traditionally , barium examinations have been the mainstay of radiological investigations of the small and large intestines . however , the standard barium follow - through ( sbft ) examination has high false - negative and false - positive rates for the detection of abnormalities this is because only 2535% of the bowel length can be adequately assessed on a standard examination due to overlapping loops , poor coating , and other factors . several new techniques have therefore evolved that provide better diagnostic capabilities and higher accuracy rates.[24 ] in this article , the newer techniques and modalities being employed for imaging of the bowel are detailed , with an overview of their strengths and weaknesses . innovations in intestinal imaging have been driven by the demand for detailed clinical information and the limitations of standard methods of small - bowel examination . alternative techniques such as the pill camera that allow high - definition color views of the bowel have appeared . at an average frequency of two frames per second , a total of > 50,000 frames are collected in 78 h as the pill camera travels through the bowel . despite the high - resolution images provided by this technique , it has become increasingly apparent that not all mucosal abnormalities seen on wireless capsule endoscopy are clinically relevant . mucosal erosions may be seen in as many as 14% of asymptomatic healthy subjects , with the rate of such abnormalities being more than double this in users of nonsteroidal anti - inflammatory drug ( nsaid ) agents . furthermore , the pill camera does not provide any information about mural and extramural abnormalities . contraindications to the pill camera include bowel obstruction , history of bowel strictures or fistulae . the most accurate conventional radiologic method in the diagnosis of small - bowel obstruction , inflammatory bowel disease ( ibd ) , gastrointestinal bleeding , and neoplasms is double - contrast enteroclysis . however , sbft has remained the most commonly performed method for the investigation of small - bowel diseases because of its ease of performance and the relative technical complexity of enteroclysis as well as the discomfort to patients during intubation . the sbft has a low diagnostic yield and low negative predictive value . in a recent study , 8% of patients with known crohn disease encountered capsule retention despite a previous normal sbft . non - intubation examinations do not fully distend the bowel , and therefore partial strictures , obstructions , and small polyps or masses can be easily missed . the advantage of enteroclysis examinations is that there is optimal bowel distension , which allows detailed evaluation of all segments [ figure 1 ] . methylcellulose bowel examination shows linear filling defects ( webs ) ( arrows ) in the distal ileum secondary to the use of nonsteroidal anti - inflammatory drugs ( nsaids ) enteroclysis examination with barium methylcellulose is the most commonly performed enteroclysis method in many countries . however , the disadvantage with this technique is that methylcellulose has a washout effect on superficial mucosal features . subtle surface abnormalities get effaced as more methylcellulose is infused to achieve a good double - contrast effect and bowel distension . therefore , this method should be reserved for detection of obstruction or mass lesions or it should be used in combination with ct scan or mri exams.[247 ] fluoroscopic evaluation with positive enteral contrast can help distinguish fixed ( stenotic ) segments of small - bowel narrowing from spasm caused by active inflammation in patients who have crohn disease and can also differentiate mild stenosis from normal peristaltic contractions . on the other hand , double - contrast enteroclysis with air / co2 and barium is by far the best method for showing mucosal details of the small bowel.[578 ] a scientific study comparing double - contrast air enteroclysis with histopathology showed good correlation between the two for the visualization of aphthae and small ulcers [ figure 2 ] . magnified view from an air  barium double contrast enteroclysis examination shows early linear mucosal breaks and ulcers ( arrow ) in a patient with proven crohn disease therefore , if capsule endoscopy is unavailable or contraindicated , double - contrast air enteroclysis should be the preferred modality for the detection of early ulcerations , irregularities , and erosions of the small bowel . this technique should also be employed if other imaging tests have been negative and a suspicion of small - bowel pathology still persists . this technique can be performed on its own or it can be combined with ct scan or mri . the pill camera remains the most sensitive method for the evaluation of gastrointestinal tract bleeding.