Introduction

There has recently been a significant incidence rise of Crohn’s disease (CD), particularly in pediatric patients.

The typical lesions of the disease that affect the whole gastrointestinal tract may extend to extra-luminal and join fistulae, abscesses and lymphadenopathy.

The gold standard for the diagnosis and follow-up of CD is an endoscopic evaluation of the colon and the terminal ileum; however, MRI is a non-invasive test that does not entail exposure to ionizing radiation, allows for an accurate study of intra- and extra-luminal of the whole gastro-intestinal tract characterized by a multiparametric and multiplanar approach and high spatial and contrast resolution.

An ever growing number of studies shows that MR enterography (MRE) is the optimal technique of cross-sectional imaging for the diagnosis and evaluation of CD activity.

DWI has recently been associated with MRE for studying CD, with a good degree of correlation between the restriction of diffusion parietal and MRI findings specifically associated with disease activity.

The purpose of MRE with DWI is to avoid using paramagnetic contrast for identifying and assessing disease activity.

MRI represents the gold standard technique for studying the pelvis and perineum, and has proved to be a highly effective means of identifying fistula perianal complications related to CD, both in adults and children.

The disease appears to be more aggressive in children over 8 years of age. The most recent studies have found significant differences between adults and children with regard to the stenotic complications that arise in 13 % of the cases involving children compared to 4 % of adult cases 4 years after the diagnosis of the disease. Similar percentages have been observed for fistula complications (4 % in adults compared to 11 % in children). These differences are of particular diagnostic and therapeutic interest [1].

Epidemiological aspect

CD is a granulomatous inflammatory bowel disease [2–6] characterized by high morbidity and mortality [7], which, together with ulcerative colitis and indeterminate colitis, are included in the spectrum of inflammatory bowel disease (IBD).

Little is known of the causes although it is thought that it derives from a combination of environmental and genetic factors related to intestinal flora [5, 8].

Diagnosis is based on clinical evidence, laboratory tests, radiological and endoscopic investigations and histological evaluation.

The incidence of CD has increased worldwide and has doubled in the pediatric population over the past 12 years [9].

There is a peak in incidence in late teenagers or young adults (up to 25 years of age), while the rate of growth appears to be more rapid in children under 10 years, which may be due to early onset or earlier diagnosis [9–11].

Phenotype

Ileum-colic localization is the most common site of disease [12–18], yet the CD phenotype differs according to the age of the patient at the onset of the disease.

Mamula et al. [19] observed that while 59 % of individuals suffering from CD aged <18 years are affected in both the small and large intestines, 89 % of children under the age of 10 are only affected in the large intestine.

Based on these observations, the classification of Paris (2009) re-examines the classification of Montreal (2005). The Very Early Onset Inflammatory Bowel Disease (VEO-IBD) is considered a form of its own and patients aged <10 years at disease onset (A1a) are classified differently from those aged >10 years (A1b) [20].

Children aged under two at diagnosis are more likely to suffer from a more aggressive phenotype [21–23] and have a clear genetic predisposition to immune deficiency.

Finally, the timeliness with which immunosuppressive therapy is carried out has a significant effect on the development of complications [12].

Clinical aspects

Abdominal pain, fever and weight loss are common symptoms of juvenile CD onset (<16 years). The main complications are intestinal perforation, progressive stricture and subsequent intestinal obstruction.

Nutritional deficiencies, stunted growth and delayed puberty occur in percentages ranging between 65 and 85 % of patients with childhood CD onset.

15–40 % of these patients continue to suffer from stunted growth throughout the course of the disease [24–26].

It is also noted a high prevalence of oral manifestations, attributable to bad absorption [12], a greater involvement of the upper gastrointestinal tract [12, 27] and a higher incidence of perianal lesions [12, 14, 27, 28]. Perianal involvement in children is observed in 25–38 % of cases [2–6, 29, 30]; it affects males four times more frequently than females [4, 30, 31] and begins with a fistula in over a third of cases. One child in 20 with CD has an isolated fistulous disease [2, 5, 6, 32].

