Emerg Radiol DOI 10.1007/s10140-014-1232-2

REVIEW ARTICLE

The role of emergency MRI in the setting of acute abdominal pain Noah G. Ditkofsky & Ajay Singh & Laura Avery & Robert A. Novelline

Received: 26 February 2014 / Accepted: 29 April 2014 # Am Soc Emergency Radiol 2014

Abstract Abdominal pain is one of the most common reasons for patients to present to the emergency department (ED) in the USA, with an estimated seven million visits in 2007–2008, a figure which represents 8 % (±0.2 %) [2] of all ED visits and a 31.8 % increase from 1999–2000. Abdominal pain has a broad differential diagnosis that encompasses multiple organ systems and can provide a significant diagnostic challenge to the ED physician. Although magnetic resonance imaging (MRI) currently plays a limited role in the assessment of abdominal pain presenting to the ED in the nongravid population, its utility in the pregnant and pediatric population has already been proven. A proven diagnostic track record, lack of ionizing radiation and the ability to provide excellent tissue contrast without the use of nephrotoxic iodinated contrast, makes MRI an attractive imaging modality. As physicians and patients become more aware of the potential risks associated with exposure to ionizing radiation, ED MRI utilization is likely to increase. In this article, we discuss the MRI appearance of some of the most common diagnoses, which present as abdominal pain to the ED.

Keywords MRI . Emergency . ED . Acute abdominal pain . Appendicitis . Ovarian torsion . Crohn’s disease . Obstructive uropathy . Nephrolithiasis . Urolithiasis . Choledocholithiasis . Cholecystitis . Pancreatitis . Small bowel obstruction . SBO

N. G. Ditkofsky (*) : A. Singh : L. Avery : R. A. Novelline Department of Radiology, Massachusetts General Hospital, 55 Fruit St. FND210, Boston, MA 02114, USA e-mail: [email protected]

Introduction Abdominal pain is one of the most common reasons for patients to present to the emergency department (ED) in the USA, with an estimated seven million visits in 2007–2008 [1], a figure which represents 8 % (±0.2 %) [2] of all ED visits and a 31.8 % increase from 1999 to 2000. As of 2010 (the last period for which figures are available), 16.2 % of all patients presenting to the ED (for any reason) received a computed tomography (CT) scan, and 0.5 % received magnetic resonance imaging (MRI) [2]. This data may in part reflect the lower availability of MRI with respect to CT, with only 66 % of EDs having access to MRI on a regular basis as opposed to 96 % having access to CT [3]. In the setting of abdominal pain, imaging can narrow the differential diagnosis and often provides the actual diagnosis. The American College of Radiology (ACR) appropriateness criteria state that in the setting of nonspecific abdominal pain, the first-line imaging modality is contrast enhanced (CE) CT scan (appropriateness criteria 8) [4]; however, this also has a relatively high associated radiation dose of approximately 15 mSv [5] with a range of 10–30 mSv [4, 6]. Even using the latest dose reduction techniques, such as iterative reconstruction, the patient will still be exposed to 4.2–8.4 mSv [7]. Ultrasound is often the second choice of imaging modality (appropriateness criteria 6) [4] but lacks the ability to screen for multiple pathologies in the same manner as CT. MRI does not entail the use of ionizing radiation, but is a more timeconsuming and expensive examination [8]. Given these drawbacks, the ACR deems it slightly less appropriate than ultrasound in the setting of nonspecific abdominal pain and fever (appropriateness criteria 5) [4]. Additional drawback that should be considered on a case-by-case basis includes the patient’s ability to both remain still during the examination (failure to do so can introduce artifacts rendering the study

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nondiagnostic) and tolerate the claustrophobic confines of a conventional MRI bore. These are a lesser issue in CT and ultrasound. Although MRI currently plays a limited role in the assessment of abdominal pain presenting to the ED in the nongravid population, its utility in the pregnant and pediatric population has already been proven [9–12]. A proven diagnostic track record, lack of ionizing radiation and the ability to provide excellent tissue contrast without the use of nephrotoxic iodinated contrast [13], makes MRI an attractive imaging modality. As physicians and patients become more aware of the potential risks associated with exposure to ionizing radiation, ED MRI utilization is likely to increase. In this context, it is important to remember that MRI is not practically used as screening modality nor is it endorsed by the ACR as the firstline imaging modality in any source of acute abdominal pain. In this article, we discuss the MRI appearance of some of the most common diagnoses, which present as abdominal pain to the ED (Table 1).

direct radiologist supervision provide the highest likelihood of a clinically useful diagnostic study. Table 2 provides a sample set of sequences with which to begin each examination.

