Current and emerging techniques in gastrointestinal imagingSE McSweeney, PM O'Donoghue, K Jhaveri
Joint Department of Medical Imaging, University Health Network and Mount Sinai Hospital, University of Toronto, 610 University Ave, Toronto, ON, M5G 2M9, Canada
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.65280
Source of Support: None, Conflict of Interest: None
This review is devoted to current and emerging techniques in gastrointestinal (GI) imaging. It is divided into three sections focusing on areas that are both interesting and challenging: imaging of the small bowel and appendix, imaging of the colon and rectum and finally liver and pancreas in the upper abdomen. The first section covers cross-sectional imaging of the small bowel using the techniques of multidetector computed tomography (MDCT) (including CT enterography) and magnetic resonance imaging (MRI). The evaluation of mesenteric ischemia and GI tract bleeding using MDCT angiography is also reviewed. Current imaging practice in the evaluation of appendix is also reviewed and illustrated. The second section reviews CT and MR colonography and imaging of the rectum. It describes CT virtual colonoscopy (CTVC) with emphasis on the advantages and disadvantages of the technique with discussion of the role of CTVC in screening. The intriguing topic of MR colonography (MRC) is also reviewed. Imaging of the rectum with emphasis on imaging of rectal cancer is described with the roles of CT, MR, endoluminal ultrasound and positron emission tomography scanning discussed. The final section reviews current and emerging techniques in liver imaging with the role of ultrasound including contrast ultrasound, MDCT and MR (including contrast agents) discussed. The new developments and applications of imaging of pancreatic disease are discussed with emphasis on the role of MDCT and MRI with gadolinium. This review highlights the current role and advancement of imaging techniques with new diagnostic and prognostic information pertinent to gastrointestinal disease continuing to emerge.
Keywords: Computed tomography, magnetic resonance imaging, ultrasound
This review is devoted to current and emerging techniques in gastrointestinal (GI) imaging. It is divided into three sections focusing on areas that are both interesting and challenging in GI imaging: imaging of the small bowel and appendix, imaging of the colon and rectum and finally imaging of liver and pancreas in the upper abdomen.
Historically, barium techniques, small bowel follow-through and fluoroscopic small bowel enteroclysis have been the only techniques available to examine the small bowel in its entirety but results of these studies have been disappointing due to the length of the small bowel and overlapping loops.
Advances in cross-sectional imaging, computed tomography (CT) and magnetic resonance imaging (MRI), have led to the use of these modalities playing an important role in imaging of small bowel disease. These modalities offer distinct advantages over fluoroscopic barium examination allowing 1) depiction of the entire length of the small bowel 2) depiction of the entire thickness of the bowel wall 3) prevention of obscuration of small bowel loops by superimposition and 4) evaluation of extra-luminal structures for detection of abscesses and fistulas. ,
CT enteroclysis and CT enterography
CT enteroclysis which involves the placement of an enteroclysis tube in the jejunum to permit contrast injection for the necessary small bowel distension can be successfully used to evaluate small bowel disease [Figure 1].
The recent introduction of neutral oral contrast agents with improved luminal distension and advances in multidetector CT (MDCT) technology have converged to provide a new CT imaging technique specific for the small bowel known as CT enterography (CTE). CTE combines neutral or low-density oral contrast agents with intravenous contrast and thin slice acquisition to depict small bowel diseases optimally. With this technique, small bowel disease such as infections, neoplasms, adhesions, polyps, vascular malformations and inflammatory bowel diseases can be detected. 
CT angiography in mesenteric ischemia and GI tract bleeding
Recent advances in CT technology allowing thinner collimation, faster scanning times, greater anatomic coverage, and better multiplanar reformatted (MPR) images have greatly expanded the diagnostic role of CT angiography (CTA) for various pathologic processes, mesenteric ischemia and GI bleeding being two of these processes.
Mesenteric ischemia is a difficult clinical diagnosis that requires a high index of clinical suspicion because the clinical and imaging features of intestinal ischemia and infarction overlap with many other intestinal disorders, and patients who have mesenteric ischemia often have coexisting diseases. When mesenteric ischemia is of clinical concern, MDCT has become the imaging modality of choice due to its ability to evaluate the mesenteric vasculature, small and large bowel and the remainder of the abdomen for other pathology. MDCT can be used in the evaluation of patients with suspected acute and chronic intestinal ischemia and can be performed quickly in critically ill patients and is less dependent on operator skill and patient factors than other imaging examinations. ,
MDCT can be used to evaluate both acute and obscure (recurrent or persistent) GI tract bleeding. Initial experience indicates that MDCT angiography is a promising first-line modality that is time-efficient, sensitive, allowing accurate diagnosis or exclusion of active GI hemorrhage and thus potentially has a profound impact on the evaluation and subsequent treatment of patients who present with acute GI bleeding.  Recent studies show that CT has depicted active GI tract hemorrhage with bleeding rates as low as 0.3 mL/min, which is below the reported threshold for selective catheter angiography. 