MR imaging

MR imaging plays an important role in the evaluation of patients with MC at diagnosis whether symptomatic or asymptomatic, in the spread of the disease, in the assessment of the degree of activity and severity of the disease, and finally for identifying the complications during follow-up and evaluating treatment response [33, 34].

When treating pediatric patients, methods that use ionizing radiation should be avoided since children are much more sensitive to radiation than the adult population [35, 36].

MR enterography in gastro-intestinal disease assessment

Procedural details

It is not easy to investigate the intestinal wall with RM.

The thinness, the movement and the presence of the intra-luminal contents, especially gases, are responsible for conditions that hamper the study.

However, the use of faster, free-motion and high-resolution sequences in breath hold, together with the administration of antiperistaltic drugs has significantly improved the situation [37].

A basic requirement for obtaining a good quality MRI is to distend the intestinal loops that can be achieved by administering mdc per os (MR enterography) or via naso-duodenal catheter (MR enteroclysis).

We only used oral administration, since it is more suitable for pediatric patients.

Moreover, placing a naso-duodenal catheter under fluoroscopic guidance results in limited exposure to ionizing radiation.

The enteroclysis allows for a greater degree of bowel distension, but this does not significantly improve the clinical value of the study [38–41].

Positive, negative or biphasic oral contrast material (contrast medium) can be used depending on the characteristics of the signal in the various MR sequences [41–43].

One hour before examination, we administrated between 1000 and 1500 ml (depending on the patient’s age) of an aqueous solution of 70 % sorbitol, divided into 3–4 doses, each to be taken within 15–20 min. Biphasic contrast medium enables us to obtain an excellent morphological evaluation of the small intestine in T2 sequences and a better characterization of inflammatory activity in the T1 sequences together with the suppression technique of the adipose signal.

It is preferable to place the patient in the prone position to reduce the movement caused by breathing and ensure better abdominal compression and more safety in the event of vomiting. Abdominal compression favors the distension of the intestinal loops and enables us to decrease the number of slices, thus reducing examination time thanks to the reduction in the thickness.

However, in the presence of abundant intraluminal gas magnetic susceptibility artefacts may occur at epigastric and peri-umbilical level.

We used N-butylbromidehyoscine (Buscopan) as anti-peristaltic drug. 0.25 mg/kg of this drug is administered intravenously to patients weighing <20 kg and from 0.5 mg/kg to a maximum of 20 mg, into patients weighing more. The dose is divided into two administrations, one after the initial dynamic cine evaluation and the other one before injecting gadolinium intravenously.

The paramagnetic contrast agent is administered in doses of 0.1 mg/kg.

MR imaging protocol

The MR enterography examinations are performed with a body coil in 1.5 T (Philips Achieva, Nova Qasar) and 3 T (Philips Ingenia 3 T) magnet, according to the protocol described in the table (Table 1).

Table 1 MR enterography imaging protocol for pediatric Crohn disease performed with a body coil in 1.5 and 3 T magnet
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Firstly, we assess the degree of bowel distension by means of very thick single-shot fast spin echo sequences, with “fluoroscopic” effect. If the intestinal distension is not sufficient, we administrate an additional dose of sorbitol solution.

Dynamic assessment of intestinal peristalsis by cine imaging is an integral part of the protocol, because it allows for a better identification of pathological features.

It is possible to identify the bowel segments affected by active inflammation, characterized by a reduced or absent motility as well as the fibro-stenotic segments that appear aperistaltic with pre-stenotic dilatation and the upper portion which is hypo-peristaltic. The cine-MRI sequences provide us with useful information even in the presence of an insufficiently relaxed intestine, since they enable us to appreciate the distension of a normal loop even if this is initially collapsed.

Correlation with some inflammatory markers such as C-reactive protein is good [44–46].

Sequences of diffusion (DWI) are very quick and can be carried out in free breathing or in breath hold and constitute a source of alternative contrast compared to the relaxation time.

Recent studies have shown reduced diffusivity in intestinal segments affected by CD.

The mechanism according to which this kind of phenomenon occurs is not known with certainty [47–49].

It is assumed that the infiltration of inflammatory cells, in particular of the aggregates of lymphocytes, lymphatic dilatation and the development of granulomas lead to a reduction in the extracellular space, thus restricting the mobility of water molecules [48].