Choosing the appropriate sequences

Table 2 A suggested set of initial sequences to asses nonspecific abdominal pain in the ED

Choosing the appropriate MR sequences in the setting of nonspecific abdominal pain can be a daunting task, as one must balance the need for rapidity of imaging with the need to acquire a diagnostically useful study. At a minimum, all exams should include T1- and T2-weighted sequences with and without fat suppression in at least one plane as well as inversion recovery imaging. Tailoring the sequences to the individual patient and performing the examination under Table 1 Indications for abdominopelvic MRI that may present as abdominal pain in the ED Pancreas

Vascular Kidney

Gallbladder and biliary tree Ovary Bowel

Evaluation of complicated acute pancreatitis and characterization of peripancreatic fluid collections* Aortic aneurysm and dissection in those unable to tolerate iodinated contrast Evaluation of obstructive uropathy in gravid patients and those unable to tolerate iodinated contrast* Evaluation of choledocholithiasis* Evaluation of cholangitis Evaluation of cholecystitis* Ovarian torsion* Appendicitis*(particularly in pregnancy) Crohn’s disease* Ulcerative colitis Bowel obstruction* Diverticulitis Infectious enteritis

Entities discussed in this paper are marked with an asterisk (*)

Appendicitis Acute appendicitis is the most common abdominal emergency [14] and the most common surgical condition encountered during pregnancy (excluding obstetrical surgical issues) [13]. Occurring at a rate of 9.38/10,000 people in the USA [15], acute appendicitis has a slight male predisposition and has the highest incidence between the ages of 10 and 19 [15]. When it comes to imaging of suspected appendicitis (in the nongravid population), contrast-enhanced CT is the imaging modality favored by the ACR receiving a rating of 8 [16] in their appropriateness criteria [6]; the drawbacks of CT are the use of ionizing radiation and the nephrotoxic potential of IV contrast. Ultrasound does not use ionizing radiation but has

1. 3 plane SSFSE without and with fat saturation Try to cover the entire abdomen and pelvis Very rapid sequences so in the event of an aborted exam, this may provide information that would otherwise not have been acquired Allows for assessment for inflammatory change in the right lower quadrant, dilated ureters and perinephric inflammation, pericholecystic and peripancreatic inflammatory change, as well as ovarian enlargement and edema Radiologist should check the images following this set of sequences to determine suitability of other sequences and the order in which they should be performed 2. Coronal T2 FSE with fat saturation Longer echo train length provides better definition than the SSFSE imaging Use the previously performed sequences to determine need for coverage of entire abdomen and pelvis or whether to focus on a specific site of pathology May be able to see a calculus in the ureter or common bile duct 3. Dual echo T1 followed by T1 fat saturated Allows for characterization of hemorrhagic lesions such as endometriomas, hemorrhagic cysts, and hemorrhage ovarian torsion. Dual echo can provide value added information about liver parenchyma and adrenal lesions Provides excellent anatomic definition (Can be useful in inflammatory bowel disease) 4. Assess need for pathology specific sequences T1 fat-saturated post-gadolinium Rectal contrast MRCP/MRU Inversion recovery Diffusion