[89 ] the most important factor for obtaining good intestinal imaging is optimal distension of the bowel with enteral contrast . enteral contrast can be positive , neutral , or negative , depending on its density ( hu ) . positive enteral contrast agents range from a 415% contrast solution to a dilute 0.36% barium solution . the author prefers an 810% iodine concentration of water - soluble contrast because this density allows diagnostic fluoroscopic observations as well as diagnostic radiographs . neutral contrast agents can be methylcellulose , mannitol , polyethyl glycol , or other bulk fibers and have a density similar to water ( 0 hu ) . negative contrast is mainly used in the imaging of the colon [ ct colonography ( ctc ) ] , where co2 or air is insufflated per rectum . enteral contrast can be administered via the intubation method ( ct enteroclysis / mri enteroclysis ) or the enterographic method [ figure 3 ] . for enterographic examination , the patient ingests a large volume of contrast over a set period of time prior to imaging . the 1300 ml is divided into two aliquots : the first lot is ingested with a prokinetic agent ( metoclopromide ) over 25 min and the second lot is consumed over the next 25 min . just prior to the scan , 200 ml of contrast the main advantage of the enterographic examination is that it is more acceptable to patients than enteroclysis as intubation may be an unpleasant experience for patients . the disadvantage of enterograpic examination is that it may not provide uniform or adequate bowel distension as compared to enteroclysis . axial ct enteroclysis examination demonstrates a segment of kinked bowel ( arrowhead ) and several adhesive bands across other segments ( arrows ) . this patient had undergone several negative ct examinations previously water is also routinely used as a neutral enteral contrast agent at many centers ; however , its use in dedicated small - bowel studies should be discontinued . water has been found to provide inadequate distension ( at various volumes and ingestion times ) of the small bowel in many studies.[1011 ] this is because water undergoes rapid reabsorption in the bowel and fails to adequately distend the distal small - bowel segments . therefore , additives ( such as contrast , mannitol or other agents ) need to be mixed with water to provide an iso - osmolar solution that does not get reabsorbed and remains in the lumen to provide distension . the use of water as a contrast agent may lead to false - negative examinations . furthermore , collapsed loops are known to demonstrate hyperenhancement on post - contrast studies and this may lead to false - positive diagnoses . therefore , the author strongly discourages the use of water as an enteral agent in dedicated studies of the small bowel . the only instance where water may be used as an enteral agent is in the enteroclysis technique where the operator exclusively controls distension of the bowel and rate of infusion . the advantage of current - generation multidetector ct ( mdct ) scanners is increased anatomical coverage with thinner sections , which provide high - quality mutiplanar ( mpr ) images and fewer motion artifacts.[1213 ] positive - contrast ct enteroclysis ( cte ) or enterography ( cteg ) may be used mainly for the diagnostic workup of small - bowel obstruction or in suspected cases of small intestinal tumors or metastases . one of the drawbacks of positive contrast is the production of streak artifacts on mpr reconstruction , particularly on older ct scanners that can not produce isotropic voxels ( slice thickness > 2 mm ) . the advantages of neutral contrast are that it does not obscure the mucosal lining and does not produce streak artifacts . therefore , neutral - contrast cte or cteg is indicated for the workup of patients with inflammatory bowel bleed , gastrointestinal bleeding , and suspected bowel pathologies . several studies have shown that ctc has high sensitivity and specificity ( > 95% ) for the detection of polyps and cancers [ figure 4a ] . datasets of the distended colon are obtained , which can then by manipulated by software to provide virtual colonoscopy images . screenshot ( a ) from a ct colonographic examination shows a polyp in the colon . top two axial images show the location of the polyp within a marker box , whereas the bottom two are virtual 3d images in the supine and prone positions which detail polyp size and distance from the rectum . mucosal views ( b ) show a virtual dissection view of the colon with a small polyp ( arrow ) in the left image . on the right , translucency rendering shows this lesion to be homogenously dense ( red ) , implying this is retained fecal matter mri is an emerging technique for comprehensive and functional bowel imaging . the absence of ionizing radiation and high - contrast resolution are the major advantages of mri over ct scan . the nonionizing aspect of mri makes it particularly suited for use in patients with ibd who may need repeated imaging . the inherent high - contrast resolution can provide high diagnostic confidence and also facilitate the detection of discriminating features of intestinal diseases . t1w and t2w imaging sequences for the imaging of the bowel have been reported in earlier studies.