The reduced diffusion may represent a new radiological non-invasive bio-marker of CD activity. Although characterized by a low spatial and contrast resolution, the diffusion sequences are able to identify the bowel segments affected by the inflammatory process, the lymph nodes and any complications such as fistulas or abscesses [50] (Fig. 1).

Fig. 1
figure 1

An 8-year-old boy with Crohn’s disease (CD). Coronal balanced FFE (BTFE) images (a, b) show concentric wall thickening of the terminal ileum with hypersignal and substenosis which determines prestenotic dilatation. The axial BTFE image (c) shows wall thickening, which appears hyperintense on diffusion-weighted image (DWI) (d) and shows restricted diffusion on ADC map (e). Coronal volume interpolated GRE (THRIVE) image (f) and axial contrast-enhanced T1-weighted with fat suppression image (g) show intense enhancement parietal with layered appearance

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Recent studies show a significant correlation between bowel wall minimum ADC value and some MRI markers of disease activity, including:

  • degree of bowel wall thickening;

  • striated pattern and degree of arterial enhancement;

  • degree of delayed enhancement;

  • quality of mesenteric inflammatory changes;

  • presence of stricture.

    There is a rather poor correlation concerning other modifications, such as

  • bowel wall T2-weighted signal intensity normalized to either spleen or paraspinal musculature;

  • length of intestinal segment affected;

  • fibrofatty proliferation of the mesentery.

If the value of positivity in DWI is confirmed and ADC cutoff values were identified, this imaging technique could prove to be a useful tool for evaluating inflammatory activity. Potentially, ADC values may make the use of intravenous gadolinium-based contrast unnecessary.

In our experience, it was possible to differentiate in the same patient intestinal tracts with active inflammation from fibro-stenotic tracts, according to ADC parameters and their correlation with the type of enhancement in the various phases of the dynamic study (Fig. 2c–f).

Fig. 2
figure 2

A 13-year-old boy with re-active Crohn’s disease (CD). Coronal (a) and sagittal (b) SSTSE T2-weighted with fat suppression images show significant pathological ileal involvement with multiple enlarged mesenterial lymph nodes. ADC map (c) and DWI image (d, e) show parietal restriction of diffusivity in the right pathological ileal loops and lower positivity in those in left quadrants with positive lymph node (e). These findings express the various degrees of disease activity in different pathological traits with coexistence both of acute inflammation and fibrosis, as confirmed by the different degree of enhancement on axial contrast-enhanced T1-weighted with fat suppression image (f)

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Submucosal edema is present in the course of active inflammation, and also in the presence of stenotic sections due to fibro-adipose involution or in sections with mural fibrosis and superimposed active inflammation.

The imaging of diffusion must be integrated with other morphological and cine sequences and with the type and degree of enhancement.

Finally, it is possible to provide documentary evidence of a reduction in diffusivity even in not dilated or collapsed loops, in which there are also aspects of wall thickening and enhancement which do not correspond to the actual situation.

Imaging diffusion must still be an integral part of MRE because of the amount of information that it provides concerning CD patients.

The speed of execution is also essential in the case of younger patients who are generally more intolerant.

Imaging findings

Some authors simply distinguish between active disease and chronic illness [41, 43].

We prefer to distinguish three main forms of CD:

  • uncomplicated active inflammatory disease (no fistulas or strictures);

  • penetrating disease (deep ulcers, fistulas or abscesses);

  • fibrostenosing disease.

This type of classification better answers the clinical needs and helps to direct clinicians toward a more or less intensified medical therapy or surgery [51].

Erythema and superficial aphthous ulcers are the earliest macroscopic lesions in active CD [52].

The spatial resolution of MRI is not yet able to detect all superficial mucosal abnormalities even if the use of high-field MR may help to overcome this limitation.

Enlarged mucosal enhancement with respect to the adjacent loops with a similar degree of distension is one of the earliest signs of inflammatory activity. This is caused by hyperemia and histologically corresponds to submucosal edema and an increased number of dilated submucosal capillaries (Fig. 3b).

