The New Era of Crohns Imaging: A Multimodality Approach

Crohns disease (granulomatous colitis) is an inflammatory bowel disease with a bimodal distribution of age at diagnosis, with peak in incidence at 15-25 years old, followed by a second smaller peak in the sixth or seventh decade¹.

The inflammation is not simply confined to the mucosa, it is transmural with skip lesions involving any part of gastrointestinal tract. However, there is a predilection for the terminal ileum. Small-bowel involvement is seen in 80 percent of patients¹.

The early age at diagnosis is associated with a family history of Crohn's disease, a complicated disease course with greater small-bowel involvement and higher frequency of surgery.

Crohn's disease has classically a relapsing, remitting course with periods of disease activity and remission within the patient's lifetime. A small minority (10 percent) of patients experience prolonged remission, up to 57 percent of patients require at least one surgical resection, and 10 percent require repeated admissions with multiple imaging investigations¹.

Unfortunately, classification of disease activity solely on the basis of clinical and laboratory parameters has not been dependable or reproducible and therefore a multifaceted approach using serum markers aided with radiological investigation and endoscopic luminal evaluation is the most effective approach used.

The investigation of Crohn's patients has historically centred on a combination of barium studies, ultrasound (USS), cross sectional imaging using CT (computed tomography) and endoscopic luminal evaluation. However due to the chronicity and multifocal nature of the condition, the imaging and management is a challenge to radiologists and gastroenterologists. The modality chosen depends upon whether a diagnosis has already been made, the acuteness or chronicity of symptoms, and age of the patient.

New techniques such as magnetic resonance imaging (MRI) imaging of the small bowel and perianal imaging have revolutionized the management of these patients obtaining detailed information of the activity and extent of disease as well complications. CT imaging continues to be used mainly in the acute setting; the use of multislice imaging combined with different oral contrast agents imaging of the colon and small bowel can be improved. Gastroenterologists have also increased their armamentarium by combining push and double balloon enteroscopy and capsule studies in investigating small bowel symptoms.

Imaging techniques

Different imaging modalities are available to radiologists and the pros and cons of each are summarized in Table A below.

Barium, CT and MRI techniques used to visualize the small bowel can be modified into two main groups:

Enterography- the small bowel is distended by the patient steadily drinking 1-2 litres of an oral contrast agent.

Enteroclysis- A nasojejunal tube is inserted via direct fluoroscopy and a contrast agent is infused steadily to ensure adequate distension throughout the small bowel.

Although enteroclysis provides better control and uniformity of distension it does add a radiation burden to the patient on inserting and screening the tube placement. The insertion is invasive and not all patients tolerate this well. Due to the chronicity of crohns a well tolerated investigation is paramount.

If the patients symptoms are mild a barium follow through examination is commonly performed looking for skip lesions, abnormal mucosal ulceration (including cobblestone ulceration-both horizontal and longitudinal ulcers), submucosal oedema and bowel wall thickening characteristically involving the mesenteric border. Ulcers may be longitudinal ulcers, apthoid ulcers, fissures and rose thorn ulcers. The bowel wall thickening and abnormal surrounding perienteric fat will cause separation of bowel loops.

The mucosal findings are less well appreciated on CT which is performed mainly in the subacute and acute setting. However increased enhancement characteristics of the mucosa and the bowel wall can be visualised combined with the perienteric findings of inflammation, increased vascularity and fat stranding.

The assessment of the mural wall can help diagnose fibrotic from more acutely inflamed crohns disease on both CT and MRI. Signs looked for in acute strictures include inflamed mesentery, enhancing nodes, engorgement of the vasa vasorum (combs sign- see figure 1) and bowel wall stratification with submucosal layer oedema.

Chronic strictures also enhanced and may have bowel wall stratification however with submucosal fat infiltration. There is a lack of mesenteric stranding or vascular engorgement.

Each modality in turn will be discussed detailing technique and indications:

USS

Ultrasound evaluation of the small bowel is particularly useful in the paediatric population and young adults as a first line investigation. It is safe, lacking in radiation and well tolerated. It can provide functional real time assessment of the bowel and assesses the bowel wall thickness, morphology, vascularity and the perienteric region. It is especially useful in differentiating tuboovarian pathology from enteric disease in young females.

Localization of disease is difficult away from the terminal ileum and mucosal detail and entero-enteral/colic fistula can not be demonstrated. However the abdominal wall and subcutaneous fistulas can be demonstrated if carefully looked for.

