Magnetic Resonance Imaging of the Anal Region: Clinical Applications
Anal canal diseases can be classified into benign and malignant.
Benign conditions include a wide range of pathologies, the most common of which are fistulas and abscesses.
The underlying cause of fistulization processes is unknown yet, although intramuscular glands’ infection with resultant intersphincteric or transphinteric spread is the most plausible hypothesis.[1-3]
Ultrasound (US) and magnetic resonance imaging (MRI) are valuable tools for anal canal assessment.
The US is a safe technique that can currently rely on three different approaches for evaluating anal canal, transperineal, endovaginal, and endoanal; however, its performance strictly depends on the operator’s experience and it has a limited field of view.
MRI has become a cornerstone in the evaluation of the anal canal and the adjacent structures as it provides a multiplanar assessment with high spatial resolution.
Therefore, MRI offers additional elements for the patient’s clinical assessment and provides a “road map” for the surgeon to intervene when required.
This pictorial essay aims to present various diseases involving the anal canal and the surrounding structures assessed through MRI performance.
MRI GROSS ANATOMY AND TECHNIQUE
The anal canal is the most distal segment of the gastrointestinal tract, comprised between the rectum and the external surface.
It is surrounded by a double layer of sphincters involved in the defecatory process: The innermost comprises smooth involuntary muscular fibers. The outer is composed of a voluntary skeletal muscle.
MRI of the anal region is performed using a surface phased array coil, without needing intraluminal contrast agents or endocavitary coils.
Our protocol includes half-Fourier acquisition single-shot turbo-spin-echo (HASTE) with and without fat suppression (slice thickness: 3 mm; TR/TE ∞/80 ms; flip angle: 90°) obtained on standard sagittal, axial-oblique, and coronal-oblique planes; diffusion-weighted imaging (DWI) (DWI; slice thickness: 5 mm; b values: 0, 400, 800) obtained on standard axial or oblique-axial planes; ultrafast three-dimensional gradient echo T1-weighted with fat suppression (TR/TE: 5.7/1.8–4.0; flip angle: 15°) before and after injection of 0.2 mL/kg gadoterate of meglumine (Dotarem); and 30 mL of normal saline obtained on standard axial or oblique-axial planes.
A broad spectrum of congenital pathologies can affect the anal region.
Therefore, several attempts have been made for classifying anorectal malformations (ARMs), on the basis of their localization (high, intermediate, and low), gender prevalence (male and female), surgical management (approach and techniques), clinical outcomes, and coexistence with other simultaneous pathological conditions (syndromic and nonsyndromic).[8,9]
Clinical picture as well as imaging findings are critical to establish the proper medical treatment or to plan surgical intervention.
However, lesions arising from the anal canal and the adjacent tissues, when asymptomatic, can go unnoticed and may be detected as incidentalomas during adulthood [Figure 2].
Benign pathology of the anal canal generally consists in fissures, fistulas, abscesses, and hemorrhoids. The related symptoms are typically non-specific, including pain, bleeding, cutaneous lumps, itching, and external drainage.
Therefore, to reach a definitive diagnosis, physical examination is mandatory.
Radiological imaging is not recommended for hemorrhoid assessment, unless a further underlying cause of bleeding is suspected within the upper part of the colon.
Due to the wide field of view, the high contrast and spatial resolution, MRI is capable in detecting mural alterations of the anal canal and extension to surrounding tissues. Limitations include the evaluation of the anal lumen, which is physiologically collapsed.
Fistulas and abscesses
Although often used as synonyms, the term “perianal fistula” refers to a chronic granulating tract connecting the anal canal and the cutaneous surface of perineum, “sinus tract” to a tunneling wound originating from the anal canal, extending into adjacent soft tissues, and ending in a blind “cul-de-sac,” “fissure in ano” to a linear ulcer confined within the anal canal, from the dentate line to the eternal margin.[12-14]
According to Parks’ classification, perianal fistulas can be classified, on the basis of their pathways and involvement of the sphincteric structures, into four types: Intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric. Fistolous tracts are visualized as linear hyperintensities on T2 weighted, DWI, and T1 weighted with fat saturation after gadolinium (Gd) injection sequences.