Fig. 3
figure 3

Coronal BTFE image (a) shows wall thickening of the terminal ileum with hypersignal compared to the psoas muscle (not shown). Coronal contrast-enhanced T1-weighted with fat suppression image (b) shows (arrow) ectasia of vasarecta from active inflammation with characteristic “comb sign”. Coronal THRIVE images show progressive parietal enhancement and an early involvement of the mucous layer (50 s) which spreads to transmural in the later stages 70 and 150 s)

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The stratified enhancement produced by alternating intense enhancement of the mucous layer and muscle/serous with an intermediate enhancement of the edematous submucosal produces the “target sign” characteristic in post-contrast T1-weighted sequences.

The thickening of the intestinal wall is another important manifestation of active CD. Bowel wall thicknesses of more than 3 mm should considered pathologic [53]. However, it is important to note that at fasting level, wall thicknesses of 3 mm measured following oral administration of the contrast agent should be considered to be standard, in relation to anatomical peculiarities and the non-optimal distension levels obtained with this technique with respect to MR enteroclysis.

The degree of wall thickening is related to the endoscopic and histological severity indexes of the disease.

However, wall thickening is not always associated with fibrosis and, therefore, does not necessarily lead to a terminal stage of the disease [54–56].

In fact in the early stages, thickening is secondary to the presence of submucosal edema.

Since only moderate or severe edema constantly produces intramural high signal intensity in T2-weighted sequences, the absence of this does not necessarily exclude active disease.

Sensitivity towards wall edema and small fluid peri-enteric collections can be increased with T2-weighted fat-suppressed sequences [57].

The fibrofatty proliferation of the mesentery, detectable in relation to the neat separation of bowel loops, is considered a specific finding of MC but not correlated with disease activity (Fig. 4a, b).

Fig. 4
figure 4

A 15-year-old boy with Crohn’s disease (CD) with extensive ileal and colic involvement. Axial turbo spin echo (TSE) T2-weighted image (a) and coronal SSTSE (single-shot turbo spin echo) T2-weighted image (b) show significant wall thickening of the ascending colon with fibro-fatty proliferation. Coronal SSTSE T2-weighted with fat suppression image (c) shows extensive ileal involvement with characteristic “cobble stoning” with prestenotic dilatation. Apparent diffusion coefficient (ADC) map (d) indicates a clear restricted diffusion of involved ileal tract shown in c. Axial contrast-enhanced T1-weighted with fat suppression (e) shows intense enhancement of the involved ileal tract

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The vascular engorgement of the vasa recta on the mesenteric side of the bowel wall (comb sign), which is more evident on post-contrast T1-weighted fat-suppressed sequences following mdc (comb sign), is an indicator of disease activity (Fig. 3).

The increase in number and enhancing lymph nodes are not specific, but are generally associated with a state of inflammatory activity of the ascending colon and terminal ileum (Fig. 2a–e).

The “cobblestone effect” is easily identifiable with RM, as long as an adequate level of distension of the intestinal lumen is achieved. It is characterized by the presence of inflammatory mucosal pseudo-polyps delimited by ulcerative lesions in transverse and longitudinal course (Fig. 1).

The mucosal islands are characterized by a significant enhancement of the T1 signal following intravenous administration of the contrast medium.

Besides helping to confirm the diagnosis in an appropriate clinical setting, this nodular pattern is an indicator of the severity of the disease which orientates us towards an intensification of the treatment [58].

Deep ulcers and intestinal fistulae are of fundamental prognostic significance with implications towards possible surgical therapeutic planning.

The extension of the intra-parietal ulcerative process (sinus tract) and the extra and inter-parietal fistulizing, both activity index of severe disease, are well detectable on post-contrast T1-weighted sequences and sometimes, in the presence of fluid content, even in the images T2w.

The accuracy of the MR enterography for detecting fistulas is 71–100 % with specificity ranging between 93 and 100 % [59–63].

Fistulas may form abscesses and phlegmons or cause fibrous adhesive processes between adjacent intestinal segments.

Phlegmons appear as extra-intestinal masses with moderate hyperintensity in T2 and T1-weighted sequences following mdc intravenous administration.