MR imaging has many unique properties that make it well-suited for imaging of the small bowel. These advantages include the lack of ionizing radiation, the improved tissue contrast that can be obtained by using a variety of pulse sequences and parameters, the ability to perform real-time and functional imaging, and the relative safety profile of gadolinium contrast agents.  As with CT, MR enteroclysis with intubation of jejunum and administration of contrast directly into small bowel versus enterography are options for study performance. The main limitations preventing more widespread implementation of MR imaging in clinical practice include inferior temporal and spatial resolution, access issues, and costs compared with CT. 
The major clinical indication for MR enteroclysis/enterography is the evaluation of patients who have suspected or known Crohn's disease. The absence of ionizing radiation, considering the young age of most of the patients and the frequency of the repeat or follow-up examinations, is an important advantage over other techniques (radiograph, small bowel studies and CT enteroclysis).  Further indications for MR imaging include evaluation of small bowel tumors and small bowel obstruction.
Because of the intrinsic properties of MR imaging, the soft-tissue contrast is better than with CT. This difference may be important for detecting subtle areas of pathology and may improve characterization of certain abnormalities. One area in which improved soft-tissue contrast has become well established is in the evaluation of perianal fistulas in the setting of Crohn's disease. MR imaging has been shown to correlate well in detecting, staging and follow-up of perianal fistulizing disease.  The MR imaging appearance of this condition shows greater concordance with surgical findings than does any other imaging evaluation. Many different MR imaging techniques have been described and MR imaging in the coronal and axial planes demonstrates fistulous tracks in relation to the sphincter complex, ischiorectal fossa, and levator muscles. Thus, MR has the potential to evaluate small and large bowel and perianal disease using a single imaging modality.
It has been recognized for a long time that early and accurate diagnosis of appendicitis improves patient outcome.  Ultrasound and CT are widely recognized as very useful in the timely diagnosis of appendicitis [Figure 2]. 
Most institutions use ultrasound as the initial imaging modality, followed by CT if sonographic diagnosis is uncertain. This approach seems to have excellent accuracy, with reported sensitivity of 94-99% and specificity of 94-95%. 
MR imaging is an attractive modality for imaging patients for whom the risks of radiation or the potential nephrotoxicity of iodinated contrast agents is a major concern, such as pregnant and pediatric patients. MR imaging is most useful for evaluating pregnant patients with acute lower abdominal pain believed to have an extra-uterine cause, such as appendicitis or ovarian torsion when ultrasound is non-diagnostic. 
Historically, optical colonoscopy (OC) has been the modality of choice for the assessment of colonic pathologies including the evaluation of colorectal masses and the depiction of bowel wall inflammation. The great advantage of OC is the ability to perform biopsy and subsequent histopathological characterization of disease at a single time point, but because it is an invasive procedure and patients' acceptance isn't optimum resultant screening programs using OC lack compliance.  The concept of virtual colonoscopy (CT and MR) is based on use of thin-slice cross-sectional imaging techniques with subsequent multiplanar and 3D reconstruction to evaluate the entire length of colon even in patients with stenotic lesions inaccessible to OC. It has the added advantage of being able to evaluate bowel lumen, bowel wall thickness, pericolonic regions and further extraluminal pathology at single examination [Figure 3]. ,
CT virtual colonoscopy
Optimum CT virtual colonography(CTVC) requires well-cleansed bowel wall and luminal distension for adequate evaluation of polyps and colorectal cancer. Recent studies have looked at positive labeling of residual material and subsequent electronic subtraction of tagged material as possible strategies to reduce and possibly eliminate need for long preparation with laxatives before study.  This would help increase patient acceptance and compliance of CTVC as cancer screening tool.