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lower sensitivity and specificity than CT [17] and receives an appropriateness rating of 6 [6]. MRI sensitivity values range from 0.85 [16] to 1.0 [18] with a specificity of 0.97 [16] to 0.99 [18], which is approximately equally sensitive to but more specific than CT (sens 0.94, spec 0.95) [17] and both more sensitive and specific than ultrasound (sens 0.86, spec 0.81 ) [17]. Currently, suspected appendicitis is the most common indication for MRI of the abdomen and pelvis in the pregnant population [9]. Appendicitis in the gravid patient typically presents in the second trimester [19] and has an incidence of 0.015–0.07 % [20]. MRI is also frequently considered in our pediatric population when appendicitis is the diagnosis of greatest concern and ultrasound is nondiagnostic. However, the use of MR in the diagnosis of appendicitis should not be restricted to the pediatric and gravid population. In the age of ALARA (Title 10, Section 20.1003, of the Code of Federal Regulations of the government of the United States of America) and the spirit of “do no harm”, radiologists should consider MRI in cooperative patients who can physically tolerate an MRI exam. In particular, MRI should be considered in the young adult population who can usually comply with examination instructions and for whom lifetime radiation dose is of greatest concern. A review of the literature yields multiple different sets of sequences that can be used to assess for appendicitis [9, 10, 13, 16, 18, 21, 22]. At our institution, we use a phased-array surface coil as it has a higher signal to noise ratio than that of the built in body coil. We begin with single-shot fast SE sequence in three planes, followed by a T2-weighted fatsaturated and STIR sequence in whichever plane best demonstrates the appendix. A radiologist then reviews the study (with the patient on the scanner), and in the nongravid patient, T1-weighted imaging before and after the administration of intravenous gadolinium contrast is performed. Although intravenous contrast material with T1-weighted fat-suppressed sequences is an excellent aid in the diagnosis of acute appendicitis, its use is contraindicated during pregnancy. The normal MRI appearance of the appendix is that of a tubular structure that measures less than 6 mm in diameter with a wall thickness of less than 2 mm [20]. In the absence of endoluminal contrast, the appendix has a cordlike appearance and intermediate signal intensity on all sequences that parallels that of the wall of the adjacent terminal ileum and cecum [23]. The normal appendix can be visualized 78 % of the time using T1-weighted spin echo sequences [24] and is unlikely to be identified on STIR imaging [25]. Morphologically, the inflamed appendix demonstrates a caliber of greater than 7 mm and a thickened appearing wall [13]. The inflamed appendix also has altered signal characteristics, with decreased signal intensity on T1-weighted images and increased peripheral signal intensity on T2-weighted sequences denoting periappendiceal fluid (Fig. 1b) [25]. An

Fig. 1 Acute appendicitis: 37-year-old gravid female with right lower quadrant pain. a Axial T2-weighted fat saturated demonstrates the appendix (arrow) in cross section with a low signal intensity wall and fluidfilled high signal intensity lumen with surrounding high signal inflammatory change (arrowhead). Note the gravid uterus (*). b Sagittal T2weighted nonfat-saturated images again demonstrates the dilated appendix (arrows), this time in long axis profile with a low signal wall and high signal lumen. Note that the inflammatory change so easily seen in a blends into the fat signal surrounding the appendix in b

inflamed and edematous appendix is well demonstrated on STIR imaging, and enhancement of an inflamed appendix is typically greater than that of the adjacent ileum [25] (barring reactive ileal inflammation) (Fig. 2). Fat-saturated T2weighted images will best demonstrate periappendiceal inflammation (Fig. 1a).

Crohn’s disease It is appropriate to discuss Crohn’s disease at the same time as appendicitis as the peak incidence of initial presentation of Crohn’s disease is age 15–29 [26], which overlap with the age

Fig. 2 Acute appendicitis: post-gadolinium T1 fat-saturated image clearly depicts the avidly enhancing appendix (identified by arrowheads) in this nongravid patient with appendicitis

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of peak incidence for appendicitis. Occurring in 0.1 % of Western populations, Crohn’s disease is characterized histologically by transmural inflammation of the bowel wall [27]. The preferred method for imaging Crohn’s disease is CT or MR enteroclysis [28] which is not typically performed in the emergency setting. As such, we will focus on the appearance of Crohn’s disease, as it would appear when imaging for appendicitis. The MRI appearance of active Crohn’s disease mirrors the appearance of Crohn’s disease on CT and includes wall thickening with increased T2 signal intensity to the bowel wall, mucosal hyperenhancement, ulcerations, mesenteric hypervascularity and vascular stratification (the comb sign), perienteric inflammation, and reactive lymphadenopathy [28] (Fig. 3). Imaging may also demonstrate some of the complications of Crohn’s disease such as abscess, fistula formation, and bowel obstruction (Fig. 4).