It has an overall sensitivity of and specificity of 93 percent and 97 percent respectively ^6,7,8.

Barium techniques

Barium can be used to outline the large bowel and small bowel. During a barium enema it is important to reflux barium into the terminal ileum to assess the ileocolic junction. Traditionally the use of barium either by enterography- a traditional follow through or using enteroclysis have been used to demonstrate the mucosa of the small bowel and map the small bowel. The limitations have been the use of radiation in young patients and the lack of extraluminal information. Barium however does demonstrate the luminal mucosa in detail demonstrating fine ulceration. Fistulas can be demonstrated when filled with contrast. However overlapping small bowel loops especially in the pelvis can be a common limitation and a challenge.

The enteroclysis technique is uncomfortable and has poor tolerance.

Until recently the small bowel follow through has been the most common modality and sometimes the only available method of evaluating the small bowel and is relatively easy and well tolerated. The accuracy varies depending on technique and can be optimized using compression, spot views and frequent fluoroscopy.

CT

CT is especially useful in the assessment of acute patients who have an increased likelihood of complications such as collections, perforations, megacolon, fistula formation and long term complications such as lymphoma. Abscesses occur in 15 percent to 20 percent of Crohn's patients and are more commonly associated with small bowel disease and ileocolitis¹. The small and large bowel can be assessed as well as extraenteric findings.

The development of multislice CT and better reformats has improved spatial resolution improving the diagnostic capacity. The radiation dose does limit CT for acute exacerbations rather than long term regular followup.

CT enterography is performed when the patient drinks an oral contrast agent either positive contrast such as barium or negative agent such as water or milk. Positive contrast agents are denser than water and allow collections to be separated from bowel loops more easily and therefore the diagnosis of interloop abscesses is made easier ⁹. Fistula tracks can also be outlined more definitively with air or contrast. However negative agents will have the same density as perienteric collections and therefore more care on interpretation is needed. Negative agents allow assessment of the mucosa where areas of increased enhancement are seen against the low density of the oral fluid which would otherwise be masked with denser positive agents ^6,8,9.

Enteroclysis can also be performed with the benefit of a more uniform distension. The images obtained via CT however are non functional.

Figure 1
Post contrast CT demonstrating terminal ileal thickening with mucosal enhancement and a stricture of the terminal ileum with proximal luminal dilatation. There is increased vascular engorgement of the vasa vasorum 'combs sign'. This is accompanied by perienteric stranding and mucosal enhancement in keeping with active disease.

Complications demonstrated on CT imaging

CT is especially good at assessing for complications such as fistulae, abscesses, perforation and the formation of lymphoma or carcinoma in chronic crohns patients. Extraabdominal findings such as sacroiliitis can also be seen.

MRI Perineum

The improved contrast resolution of MRI allows the assessment of the perineum, the evaluation of the sphincters and ischioanal and ischiorectal fossa. Accurate classification of perianal fistulas and there anatomical location in relation to the sphincter complex can be carried out in order to aid treatment and surgical intervention ¹⁰.

MRI small bowel

This new modality allows for a safe and functional assessment of the small bowel. Once again an enterography or enteroclysis technique can be used with different oral contrast agents. There are traditionally positive (high signal on T2 and T1 imaging), negative (low signal on T2 and T1 imaging) and biphasic agents (high signal on T2 and low on T1 imaging) agents. The signal characteristics are important to help assessing mucosal enhancement, interloop fluid and fistula tracks⁵.

At our institution a biphasic agent using Klean prep solution macrogol (polyethlene glycol '3350') a osmotic laxative (Norgine), is used to ensure adequate distension and an isoosmolar effect. The patient drinks 1.5 litres of solution steadily over 45 minutes and is then imaged prone to ensure adequate separation of the small bowel loops (see protocol, Figure 2 and images-Figure 3 and 4). Due to different sequences being acquired at different times this is a functional investigation and any areas of interest can be imaged using a cine loop to assess bowel movement and help distinguish stricture from spasm and degree of true stricture.

A bowel relaxant such as Hyoscine butylbromide, Buscopan, is administered to reduce spasm.