A recent meta-analysis of sensitivity and specificity of CTVC for polyps 10 mm or larger, between 6 and 9 mm and smaller than 6 mm was 85%, 70%, and 48%, and 97%, 93%, and 91% respectively.  The recently published ACRIN trial also found that CTVC is highly accurate for the detection of intermediate and large polyps and using OC as the gold standard, the study reported that CTVC detected polyps 10 mm or larger in 90% of all participants who were confirmed to have a polyp of this size by OC. They concluded that their findings augment published data on the role of CT colonography in screening patients with an average risk of colorectal cancer.  The major limiting factor in the use of CTVC as a screening tool is that CTVC uses ionizing radiation., as a result researchers continue to change imaging parameters to allow the effective radiation dose to the patient to be substantially lower than that compared with routine CT examinations of the abdomen and pelvis without significant loss in image quality or sensitivity in detection of colonic polyps due to the inherent high contrast between the intraluminal gas and the soft tissue of the colonic wall.,  As result CTVC continues to be an evolving technique for evaluation of the colon in both diagnostic and screening settings.
Although CTVC has received most attention to date, MR colonography (MRC) also provides a less invasive alternative study compared to OC. It has major advantages over CTVC in that it avoids exposure of a screening population to ionizing radiation and provides better soft-tissue contrast.  The main current indications for MR colonoscopy include detection of colorectal masses, with MRC achieving a high accuracy, with sensitivity and specificity of 93% and 97% respectively for the detection of colorectal lesions independent of their size. , Further indications include diagnosis and characterization of inflammatory bowel disease (IBD), ulcerative colitis and Crohn's disease and providing additional information in patients after incomplete endoscopy.
The current disadvantages of the technique include the limitation of MRC to a few specialist centers and similar problems previously mentioned in regard to MR enteroclysis such as access and cost when compared to CT. There is also lack of consensus as to the optimal technique for MRC. A recent paper in relation to MRC without bowel cleansing and fecal tagging showed acceptable per-patient sensitivities, but the per-polyp sensitivities were compromised by the fecal tagging and investigators concluded that MRC at the present time with fecal tagging is not ready for widespread use, although it remained a very promising diagnostic tool. 
Imaging of rectal cancer
Developments in rectal cancer imaging have revolutionized the management of this condition. Rectal cancer outcomes have improved due to modern surgical techniques, namely total mesorectal excision (TME). Meticulous preoperative assessment with imaging is the key to obtaining this with TNM staging less crucial compared to assessing tumor distance from the potential plane of surgical resection and this is reliant on high-quality imaging. 
Endorectal ultrasound (ERUS) can be used for early T-staging of rectal cancer with accuracy of 80-90% but is limited in distinguishing between T2 and early T3 disease and has limited ability to visualize the mesorectal fascia. 
High-resolution MRI is the standard staging modality for rectal cancer and has been shown to be superior to clinical examination (digital rectal examination), CT, and endoluminal ultrasound (EUS) for staging of rectal tumors. Tumors of the distal sigmoid, rectosigmoid, and upper rectum can all be staged accurately using MRI.  Nonetheless, the same limitations as with ERUS do exist for distinction between T2 and early T3 lesions, because stranding into the mesorectal fat may represent inflammation and be overstaged as T3.  Using a high-resolution protocol, MRI can not only distinguish tumor from rectal wall, but also consistently depict the mesorectal fascia and most importantly the distance from the tumor to the fascia which is essential for clear resection margins post TME. ,
CT is limited for local staging because of its inherent low soft-tissue contrast, which does not allow for accurate approximation of T stage unless there is gross invasion of adjacent organs (T4), and even here many false-positive cases are seen.  CT is used primarily for the detection of metastatic disease. Although MRI can be used, CT is more practical as it is more widely available, and has shorter waiting lists and significantly faster examination acquisition times. 
Positron emission tomography (PET) and CT/PET don't appear to have a significant role in local staging of disease and are most helpful in assessment of metastatic disease and patients with potential recurrence. ,
Imaging of the liver and pancreas has progressed rapidly during the past decade with continued advancement of US, CT, and MRI. Each modality has not only seen refinement enabling better anatomic characterization of disease but has also received strength from the addition of new techniques. Imaging has been further enhanced with new contrast agents in both MRI and ultrasound becoming available.
US, CT, and MRI are each based on different basic physical principles and currently play complementary roles in comprehensive imaging of the liver.
Today, US, augmented by contrast-enhanced ultrasound (CEUS), allows for excellent detection and characterization of most focal liver disease, both benign and malignant.  Contrast-enhanced sonography requires intravenous injection of microbubbles, which are detected when they are destroyed by interaction with the US wave [Figure 4]. One of the disadvantages of this technique is in patients with more than one lesion; unlike CT or MRI, CEUS requires a separate injection to evaluate each lesion. 
The ability to noninvasively image hepatic blood flow with color Doppler technique is a major benefit of US and has become the initial test of choice in detecting vascular complications following liver transplantation. 