Ovarian torsion Nonspecific pelvic pain in the reproductive age female population is a common ED presentation, with a broad differential diagnosis. Currently, the ACR recommends ultrasound as the first-line imaging (appropriateness criteria 9) modality, irrespective of patient ß-HCG status if a gynecologic abnormality is suspected [29], and in the gravid population. In those for whom ultrasound is not diagnostic, but a gynecologic etiology can be ruled out by adequate visualization of the ovaries and uterus (including the fundus), CT should be the second-line imaging modality (considering of course patient age and the use of ionizing radiation) [29]. MRI is particularly useful in the setting of clinically suspected ovarian torsion and nondiagnostic ultrasound exam, as the tissue contrast inherent to MRI permits better characterization of the abnormal ovary.

Fig. 3 Crohn’s disease: coronal T1-weighted nonfat-saturated image demonstrates thickening of the terminal ileum (arrowheads) as well as reactive lymph nodes and the stratified vasculature of the comb sign (arrows)

Fig. 4 Fistulising Crohn’s disease with abscess: axial T1-weighted fatsaturated post-gadolinium image shows enhancing intra-abdominal collection (arrow) extending into right lateral abdominal wall (arrowheads)

The normal ovary is almost always identifiable on MRI in reproductive age females measuring approximately 3×3× 2 cm [30]. Typically, T1-weighted imaging demonstrates a homogenous soft tissue intensity signal isointense to that of the uterine myometrium [30, 31], which can make the normal ovary difficult to differentiate from adjacent bowel. The T2weighted imaging appearance of the normal ovary is affected by the patient’s menopausal status. The premenopausal ovary demonstrates a zonal anatomy with a low signal intensity cortex and high signal intensity medulla. Within the medulla, there are normally scattered well-circumscribed rounded foci of fluid intensity signal, which correspond to follicles [30, 31]. The normal ovary enhances less than or equal to that of the uterine myometrium [30, 31]. In ovarian torsion, the ovary twists on its vascular pedicle, resulting in partial to complete obstruction of arterial inflow and venous outflow, which, when left untreated, can lead to ischemia, edema, hemorrhage, and necrosis. T2-weighted imaging is the mainstay of diagnosis demonstrating an enlarged ovary with increased signal and peripherally arrayed follicles [32] (Fig. 5a). This appearance is caused by ischemia, leading to central ovarian edema. This results in peripheral displacement of developing follicles [33]. As ischemia progresses to hemorrhagic infarct of the ovary, the T1 appearance of the ovary will change. Initially, the ovary will be of low signal and have a homogenous appearance; however, as the ischemia progresses to hemorrhagic infarction, the ovary will begin to exhibit increasing T1 signal [34, 35] (Fig. 5b) which is best appreciated on fat-suppressed sequences. As torsion is a process resulting from disruption of vascular supply, following the administration of intravenous gadolinium, the torsed ovary will exhibit decreased, minimal, or absent enhancement (Fig. 5c). The degree of enhancement will relate to the degree of torsion. An infarcted and completely avascular ovary will demonstrate no enhancement, and an ovary with arterial inflow and no vascular outflow will demonstrate some mild, heterogeneous enhancement. This is best quantified by comparison with the contralateral non-torsed ovary [36].

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* * * C Fig. 5 Ovarian torsion: sagittal T2-weighted image (a) demonstrates enlargement of the left ovary with multiple peripherally arrayed follicles and low internal signal secondary to hemorrhagic infarct. Note the normal right ovary (*). T1-weighted fat-saturated pre (b) and post (c) gadolinium images demonstrate increased T1 signal consistent with hemorrhagic left ovarian infarct as well as lack of enhancement following gadolinium

Urolithiasis Urolithiasis frequently presents with lower abdominopelvic pain and thus it is commonly included in the same clinical differential diagnosis as both appendicitis and ovarian torsion. While CT is the preferred method for assessment of both appendicitis and urolithiasis, MRI is used in the setting of clinically suspected ovarian torsion and nondiagnostic ultrasound. Consequently, it is not unreasonable to expect the occasional urolithiasis to present on an MRI assessing for ovarian torsion. Additionally, in the gravid patient, the enlarged uterus may prevent adequately ultrasound visualization of the distal ureters necessitating MR when ureteric calculus is clinically suspected. Thus, a familiarity with the MR appearance of urolithiasis is useful in the setting of nonspecific abdominopelvic pain. The basic principle of ED MR urography (MRU) is that urine is a simple fluid which will have a very long T2 relaxation time. The application of heavily T2-weighted sequences will consequently result in high signal from the urine in a dilated collecting system while simultaneously nulling the signal from adjacent soft tissues with shorter T2 relaxation times [37]. This is particularly useful in settings where gadolinium cannot be administered (renal failure, pregnancy, and allergy). The most commonly used T2 MRU sequence for imaging an obstructed ureter is single-shot fast spin echo [37] (SSFSE, GE; HASTE, Seimens; FSE-ADA, Hitachi; FASE, Toshiba). A heavily T2-weighted sequence of the abdomen and pelvis in the coronal plane should be added to any study in which urolithiasis is a clinical concern. If gadolinium has been administered, fat-suppressed T1-weighted imaging should occur

