
Sonography can depict areas of bowel wall thickening and areas of inflammation; however, there are limitations, such as operator dependence, inter-observer agreement and even intra-observer agreement in the same patient during follow-up imaging. Additionally, detection and characterization of complex entero-enteric fistulas can be difficult [11]. Thus, we have developed a low-radiation dose, modified bowel CT (MBCT) enterography technique using hyperdense oral contrast, without intravenous contrast. MBCT can specifically evaluate small bowel while exposing the patient to less radiation compared to SBFT and CT enterography [4]. MBCT can characterize small bowel-related abnormalities, such as hernias, acute versus chronic Crohn’s disease, postoperative fistulas or bowel leaks and small bowel intraluminal masses including the site of chronic, low-grade bowel obstructions.
Applications
Follow-up of Crohn’s disease
Active Crohn’s disease on CT enterography is identified by increased mural attenuation, increased mural thickness, mural stratification and vasa recta hypervascularity [12, 13]. On SBFT, there is luminal narrowing with irregularity, but differentiation from chronic disease is not always possible [14]. On MBCT, increased mural thickness, increased mesenteric fat attenuation and prominent vasa recta can be identified, indicating active disease (Fig. 1). MBCT imaging for Crohn’s disease can delineate extraluminal complications such as fluid collections and enterocutaneous/enteroenteric fistulas. MBCT can also identify widening and fatty infiltration of the submucosal layer of the bowel due to chronic inflammation, consistent with the fat-halo sign [15]. Extra-enteric abnormalities associated with Crohn’s disease, including gallstones, renal stones and sacroiliitis, can often be identified.
Small bowel hernias
Abdominal hernias are evaluated using MBCT, especially with provocation techniques such as Valsalva manoeuvres, to enhance visualisation of the area of herniation.
Post-operative small bowel leaks/fistulas
Compared with fluoroscopic techniques, MBCT is advantageous in the assessment of enteric fistulas because of its ability to define the cross-sectional location of the underlying disease (Fig. 2a, b).
Chronic low-grade obstructions
Modified bowel CT is useful in identifying chronic low-grade obstructions, including the dilated segment and location of obstruction. Often, the diagnosis is that of adhesions, with abnormal tethering of small bowel loops and an associated distinct transition point indicating the site of obstruction. Strictures can also be characterised, including the length of bowel involvement and the severity of luminal narrowing.
Small bowel tumours
Small bowel neoplasms can be identified as a filling defect or an area of narrowing on MBCT. MBCT can depict areas of irregular luminal narrowing and shouldering of the margins, raising the suspicion of an underlying malignancy (Fig. 3a, b). Extrinsic masses, such as adenopathy and abnormal mesenteric masses, are also identifiable on MBCT.
Limitations
This technique is not recommended for evaluating patients with non-specific abdominal pain, unexplained weight loss or possible metastatic disease. As MBCT does not use intravenous contrast material, solid organs cannot be adequately evaluated. Differentiating a phlegmon from a mature abscess can be challenging without the use of intravenous contrast material. This technique should not be used in acute GI bleeding since the hyperdense oral contrast will obscure visualisation of the luminal blood.