Intraoperative sonography remains one of the most accurate modalities in detecting tumor deposits in the liver with several studies showing intraoperative US to be more accurate than CT in detecting lesions. 
CT is an extremely valuable modality for assessing the liver. Continued technological improvements, including MDCT, permit accurate detection and characterization of liver lesions via improved separation of the vascular phases of liver enhancement. The isotropic nature of the acquired CT data permits high-quality multiplanar and 3D reconstruction of the pertinent anatomy or pathology which can be helpful in surgical planning [Figure 5]. 
CT cholangiography is performed post IV administration of a biliary excreted contrast agent and with advancement in multislice CT technology there is capacity for thin-section scanning and multiplanar reformation, which clearly demonstrate biliary anatomy, anatomic variants and extent of disease [Figure 5]. 
MRI can be used for lesion characterization or as a problem-solving examination in cases in which the results from multidetector CT or US examinations are inconclusive or incomplete. Of available modalities, MRI may provide overall the most accurate detection and characterization of hepatobiliary disease and can be used for sophisticated assessment of benign and malignant hepatic tumors. The excellent inherent soft-tissue contrast of MRI can be further improved by non-specific extracellular contrast agents (gadolinium) and by liver-specific contrast agents such as primovist, some of which are excreted by the biliary system. These contrast agents are now routinely used for liver imaging and improve the sensitivity and specificity of hepatobiliary MRI.  Hepatobiliary imaging with 3T MR systems provides both opportunities and challenges that are currently being investigated. The use of 3T MR compared with that of 1.5-T systems shows improved spatial resolution resulting in better detection of malignant liver lesions. 
Magnetic resonance cholangiopancreatograpy (MRCP ) is the noninvasive imaging study of choice to investigate the biliary tree. New high-resolution 3D MRCP acquires very thin sections through the biliary tree using fluid-sensitive (long echo train, heavily T2-weighted) magnetic resonance pulse sequences. , With the intrinsic high signal from bile, an MRCP can produce images of the intrahepatic and extrahepatic bile ducts that rival endoscopic retrograde cholangiopancreatography in image quality.
The mainstays of imaging for pancreatic disease have been CT, MRI, endoscopic retrograde pancreatography, and endoscopic ultrasound. Recently, there have been new advancements and refinements in all of these techniques, resulting in improved imaging of the pancreas.
MDCT as elsewhere within the abdomen has become the mainstay of pancreatic imaging. Recent advances in technology allow faster image acquisition and improved resolution. Furthermore, these improved detailed images can be converted into multiplanar and three-dimensional reconstructions to further evaluate the pancreas and pancreatic duct. In acute pancreatitis multiphase contrast-enhanced MDCT is the modality of choice to confirm the diagnosis, assess for necrosis and determine complications of pancreatitis including developing fluid collections/abscesses and vascular complications.  MDCT evaluation of pancreatic tumors has led to more accurate detection, staging, assessment of resectability of tumor, and differentiation of pancreatic tumors such as adenocarcinoma from intraductal papillary mucinous tumors. ,
MR imaging has the unique capability of allowing noninvasive evaluation of the pancreatic ducts, pancreatic parenchyma, adjacent soft tissues, and vascular network in a single imaging modality without use of ionizing radiation.
Imaging the pancreatic duct with MRCP uses the same technique as previously described for the biliary tree and is dependent on heavily T2-weighted imaging that selectively displays fluid-filled structures. These images of the pancreatic duct can be enhanced by use of secretin-enhanced MRCP. Secretin stimulation causes increase in fluid secretion by the ductal cells and simultaneously increasesthe tone of the Sphincter of Oddi More Details. Consequently, the volume of stationary fluid in the pancreatic duct increases and its delineation may be improved at MRCP.  Fat-suppressed T1-weighted sequence provides excellent delineation of pancreatic borders and the pancreas itself, which appears homogeneously bright compared with surrounding low-intensity fat and is excellent for identifying pancreatic masses or focal pancreatitis (with both appearing less intense than normal high-intensity pancreas) [Figure 6]. In patients with pancreatic cancer, high-resolution, axial or coronal, gadolinium-enhanced T1 sequences are used allowing the study of the pancreas parenchyma and tumor combined with dynamic phase MR angiography (after maximum tntensity projection (MIP ) reconstruction) allowing assessment of possible vascular involvement. ,
This review highlights the current role and advancement of imaging techniques with new diagnostic and prognostic information pertinent to gastrointestinal disease continuing to emerge.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]