in the excretory phase at 5 min post administration in multiple planes. The MRI appearance of obstructive uropathy parallels the appearance of obstructive uropathy on CT, with dilatation of the collecting system. Although nonobstructing ureteric calculi are not felt to be reliably detected on MRI, when imaged, they appear as filling defects within the fluid column [38] (Fig. 6). In the setting of obstruction, MRI is 94–100 % [37] sensitive to the detection of the culprit calculi. One can often trace the dilated ureter to the point of the obstruction. Ancillary features of obstructive uropathy relate to perinephric and periureteric edema, which is best observed on fat-suppressed T2-weighted images. The appearance of an obstructing calculus on excretory phase T1-weighted MRU is again similar to the appearance of CT, with a calculus (present as a signal void) obstructing the contrast column.

Acute cholecystitis and pancreatitis It is appropriate to discuss acute cholecystitis and pancreatitis together as obstruction of the common bile duct (CBD) can lead to both of these entities. Additionally, pancreatitis can lead to a reactive cholecystitis. Acutely, pancreatitis has two phases, acute interstitial edematous pancreatitis (IEP) and necrotizing acute pancreatitis (NAP) [39]. NAP affects 6– 20 % of patients with pancreatitis and is associated with increased morbidity and mortality [39, 40]. In the setting of clinically suspected acute pancreatitis, the ACR currently recommends ultrasound to assess for cholelithiasis (appropriateness rating 9) and CECT or MRI to evaluate pancreatitis (appropriateness rating 4 for both) acknowledging that MRI provides excellent visualization of both the pancreatic parenchyma and the pancreatic duct as well as the

Fig. 6 Obstructive uropathy: coronal T2-weighted image demonstrates a large staghorn calculus (arrow) obstructing the right kidney at the level of the renal pelvis. Note the right sided hydronephrosis and compare to the relatively normal appearance of the left kidney with associated exophytic cyst (arrowhead)

Emerg Radiol Fig. 7 Acute cholecystitis: a Axial T1-weighted out of phase image demonstrates thickening of the gallbladder wall. b Axial T2weighted fat-saturated image at the same level demonstrates the high signal bile within the gall bladder as well as inflammatory change in the adjacent fat. c Axial T2-weighted image at the level of the gallbladder neck demonstrates a large gallstone (arrow)

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common bile [41]. The previous version of the appropriateness criteria for acute pancreatitis noted MRI to be more time consuming and less available than CT. This resulted in a lower appropriateness rating. However, the most recent iteration notes that MRI is gaining greater acceptance and endorses MRI and CT equally if pancreatitis is suspected and ultrasound is nondiagnositic. In the setting of right upper quadrant pain, as in the setting of pancreatitis, the ACR recommends initial assessment with ultrasound (appropriateness rating 9) as the diagnosis of exclusion is acute cholecystitis [42]. MRI receives a cursory discussion (and an appropriateness rating of 6) as does CT. In the setting of acute cholecystitis, ultrasound has a sensitivity of 81 % and a specificity of 83 % [43], and given its low cost, we support this as the first-line imaging modality. Nuclear medicine cholescintigraphy has a higher sensitivity and specificity [44]; however, it can take up to 4 h to perform and requires exposure to ionizing radiation. Moreover, though excellent in diagnosing cholecystitis, it offers limited means of diagnosing other pathologies. MRI has a sensitivity of 88 % and specificity of 89 % [44] exceeding those of ultrasound. MRI assessment of the right upper quadrant and the pancreas is best performed using a phased-array coil. Initial sequences should include T1-weighted sequences to delineate anatomy and assess the best subsequent imaging planes. Fluid sensitive sequences (such as inversion recovery imaging or fat-suppressed T2-weighted imaging) should then be performed to assess both the size of the gallbladder and for the presence of pericholecystic and peripancreatic edema. Magnetic resonance cholangiopancreatography (MRCP) utilizes the same principles as MRU to depict static fluid; in this case, the biliary ducts, gallbladder, and pancreatic duct. This is best acquired using respiratory-gated thin sections or alternatively