Abscesses are represented by fluid collections, possibly containing gas (with strong parietal enhancement on post-contrast T1-weighted fat-suppressed sequences).

The restriction of the diffusion documented in DWI may be useful for differentiating small abscesses from loops filled with fluid or pseudo-collection.

Fibro-stenotic lesions are indicative of chronic disease, which occur after recurrent episodes of inflammatory activity alternated with granulomatous reaction and deposition of reparative fibrotic tissue and result in progressive stricture of the intestinal segment, stenosis and pre-stenotic dilation.

The sensitivity of MRI for detecting stenosis ranges between 75 and 100 % with a specificity of 91–100 % [64].

As a loop affected by a severe inflammatory process may become stenotic, the differentiation between inflammatory stenosis and fibro-stenotic lesion significantly influences the choice of treatment.

Usually bowel wall thickening in fibrostenosing disease is less pronounced and enhancement is more homogeneous and unstratified.

Fat-saturated and non-fat-saturated T2w sequences enable us to distinguish between edema and fat deposition at submucosal level. Submusal fat infiltration causes the separation of the collagen fibers that, with an orientation parallel to the surface, create a characteristic layering effect known as “halo sign” that must be distinguished from the so-called “target sign” of the submucosal edema [57].

The cine imaging completes the study showing a marked reduction or even the disappearance of peristaltic activity at the stricture.

The reliability of imaging criteria alone is not absolutely certain, since there are often both inflammatory and fibrotic changes in the same intestinal segment.

Comparison with histological standards confirms a high level of accuracy (87 %) of the MRE in the identification of disease and it is significantly lower in the evaluation of mural fibrosis [43].

The regenerative processes cause mucosal atrophy associated with the emergence of regenerative polyps.

In these cases, there is neither a significant thickening of the wall nor a significant increase of enhancement and unlike what occurs in chronic disease, stenosis and pre-stenotic dilation are not characteristic findings.

Pelvic MR in perianal disease assessment

In radiological imaging, the correct interpretation of perianal disease is based on pelvic anatomy evaluation, pathophysiology aspects and the correct classification of the various types of fistula [65]. In children inadequate treatment, following a wrong diagnostic classification, almost always leads to a significant increase in morbidity, thus causing deep extension of the pathological process that affects the pelvic muscles and perianal areas. In children, the presence of fistulas and/or perianal abscesses is one of the main causes of incomplete response to medical or surgical treatment and relapse.

Anatomy of the anal canal [66]

The anal sphincter is composed of two types of muscles: the external voluntary anal sphincter (ES) and that internal involuntary anal sphincter (IS).

Between the ES and the IS lies the inter-sphincteric space (ISS), consisting in a thin layer of mainly adipose connective tissue. The ES merges anteriorly with the perineal body and above with the puborectalis muscle (PR), the latter continuing posteriorly with the levator ani muscle (LA).

The IS extends from the ano-rectal junction up to 1–1.5 cm below the dentate line and is an offshoot of the circular musculature of the rectum.

The ES is the medial edge of the ischio-anal fossae (IAF) or ischio-rectal fossae (IRF), composed of loose and fibrous connective tissue, vessels and some nerves (inferior rectal and perineal branch of the fourth sacral nerve).

The lateral border of IAF is represented by the internal obturator muscle (IO), while the superior border is represented by the LA; the posterior border is formed by the sacrotuberous ligament and gluteus maximus muscle (GM), while anteriorly it is delimited by the superficial and deep transverse muscles.

Between the LA and the ano-coccygeal ligament, there is a horseshoe connection that links the ischioanal space and deep post-anal space.

MR anatomical landmarks [2, 67]

The axial and coronal planes are oriented at right angles and parallel to the anal sphincter, respectively, to allow for an accurate definition of the anatomical structures of reference.

The T2-weighted images enable us to distinguish the various components of the sphincter in relation to the characteristics of the signal, which is relatively hyperintense at IS level compared to ES. IS, ES, LA and the IAF are well visualized in T1w unenhanced sequences and post-contrast medium acquisitions and define the relationships with abscess vehicles, especially on the coronal plane which is the main anatomic reference.