[101718 ] however , in recent years , advances with ultra - fast sequences based on steady - state precession have revolutionized bowel imaging . these sequences can be performed within a single breath - hold , and when combined with bowel paralysis ( using antiperistaltic agents , e.g. , buscopan ) , the technique allows rapid imaging of the small bowel . these sequences are called true fast imaging with steady - state precession ( true - fisp ) , balanced fast field - echo ( ffe ) , or fast imaging employing steady - state acquisition ( fiesta ) . these sequences are relatively insensitive to motion artifacts and provide high contrast between the bowel wall , lumen , and the mesentery . the problem with these sequences is the presence of a  black boundary  artifact along the bowel wall , which may mask small lesions or abnormalities . the addition of fat suppression may help in reducing the effects of the black boundary artifact [ figure 5 ] . fast sequences also obviate the need for long breath - holds , and thus reduce motion or respiratory artifacts . a patient with crohn disease . mri enterography examination shows good opacification of the small and large bowel with thickening of the inflamed cecal wall ( arrow ) t2w fast sequences based on rapid acquisition and relaxation , such as half - fourier single - shot turbo spin - echo ( haste ) or single - shot fast spin - echo ( ssfse ) sequences , are also used and can provide high contrast between the lumen and the bowel wall , producing images akin to a conventional barium study . fat saturation can be used to increase contrast resolution and also for better assessment of bowel enhancement . it is possible that with this technique , high - resolution three - dimensional datasets comprising isotropic voxels of the entire abdomen could be produced within a single breath - hold . three - dimensional datasets allow high - quality mpr reconstructions , which can increase diagnostic confidence . usg also allows observation of normal or abnormal bowel peristalsis , fixity , and compressibility . some studies have employed bowel distension using oral contrast ( hydrosonography ) although its use remains limited . the current - generation high - frequency probes provide greater spatial resolution than ct scan or mri [ figure 6 ] . contrast - enhanced usg can provide detailed information about vascularity and inflammation of the bowel . high - resolution usg image shows an inflamed bowel segment , with marked enhancement of the mucosal vessels ( arrowheads ) and engorgement of the penetrating blood vessels ( arrow ) the most important factor for obtaining good intestinal imaging is optimal distension of the bowel with enteral contrast . enteral contrast can be positive , neutral , or negative , depending on its density ( hu ) . positive enteral contrast agents range from a 415% contrast solution to a dilute 0.36% barium solution . the author prefers an 810% iodine concentration of water - soluble contrast because this density allows diagnostic fluoroscopic observations as well as diagnostic radiographs . neutral contrast agents can be methylcellulose , mannitol , polyethyl glycol , or other bulk fibers and have a density similar to water ( 0 hu ) . negative contrast is mainly used in the imaging of the colon [ ct colonography ( ctc ) ] , where co2 or air is insufflated per rectum . enteral contrast can be administered via the intubation method ( ct enteroclysis / mri enteroclysis ) or the enterographic method [ figure 3 ] . for enterographic examination , the patient ingests a large volume of contrast over a set period of time prior to imaging . the 1300 ml is divided into two aliquots : the first lot is ingested with a prokinetic agent ( metoclopromide ) over 25 min and the second lot is consumed over the next 25 min . just prior to the scan , 200 ml of contrast the main advantage of the enterographic examination is that it is more acceptable to patients than enteroclysis as intubation may be an unpleasant experience for patients . the disadvantage of enterograpic examination is that it may not provide uniform or adequate bowel distension as compared to enteroclysis . axial ct enteroclysis examination demonstrates a segment of kinked bowel ( arrowhead ) and several adhesive bands across other segments ( arrows ) . this patient had undergone several negative ct examinations previously water is also routinely used as a neutral enteral contrast agent at many centers ; however , its use in dedicated small - bowel studies should be discontinued . water has been found to provide inadequate distension ( at various volumes and ingestion times ) of the small bowel in many studies.