Figure 2
Small bowel enterography protocol (Phillips 1.5Tesla):

1. Coronal BTFE-M2D (FISP) 6mm
2. Coronal T2 Ssh TE60 Cor 6mm
3. Axial sFSSSh-TE90 (FS HASTE) 6mm Axial
4. Coronal 3D THRIVE -T1FS pre and
post contrast- 30,60,90 sec
5. Axial 3 minute post contrast - T1FS THRIVE
6. Region of interest (ROI) MRI fluoroscopy   

Figure 3
Coronal Balanced gradient echo (FISP) demonstrating adequate full distension of the entire small bowel. This sequence demonstrated mesenteric vessel engorgement and any reactive nodes well. A coronal T2 image is also obtained which does not have the low signal wall interface and less flow voids intraluminally.   

Figure 4
Axial T2 fat suppressed sequence to demonstrate the bowel wall and mesentery. The fat suppression demonstrated oedema well.   

Figure 5
There is a long segment of transverse colon and hepatic flexure, with wall thickening, post contrast enhancement and pericolic vascularity- Combs sign (Figure 5 post-contrast THRIVE).

The advantages of MRI include excellent contrast resolution and mapping of the whole small bowel and depending on the technique used and timing the large bowel can also be assessed. Both the perienteric and pericolic spaces can be assessed and strictures can be categorized accurately between active and fibrotic thus helping aid management and outcome.

The follow up of all young known crohns patients is likely to be with small bowel MRI studies in view of accuracy and volume of information it provides.

Figure 6
There is a tight stricture at the terminal ileum demonstrating enhancement, concentric wall thickening but no stranding, increased vascularity or perienteric fat inflammation. There is no stratification of the bowel wall. The appearances are that of a fibrostenotic stricture. There is secondary subacute obstruction with distended small bowel loops with mottled faeculant matter -"small bowel faeces sign'. The pelvic loops measure up to 4.5 cm. (Figure 6 -coronal T2).

The sensitivity for detected crohns disease has been reported as 84 percent to 90 percent and a specificity of 90percent to 100 percent for the detection of wall thickening, prestenotic dilation and luminal stenosis^11,12. The sensitivity for linear ulcers is 88 percent.^11,12

There is however an inferior spatial resolution compared to conventional enteroclysis and therefore MRI is not capable of detecting more subtle mucosal disease leading to the recommendation by Fidler et al 5 for MRI not to be the initial investigation for the initial diagnosis of crohns disease. One study demonstrated the sensitivity for the detection of terminal ileal Crohns using MRI shows 89 percent sensitivity compared with 72 percent with conventional enteroclysis ^12,13,14.

Figure 7
THRIVE coronal MRI shows tethering of the antrum of the stomach to the transverse colon forming an enhancing fistula track. There is also abnormal enhancement of the inflamed portion of the transverse colon and ascending colon.

MRI detected 80-85% abnormal small bowel segments compared with CT 60 percent to 65 percent 14 and helps to differentiate active versus fibrotic changes (see Figure 5 and 6)¹².

Due to the good contrast resolution there are higher rates of fistula detection compared with both CT and fluoroscopy, the track can be demonstrated without the need for air or barium to fill the track (Figure 7).

Discussion

MR enterography in our experience is a well tolerated, accurate additional tool in the armementarium of small bowel imaging. The use of this technique combined with other modalities can help in the diagnosis and followup of Crohn's patients depending on the clinical setting. In the new diagnosis of crohns patients a barium follow through or barium enteroclysis can assess for more subtle mucosal abnormalities and in younger patients USS can assess the vascularity and bowel wall thickness of the terminal ileum in experienced hands. Patients in an acute setting may warrant a CT to assess for large and small bowel abnormalities and complications such as abscesses and perforation. Crohns disease varies from active inflammation to fibrostenotic subtype, fistulizing/perforating and reperative types. The role of small bowel MRI is especially good in the follow up of relapsing cases to assess the small and large bowel and disease activity and subtype without the additional burden of radiation.

The multidisciplinary approach in managing crohns patients has warranted inflammatory bowel disease meetings where by surgeons, gastroenterologist and radiologists meet often weekly to discuss patients similar to the already established cancer MDT format. This will provide an accurate and patient centered approach. The choice of imaging modality used to investigate a patient is geared towards the clinical scenario and age. Radiologists have a pivotal role in this and familiarity with all imaging modalities and their benefits and limitations is vital.

The success is dependent on how we can integrate this investigation in clinical setting practically for patients, clinicians and imaging departments.