The IS also shows a normal homogeneous signal enhancement on post-contrastographic T1w sequence.

The ISS shows a fat-type signal in all sequences, therefore on T2w sequences with suppression of fat signal it differs greatly from IS.

The main fistulae, their secondary branches and abscesses appear hyperintense on long TR sequences, which allow for a good differentiation of the muscles and the sphincter.

The course and relationships of fistulae and abscesses are best evaluated on post-intravenous contrast agents T1w sequences, particularly in the coronal plane, which is the main anatomical reference.

MRI technique

Let us outline the fields of application of the MR in the diagnostic and treatment procedure of perianal disease in CD:

  • diagnosis and classification of fistulas (disease status),

  • planning, pre-surgical treatment and

  • follow-up.

Unlike what occurs for MRE, pelvic MR does not require specific preparation and is, therefore, well tolerated by young patients. The study of the pelvis can be integrated with MRE which completes the evaluation of the status of the disease (in particular at diagnosis of CD).

The study of perianal disease by means of MRI on supine patient with pelvic body coil is essentially based on axial and coronal T2w sequences with and without fat saturation sequences, axial/coronal T1w unenhanced sequences and dynamic post-contrastographic T1w sequences.

Our standard protocol, applied with MRI 1.5 T (Philips Achieva, Nova Qasar) [68, 69] and 3 T (Philips Ingenia) systems, foresees [70, 71]:

  • T2-weighted TSE sequences (single-shot) and T1 Fat Sat SPIR on axial and coronal planes, with sagittal optional with respect to number and location of the fistula;

  • STIR/SPAIR sequences on multiple scanning planes;

  • DWIBS (b value 1000) with axial acquisition and MPR reconstruction;

  • T1 TSE Fat Sat SPIR sequence on coronal plane pre- and post-contrastographic and subtraction of images;

  • Axial T1 FS THRIVE volumetric and dynamic contrast-enhanced (DOTAREM 0.2 mg/kg).

Optional GE (BTFE) sequences on axial/coronal plane can be carried out to improve the definition of the anatomic relationships of the lesion against IS and ES, using the distinctive “edge effect”. The DWIBS sequence acquired in free breathing (Diffusion-Weighted Imaging with background body subtraction, TR >5000 MS, TE shortest ≈70 ms, 180 ms TI, EPI factor 47, B-factor 1000 s/mm2) represents something of a compromise between DWI and STIR; it suppresses the normal tissues and organs and it enhanced through a strong hyperintense signal pathological tissues, providing striking images-like PET [72, 73].

Pathophysiology and epidemiology of perianal fistulas

At present, the “crypto-glandular” etiopathogenetic hypothesis is the most accepted even if controversial [8, 74]. The anal glands, located on the same level as the dentate line between the columns of Morgagni, are involved in the lubrication process of the anus through the secretion of mucus in the anal crypts. The alteration or obstruction of the drainage of crypts is probably at the origin of a local inflammatory/infectious process that results in an abscess which may resolve itself spontaneously due to direct drainage into the anal canal or lead to the formation of fistulas [75].

Abscess and fistula would then be acute or chronic manifestations of a single disease process, since 87 % of the patients develop fistulas following an acute perianal abscess [67].

Other causes of perianal fistula, less significant in statistical terms than CD, are tuberculosis, diverticulitis, pelvic infections, trauma and complications of surgery [76, 77]. There is perianal involvement in patients with more than 38 % CD with a ratio of four males to one female (due to the fact that males have more anal glands than females). More than 30 % of pediatric patients with CD develop a perianal inflammation that will lead to the formation of a fistula in a high percentage of cases. For over 5 % of pediatric patients, the perianal fistula is the expression of onset of CD in the absence of intestinal involvement at diagnosis [76, 77].

Classification of fistulas

Parks’s classification (1976) is the first and main classification of perianal fistulas; it is based on principles of surgical anatomy [78, 79] and classifies the various types of fistulas according to their course and relationship with the ES.

Parks’s classification identifies four types of fistula: inter-sphincteric, trans-sphincteric, supra-sphincter and extra-sphincter according to the relationship of the sphincter complex in the coronal plane of the anal canal.