[1011 ] this is because water undergoes rapid reabsorption in the bowel and fails to adequately distend the distal small - bowel segments . therefore , additives ( such as contrast , mannitol or other agents ) need to be mixed with water to provide an iso - osmolar solution that does not get reabsorbed and remains in the lumen to provide distension . the use of water as a contrast agent may lead to false - negative examinations . furthermore , collapsed loops are known to demonstrate hyperenhancement on post - contrast studies and this may lead to false - positive diagnoses . therefore , the author strongly discourages the use of water as an enteral agent in dedicated studies of the small bowel . the only instance where water may be used as an enteral agent is in the enteroclysis technique where the operator exclusively controls distension of the bowel and rate of infusion . the advantage of current - generation multidetector ct ( mdct ) scanners is increased anatomical coverage with thinner sections , which provide high - quality mutiplanar ( mpr ) images and fewer motion artifacts.[1213 ] positive - contrast ct enteroclysis ( cte ) or enterography ( cteg ) may be used mainly for the diagnostic workup of small - bowel obstruction or in suspected cases of small intestinal tumors or metastases . one of the drawbacks of positive contrast is the production of streak artifacts on mpr reconstruction , particularly on older ct scanners that can not produce isotropic voxels ( slice thickness > 2 mm ) . the advantages of neutral contrast are that it does not obscure the mucosal lining and does not produce streak artifacts . therefore , neutral - contrast cte or cteg is indicated for the workup of patients with inflammatory bowel bleed , gastrointestinal bleeding , and suspected bowel pathologies . several studies have shown that ctc has high sensitivity and specificity ( > 95% ) for the detection of polyps and cancers [ figure 4a ] . datasets of the distended colon are obtained , which can then by manipulated by software to provide virtual colonoscopy images . screenshot ( a ) from a ct colonographic examination shows a polyp in the colon . top two axial images show the location of the polyp within a marker box , whereas the bottom two are virtual 3d images in the supine and prone positions which detail polyp size and distance from the rectum . mucosal views ( b ) show a virtual dissection view of the colon with a small polyp ( arrow ) in the left image . on the right , translucency rendering shows this lesion to be homogenously dense ( red ) , implying this is retained fecal matter the absence of ionizing radiation and high - contrast resolution are the major advantages of mri over ct scan . the nonionizing aspect of mri makes it particularly suited for use in patients with ibd who may need repeated imaging . the inherent high - contrast resolution can provide high diagnostic confidence and also facilitate the detection of discriminating features of intestinal diseases . t1w and t2w imaging sequences for the imaging of the bowel have been reported in earlier studies.[101718 ] however , in recent years , advances with ultra - fast sequences based on steady - state precession have revolutionized bowel imaging . these sequences can be performed within a single breath - hold , and when combined with bowel paralysis ( using antiperistaltic agents , e.g. , buscopan ) , the technique allows rapid imaging of the small bowel . these sequences are called true fast imaging with steady - state precession ( true - fisp ) , balanced fast field - echo ( ffe ) , or fast imaging employing steady - state acquisition ( fiesta ) . these sequences are relatively insensitive to motion artifacts and provide high contrast between the bowel wall , lumen , and the mesentery . the problem with these sequences is the presence of a  black boundary  artifact along the bowel wall , which may mask small lesions or abnormalities . the addition of fat suppression may help in reducing the effects of the black boundary artifact [ figure 5 ] . fast sequences also obviate the need for long breath - holds , and thus reduce motion or respiratory artifacts . a patient with