Decrease in signal intensity can be related to fibrotic degeneration in long-standing diseases.
On the other hand, abscesses are inhomogeneous fluid collection, in which also small gas bubbles can be appreciated that can arise anywhere within the perianal tissues. They are generally typical of acute phases.
Therefore, perianal complications are frequently related to Crohn’s disease (CD) (up to 28% of CD patients) due to the transmural involvement and, tough more rarely, to ulcerative colitis 4% [Figure 3].
Anal canal affection in vasculitis, such as Behcet disease (BD), has also to be taken into account. In fact, a gastrointestinal localization has been recorded in up to 30% of BD patients. However, only a few cases of perianal involvement have been described so far [Figure 4].[17,18]
In all the cases supramentioned, the goals of MRI consist in detecting the internal orifice within the anal canal, showing the fistulous route, with its caliber and extension, and the structures involved, to establish the most proper treatment.
A strong correlation (approximately 90% of the cases) has been established with human papilloma virus (HPV), mainly HPV-16 subtype, and the human immunodeficiency virus.
Other risk factors include cervical dysplasia, autoimmune disorders and immune suppression in transplant recipients, cigarette smoking, and long-standing perianal fistulizing CD.
According to their localization, they can be classified into “anal canal” and “anal margin” carcinomas. Anal canal tumors include three main types; squamous cell carcinoma (SCC), which is the most common type (up to 80%), adenocarcinoma, which is rarer (10–20% of the cases) but more aggressive than SCC, and melanoma (1–4%).
On the other hand, tumors of the anal margin consist on extramammary Paget’s disease, Bowen’s disease, anal margin SCC, basal cell carcinoma (0.2%), and melanoma (2–4%).
Typical symptoms are pain, rectal bleeding, constipation, and mass sensation.
MRI features of anal cancer are not specific for a histological type or subtype and include iso- to hypointensity on T1-weighted scans and iso- to hyperintense on T2-weighted and DWI sequences. After Gd injection, the mass is generally characterized by an intense contrast enhancement.
Perianal diseases include pathological conditions that originate from the tissues surrounding the anal canal.
Beyond rare and often benign tumors (i.e., lipomas or teratomas), the most frequent pathologies are associated with inflammation of the hair follicles.
In these cases, differential diagnosis from perianal fistulas and abscesses is mandatory.
MRI takes advantage of its comprehensive overview of the tissues surrounding the anal canal: If a connection with the anal structures is not visible, perianal fistulas’ diagnosis can be excluded.
As well as for the other pathologies, MRI can provide an accurate assessment of treatment outcomes.
Hidradenitis suppurativa is caused by an obstruction of cutaneous hair follicular ducts located near to apocrine glands. The disease course is usually long term with frequent relapses and consists of abscesses and sinus tract formation.
Chemoradiotherapy is usually the first-line treatment modality in anal cancer. When not sufficient in reaching complete regression, it can serve as a neoadjuvant approach before surgery [Figure 9].
MRI features of a positive response are decrease in size and signal intensity on T2-weighted sequences, due to fibrosis.
However, thickening or hyperintensity of the mucosa related to mucosal edema can be normal effects of chemoradiotherapy and should not be misinterpreted as residual tumor.
Nevertheless, perianal MRI can help detect outcomes after colonic or pelvic surgery.
Furthermore, although CT scan and fluoroscopy can better detect the presence of dehiscence, MRI can rely on its higher soft-tissue resolution for detecting acute (i.e., fistulas/ abscess formation and urinary retention) or chronic (i.e., strictures/stenosis and fecal incontinence) complications [Figure 10 and 11]. Moreover, sequences obtained before and during Valsalva maneuver can additionally assess pelvic floor continence and visceral prolapses when the patient complains about defecatory dysfunctions (constipation or incontinency) [Figure 12].
The anal canal and the adjacent structures can be involved in different clinical conditions.
Perianal MRI is an imaging tool that requires a specific technical approach; it is provided with high soft-tissue resolution and offers a complete overview of anal and perianal structures.
Radiologists and clinicians should be familiar with this technique’s capabilities in different clinical scenarios.