St. James’ University Hospital Classification (Morris et al.) is an adaptation of Park’s classification of diagnostic imaging [31].

In this classification, the categories identified by Park are maintained, while the surgical references are replaced with radiologic anatomy on pelvic MRI and additional parameters are introduced, such as the presence of secondary sinus tract and/or abscess formation.

According to Morris’s classification, there are five different types or degrees of perianal fistulas:

  1. 1.

    Simple linear intersphincteric fistula (grade 1) that penetrates the IS and ISS runs through it to the skin in the absence of secondary tracts, abscesses or extensions to the LA (Fig. 5);

    Fig. 5
    figure 5

    Grade 1 (Morris’s) simple intersphincteric fistula which passes through the intersphincteric space (ISS). The fistula tract does not involve the external sphincter (SE) as evident in coronal T2-weighted image (a) and coronal short T1 inversion recovery (STIR) image (b). Axial diffusion-weighted whole-body imaging with background body signal suppression (DWIBS) image (c) shows a high signal tract which matches with intense enhancement shown on axial contrast-enhanced T1-weighted with fat suppression image (d) consistent with active disease. SIS intersphincteric space, EA levator ani, SI internal sphincter

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  2. 2.

    Intersphincter fistula with intersphincteric abscess or secondary fistulous tract (grade 2) consists of an intersphincteric fistula associated with an abscess or a secondary tract that lies within the ISS. When process extends to the opposite side, spreading partially circumferentially around the IS, it is termed a horseshoe fistula. The profile of ES is always intact (Fig. 6);

    Fig. 6
    figure 6

    Grade 2 (Morris’s) intersphincter fistula (a, arrow) which is limited by the external sphincter (SE) with cryptoglandular abscess (b, dashed arrow). OI internal obturator muscle, FIA ischioanal fossae

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  3. 3.

    Simple transphincteric fistula (grade 3), which penetrates both the IS and the ES and penetrates the IRF or the IAF down to the skin. There are no secondary tracts, abscesses or signs of involvement of the LA (Fig. 7);

    Fig. 7
    figure 7

    Grade 3 (Morris’s) transphincteric fistula with an internal opening at 9–10 o’clock and which penetrates the sphincterial complex and in particular the external anal sphincter (SE) as shown on axial T2-weighted image (a) and on the corresponding axial DWIBs image (b). Coronal STIR image (c), coronal DWIBs image (d) and coronal contrast-enhanced T1-weighted with fat suppression image (e) show penetration through the ischio-rectal fossae (FIR). EA levator ani

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  4. 4.

    Complex transphincteric fistula with abscess or secondary fistulous tract within the IAF or IRF (grade 4) that has similar characteristics to simple trans-sphincteric fistula, but it is complicated by secondary tracts or abscesses. There is no involvement of the LA (Fig. 8);

    Fig. 8
    figure 8

    Grade 4 (Morris’s) transphincteric fistula: it combines a transphincteric fistula (as Fig. 7) with a secondary tract (a, b) that extends caudally to the right large lip (GL) with marked enhancement(c–e). EA levator ani muscle, unscathed, FIA ischio-anal fossa, SE external anal sphincter, SI internal anal sphincter

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  5. 5.

    Supra- and extra-sphincteric fistula that forms complex fistulas (grade 5): supra-sphincter fistula exits the ISS above the puborectalis muscle and penetrates the LA before passing through the IRF towards the skin. Extra-sphincter fistulas are independent of pathologic entities, since there is no involvement of the sphincter complex and they pass caudally through the plane of the LA through the IRF until they reach the skin (Fig. 9).

    Fig. 9
    figure 9

    Grade 5 (Morris’s) supralevator fistula with abscess: right supralevator fistula that penetrates levator ani (EA) above the puborectal muscle which shows an anomalous signal on axial T2-weighted image (a) and on axial T1-weighted image (b), hyperintense and discontinued signal on coronal STIR image (c) and abnormal enhancement on T1-weighted image with fat suppression following contrast agent administration (d). SE external sphincter, OI internal obturator muscle

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Although the classification of St. James ‘University Hospital is more descriptive and is based on MR anatomy, Park’s classification is preferable in pediatrics as it is based on the coronal plane rather than the axial plane. In fact in children, the anal canal is much shorter than in adults, for which it is particularly difficult to classify a type of fistula on the axial plane correctly.