crohn disease . mri enterography examination shows good opacification of the small and large bowel with thickening of the inflamed cecal wall ( arrow ) t2w fast sequences based on rapid acquisition and relaxation , such as half - fourier single - shot turbo spin - echo ( haste ) or single - shot fast spin - echo ( ssfse ) sequences , are also used and can provide high contrast between the lumen and the bowel wall , producing images akin to a conventional barium study . fat saturation can be used to increase contrast resolution and also for better assessment of bowel enhancement . it is possible that with this technique , high - resolution three - dimensional datasets comprising isotropic voxels of the entire abdomen could be produced within a single breath - hold . three - dimensional datasets allow high - quality mpr reconstructions , which can increase diagnostic confidence . the main advantages of usg are its nonionizing character and general availability . the dynamic , real - time qualities of usg provide high temporal resolution . usg also allows observation of normal or abnormal bowel peristalsis , fixity , and compressibility . some studies have employed bowel distension using oral contrast ( hydrosonography ) although its use remains limited . the current - generation high - frequency probes provide greater spatial resolution than ct scan or mri [ figure 6 ] . contrast - enhanced usg can provide detailed information about vascularity and inflammation of the bowel . high - resolution usg image shows an inflamed bowel segment , with marked enhancement of the mucosal vessels ( arrowheads ) and engorgement of the penetrating blood vessels ( arrow ) mri fluoroscopy : changes in bowel kinetics can be evaluated on mri fluoroscopy to demonstrate either an obstructive element or abnormalities in peristalsis . mri fluoroscopy can provide functional information regarding bowel motility and may help distinguish between fibrotic strictures and functional bowel spasm . pet / ct : positron emission tomography ( pet)/ct depends on the uptake of tracer by abnormal or highly metabolic tissue . typically , cancers or metastases show up as hot spots due to their higher metabolic rate as compared to the surrounding tissues . the role of pet / ct in intestinal imaging is mainly related to cancer imaging and the detection of distant metastases . pet - ct also has a role is the detection of recurrent cancers and in quantifying malignancy in suspicious nodes [ figure 7].[2324 ] pet / ct image shows a recurrent rectal tumor as a hot spot ( arrow ) just anterior to the presacral fascia diffusion - weighted mri imaging : diffusion - weighted mri signal is derived from the motion of water molecules within cells or extracellular spaces . highly cellular tumors have restricted water diffusion and show up as areas retaining high signal intensity on high b value ( 8001000 s / mm ) images . by performing diffusion imaging using different b values , it is possible to calculate the apparent diffusion coefficient ( adc , measured in m / s ) . areas of restricted diffusion show low adc values and adc values are inversely correlated with tumor cellularity . therefore , reduction in adc has been shown to have good correlation with response to cytotoxic therapy . currently , diffusion imaging is mainly used for detection of abnormal cancerous tissue and involved lymph nodes . studies have also been conducted on the detection of active inflammation in ibd patients using diffusion imaging [ figure 8 ] . diffusion - weighted mri image shows high signal in the cancer tissue ( arrow ) and involved lymph node in the mesorectum ( short arrow ) perfusion imaging : perfusion ct is a technique that integrates anatomic detail with assessment of vascular physiology . analysis of tumor enhancement , tumor blood flow , blood volume , mean transit time , and permeability  surface area product are possible . perfusion ct is a reflection of angiogenesis in tumors , and therefore its main use is in the assessment of colorectal tumors and their response to treatment .
in recent years , advances in scanner technology and competition from other specialties have produced rapid changes in the way the intestines are imaged . 
 mri and ct scan along with the traditional enteroclysis examination have emerged at the forefront of intestinal imaging . 
 functional modalities such as diffusion and perfusion imaging are also changing the way tumors and inflammatory bowel diseases are evaluated . 
 ct colonography is now a valid alterative to optical colonoscopy . 
 contrast - enhanced usg is being used for the assessment of inflammation and post - treatment changes . in this review , 
 recent advances in intestinal imaging are described .