a 3D acquisition so that isotropic data is acquired allowing for multiplanar reconstruction and thick slab reformatting. Finally, T1-weighted fat-suppressed imaging is performed to assess for hemorrhage prior to the intravenous administration of gadolinium. Dynamic, T1-weighted, fat-suppressed postgadolinium imaging is then performed to assess for pancreatic necrosis and gallbladder hyperemia. The normal gallbladder wall measures less than 3 mm in thickness and is best identified by its low signal on nonfatsuppressed T2-weighted sequences [45]. The gallbladder wall has intermediate signal intensity on T1-weighted imaging. Following the administration of intravenous gadolinium, the gallbladder wall enhances uniformly [45]. Gallstones are low signal on both T1- and T2-weighted sequences but are best appreciated on T2-weighted imaging, as low signal against the high signal of the bile in the gallbladder [45–47]. Gallbladder sludge, formed by the concentration and precipitation of cholesterol and bile salts, is typically isointense to mildly hyperintense on T2 and hyperintense on T1-weighted imaging relative to bile [45, 46]. The MRI appearance of acute cholecystitis parallels that observed on ultrasound. The gallbladder demonstrates distension to greater than 4 cm, and the wall becomes thickened to greater than 3 mm (Fig. 7a). The normal low T2 signal intensity wall of the gallbladder increases in signal, appearing ill defined or stratified on nonfat-suppressed T2 imaging (Fig. 7b). Pericholecystic fluid will also be present. Obstructing gallstones are the most common cause of acute cholecystitis and can be identified in many cases as low signal foci obstructing the gallbladder neck at the distal aspect of a distended duct on T2-weighted sequences (Fig. 7c) [47]. Following the administration of intravenous gadolinium, the inflamed gallbladder wall demonstrates increased

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enhancement (Fig. 8). Transient increased enhancement of the undersurface of the liver can also be observed [46, 48]. Ischemic necrosis of the gallbladder resulting in gangrene will result in asymmetric foci of intramural T2 hyperintensity on fat-suppressed imaging as a result of ulceration, hemorrhage, or micro-abscess formation within the gallbladder wall [49, 50]. The gangrenous gallbladder will demonstrate little or no enhancement following gadolinium administration [49]. Choledocholithiasis is a feature of 10–15 % of cases of acute cholecystitis [46]. Though ultrasound is of limited utility in assessing the distal CBD, MRCP is both sensitive (89– 100 %) and specific (83–100 %) to detection of any CBD stone greater than 3 mm in size [46]. This is important, as the most common etiology in acute pancreatitis is gallstones [51–53]. The normal pancreas is best appreciated on T1-weighted fat-suppressed imaging [54] as a high signal structure in the anterior pararenal retroperitoneal space. The normal pancreatic parenchyma is dark on T2-weighted sequences, which are most useful in assessing the pancreatic duct, as well as for complications of pancreatitis such as pseudocysts. The normal pancreas enhances avidly to become brighter than the liver on the pancreatic phase, approximately 15–20 s after intravenous gadolinium administration [54] before washing out to become isointense to the liver on portal venous phase. In pancreatitis, there is breakdown of the normal pancreatic parenchyma, which releases proteolytic enzymes. This leads to edema, fluid collections, and in some patients, pancreatic parenchymal necrosis. T1-weighted imaging is insensitive to edema; however, stranding in the peripancreatic fat can be appreciated on the in-phase portion of the dual echo gradient T1 sequences [54]. T1-weighted fat-suppressed sequences can demonstrate an enlarged pancreas with a decrease in (or heterogeneity to) the normal high intensity pancreatic signal (Fig. 9b). Peripancreatic fluid collections and pancreatic edema are best identified on T2-weighted fat-suppressed images by their elevated signal (Fig. 9a). T2-weighted nonfatsuppressed imaging is also useful in the identifying pancreatic