Therefore, the description of secondary vehicles and the associated abscesses must be included in the radiological reports when referring to Park’s classification as they are the main causes of inadequate response to treatment (medical or surgical) and relapse in children.

For descriptive purposes when reporting MR, the so-called “anal clock” [80] of the surgical field is generally used to locate the origin of the fistula with respect to the anal canal exactly. The “anal clock” is an axial section comparable to a clock face in which the pubic symphysis indicates 12 o’clock, the buttock crease 6 o’clock, and the right and left buttocks 9 and 3 o’clock, respectively.

Imaging findings

The first element to be assessed is whether there is a main tract or not and the total number of fistulas present [32, 81].

In the structure of the sphincter complex and pelvic muscles, a fistula is distinguished as a hypointense linear structure on T1-weighted sequences and hyperintense on T2w and T1-weighted sequences (especially with fat saturation). The intrinsic characteristics of the MRI method, based on multiparametricity, enable us to assess the degree of activity of perianal disease. Active fistula or abscesses are characterized by a marked positivity on DWIBS images and a corresponding enhancement on T1 post-contrastographic sequences.

A loss of T2-weighted imaging hyperintense signal or lack of enhancement on T1-weighted post-contrastographic sequences correlated with a progressive inactivation of the disease process [82]. These changes are reliable predictors and make the MR study particularly useful in the follow-up of peri-anal disease, especially as an instrumental assessment of the response to treatment [4, 83, 84].

Equally important is the contribution provided by MRI in the evaluation of deep tissue, peripheral to the fistula, with particular regard to the IRF or IAF. Edema, signal and structural distortion of fibro-adipose components and post-contrast-enhancement of the signal indicate deep and persistent inflammation in the tissue.

The fistula must be characterized and classified according to its course and its relationship with the sphincter complex. Most fistulas are of posterior origin and must be localized according to the “anal clock” [85].

Therefore, the description of secondary tracts and the associated abscesses must be included in the radiological reports when referring to Park’s classification as they are the main causes of inadequate response to treatment (medical or surgical) and relapse in children. The secondary tracts are identified separately and described according to their relationships with the main fistula, localizing them with respect to the anal sphincter, the LA and any skin outlet skin, if independent. The perianal abscesses tend to show high signal intensity on T2-weighted sequences as fistulas, but may appear uneven and hyper intense in T1-weighted baseline sequences with peripheral characteristic peripheral enhancement in post-contrastographic sequences. MR images enable us to view and correctly assess post-surgical findings, particularly in the presence of setons [86, 87].

The identification of collateral findings is particularly important in children, especially in pelvic MRI. Here, are some extra-anal and extra-sphincter findings that frequently affect the clinical development of the disease in children:

- sacroiliitis or hip joint articular involvement;

- enterocolitis;

- abdominal or pelvic fluid collection;

- lymphadenopathy;

- post-medical or surgical complications.

Conclusions

The MRE is a highly sensitive and specific method that is preferable for evaluating pediatric patients suffering from Crohn’s disease.

The MRE provides us with results comparable to those obtained with endoscopy for the identification and assessment of disease activity and is better than other cross-sectional imaging techniques, such as CT, for evaluating the degree of fibrosis and complications of disease.

MR Diffusion Imaging is a technique that enables us to add important additional information and which may replace the use of intravenous contrast agents.

Although perianal fistulas can be identified with high sensitivity and specificity during a pelvic MR examination, a pelvic MR is optimal and can provide all the necessary information for a correct diagnostic and therapeutic management of disease.

In children, pelvic MRI reaches specificity levels above 81 % that can reach 100 % and can easily be integrated with MRE, while causing a relative increase of the duration of the examination.

Pelvic MRI also helps to identify extra-anal and extra-sphincteric complications included in the field of view acquired and above all to detect, assess and classify secondary fistulas that are often responsible for complications and recurrences [88, 89].