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Fig. 9 Acute pancreatitis: a Axial T2-weighted fat-suppressed image demonstrates increased signal in the pancreatic parenchyma (arrow) with associated peripancreatic fluid (arrowhead). b Axial T1-weighted fatsuppressed image in the same patient at approximately the same level demonstrates heterogeneously decreased signal to the pancreatic parenchyma (arrow)

necrosis in NAP, as these areas will appear dark relative to the normal pancreatic parenchyma at 1.5 T [53, 54]. Contrastenhanced MRI is also useful for assessment for the acute complications of pancreatitis which include pseudoaneurysm

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Fig. 8 Acute cholecystitis: axial T1-weighted fat-saturated post-gadolinium images in another patient demonstrate marked thickening and enhancement of the gall bladder wall

Fig. 10 Small bowel obstruction secondary to stricture in a patient with Crohn’s disease: a Axial T2-weighted fat-suppressed image demonstrates multiple dilated loops of small bowel with air fluid levels (arrowheads). b Axial T2-weighted fat-suppressed image slightly more inferiorly demonstrates an abrupt transition point in the right lower quadrant at site of stricture (arrow). Note the inspissated fecal material in the loop proximal to the obstruction (*)

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formation and vascular thrombosis (most commonly involving the splenic vein).

also occasionally mimic a mass and should be considered in the appropriate clinical context.

Conclusion Small bowel obstruction Although infrequently imaged emergently with MRI, small bowel obstruction (SBO) is found to be the etiology of acute abdominal pain in approximately 2 % of ED patients [55] and 71 % of patients with SBO initially present via the ED [56]. Traditionally, radiography has been the initial imaging modality in the assessment of suspected SBO given its availability, low cost, and rapidity. However, radiography has a sensitivity and specificity of only 75 and 66 %, respectively [55] and has limited ability to demonstrate the underlying etiology of the obstruction when positive. CT (sensitivity 87 % and specificity 81 %) [55] and MRI (sensitivity 90 % and specificity 89 %) [55] offer increased sensitivity and specificity over radiography with the ability to determine the site and etiology of the obstruction. In the setting of suspected high-grade SBO, the ACR currently recommends CT scan of the abdomen and pelvis with IV contrast (appropriateness rating 9) or without IV contrast (appropriateness rating 7) over MRI abdomen and pelvis with and without IV contrast (appropriateness criteria 6) [57]. The ACR further states that given the expense and lack of evidence demonstrating a diagnostic advantage over CT, MRI should not be routinely used; however, its use should be considered in young adults who have previously undergone multiple CT scans, pregnant women, and in children [57]. In the setting of intermittent or low grade SBO, the ACR equally endorses CT abdomen and pelvis with contrast, CT enteroclysis and MR enteroclysis (appropriateness rating 8) [57]. However, given the involved nature of MR enteroclysis, it is infrequently performed in the ED setting. The MRI appearance of high-grade SBO parallels that of CT, demonstrating dilated loops of bowel with air fluid levels (Fig. 10), which are best appreciated on axial T2-weighted images [58]. Benign etiologies for SBO will lack a focal mass but may demonstrate diffuse mural thickening [59]. The most common etiology for benign SBO is post-operative adhesive disease, representing 74 % of cases in western populations [28, 55, 60, 61]. The presence of adhesions can be inferred by a lack of focal mass coupled with clustering and deformation of bowel loops at a point of abrupt bowel loop caliber change, “the transition point” [28, 59]. Occasionally, adhesions can be identified on nonfat-suppressed T2-weighted sequences as low signal intensity chords traversing the high signal intensity mesenteric fat [28, 59]. Other benign causes of SBO include hernia, gallstone ileus, and stricture in Crohn’s disease. Malignant etiologies may demonstrate a focal mass, segmental mural thickening, or evidence of locoregional or distant metastatic disease [59]. Focal structuring in Crohn’s disease may

We expect the emergency department utilization of MRI to increase as availability of MRI increases. Additional driving factors are increased physician and patient awareness of the detrimental effects of ionizing radiation. MRI has the advantage of offering excellent tissue contrast resolution without the use of ionizing radiation as well as in many cases without the use of intravenous contrast. This last point is important, as with an aging population and increasing prevalence of diabetes [62], rates of renal dysfunction are likely to increase.

Conflict of interest The authors declare that they have no conflict of interest.

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