The ultrasound probe is placed on the site of interest. Ultrasound jelly is used to ensure a good contact between probe and skin (or whatever the relevant surface is) which is essential for it to work. High frequency sound waves pass from the probe into the patient. Depending on the reflectivity of the tissues that the sound passes through, the sound is reflected and collected by the probe. The time to echo is one of the variables used to generate a 2d image on the screen.

How does Doppler ultrasound work and what is it used for?

This makes use of the Doppler effect. Fluid moving toward the probe increases the frequency of the reflected sound collected by the probe and fluid moving away from the probe decreases the frequency. This information is processed by the machine and is depicted in colour to the operator.

Doppler ultrasound is used for imaging flow, usually in blood vessels and the heart. A gastroenterologist might be interested in imaging portal venous flow, and Doppler is highly relevant in the transplanted liver.

What are the risks of ultrasound?

Ultrasound is relatively free of risks. There is a theoretical risk to the fetus of heating in prolonged use of Doppler in early pregnancy but this is unlikely to be of relevance to gastroenterologists. There is no radiation involved.

What are the advantages and disadvantages of ultrasound?

Ultrasound is free of risks, is acceptable to the patient. It is however very user dependent. In the abdomen, it can be of less value in patients of high BMI, and does not image the retroperitoneum very well. CT is better in these patients.

What are the uses of ultrasound that might be relevant to a gastroenterologist?

Routine abdominal ultrasound is often the first line investigation in a range of presentations, such as jaundice, weight loss, abdominal pain, suspected abdominal mass. The bowel can be imaged and inflammatory processes can be diagnosed. Hernias can also be detected with ultrasound.

What does a standard ultrasound of the abdomen include? Which organs are not well seen?

Kidneys, liver, gallbladder, biliary system, pancreas, spleen, abdominal aorta, urinary bladder.
Ultrasound is the gold standard for diagnosing gallstones, and is better than CT. The gallbladder itself is also better seen than on CT. For liver pathology, CT is generally more sensitive, but both modalities may provide information, and ultrasound can sometimes be used to characterise a lesion which is indeterminate on CT. Sometimes lesions such as liver metastases are difficult to see on a CT scan and may be better visualised at ultrasound.

The GI tract is not generally well seen as air within it is highly reflective. However see below. The adrenals are also not seen. The retroperitoneum is not well visualised and if you suspect pathology there a CT might be better.

If you ask for an US abdomen, the female reproductive organs (uterus,ovaries) are not routinely included on the study.

What are the indications for ultrasound of the bowel?

Bowel ultrasound is a specialised test which is performed by an experienced radiologist. It is a relatively new technique, as the bowel is not routinely included in an abdominal ultrasound examination owing to difficulties in assessment, largely attributable to the reflectivity of intraluminal air.

Ultrasound can be used to detect areas of inflammatory bowel disease, in which the patient will have thick walled bowel loops with increased Doppler flow (vascularity). These findings are non specific, and are shared with other bowel pathologies, but can guide the need for colonoscopy and biopsy to make a histological diagnosis. Ultrasound may have a more important role in assessing the location of active disease in the patient with known inflammatory bowel disease. It has the advantage here of being non invasive, without the risks that colonoscopy carries, particularly in actively inflamed bowel. Ultrasound can also assess for complications of IBD, such as colections.

Ultrasound (for this indication as well as others) is of limited use in obese patients, although those with inflammatory bowel disease are often thin. The colon may be less well visualised than the small bowel. Strictures are also more difficult to detect. Contrast enhanced ultrasound can increase the sensitivity of this technique for active disease.

Edited by Dr Iain Au-Yong

Gastric emptying scintigraphy of a radiolabeled solid meal is the gold standard for the diagnosis of gastroparesis.   Measurement of emptying of solids is more sensitive by scintigraphy. This is due to the fact that liquid emptying may remain normal despite advanced disease.

Consensus recommendations for a standardized gastric emptying procedure have recommended a universally acceptable 99-m technetium sulphur-colloid labelled low fat, egg-white meal. Retention of over 10% of the solid meal after 4 h is abnormal. A grading of severity based on 4 h values might be used: grade 1 (mild), 11%-20% retention at 4 h; grade 2 (moderate), 21%-35% retention at 4 h; grade 3 (severe), 36%-50% retention at 4 h.

What is gastroparesis and how is it diagnosed?

Gastroparesis is diagnosed when symptoms such as nausea, vomiting, early satiety, postprandial fullness, abdominal discomfort, and pain are associated with objective evidence of delayed GE in the absence of obstruction, and typically with impairment in maintenance of normal nutrition using standard food (see reference).

Rapid gastric transit (dumping syndrome) can also be diagnosed.

What are the potential problems with interpretation of gastric emptying scintigraphy?

As Gastric emptying scintigraphy is the gold standard, and there is difficulty in comparing it with any other investigation, the difficulty is establishing normal values in normal patients. There is obviously an ethical concern in doing the study in normal patients to establish normal values, owing to the radiation dose.

The recent consensus statement recommends imaging at 0, 1, 2 and 4 hours, although an older measure of gastric emptying was the T50, or time for the gastric activity to reach half of its original value. By standardizing the protocol (for timings of imaging and the nature of the meal) wherever it is used, it is hoped that normal values can be established and interpreted. Variations in the imaging protocol result in variations in the values obtained, caused by test rather than patient factors and decreases the reliability of the test.

Reference

Consensus Recommendations for Gastric Emptying Scintigraphy: A Joint Report of the American Neurogastroenterology and Motility Society and the Society of Nuclear Medicine

Edited by Dr Iain Au-Yong

• A blood sample is obtained from the patient. The leucocytes are separted from erythrocytes and platelets. Leucocytes  labelling is achieved by incubation with the radioisotope  in plasma or saline. The radioisotopes used are either Indium-111 tropolonate (111 In) or Technetium-99m hexamethyl propylene  amine oxime (99m Tc HMPAO). This process takes a couple of hours (thus the patient is asked to return after a couple of hours). Results from 99m Tc HMPAO  in small bowel disease are better than those obtained from 111 In
• The tagged white cells are injected intravenously in the patient.  After injection, the labelled white cells migrate into the inflammed lesion before being shed into the bowel lumen.
• Images are obtained with a gamma camera at 2-6 hrs later. A late image (24 hrs) may be required if  the technique is used to detect infection.

What are the disadvantages of the white cell scan?

• False negative scans. However, this is rarely seen in symptomatic patients with active disease. A negative scan in a symptomatic patient virtually excludes Crohn’s disease.
• Useful only for active disease
• Difficulty in identifying the various bowel segments

What is the indication for SeHCAT test?

Bile Acid Malabsorption

This can cause symptoms of chronic diarrhoea. Three types of bile acid malabsorption are recognised:
Type 1: following ileal disease or resection or bypass surgery.
Type 2: primary idiopathic malabsorption.
Type 3: associated with cholecystectomy, peptic ulcer surgery, chronic pancreatitis, coeliac disease and diabetes mellitus.
Bile acids are cathartic to colonic mucosa, and impair sodium and water absorption.

• Upper GI bleed: limited role due to the widespread use of endoscopy as a first-line modality for such bleeding.
• Lower GI bleed: Tc-99m RBC scintigraphy plays a larger role in the evaluation of lower GI bleeding due to the limited sensitivity of endoscopy within the lower GIT and has typically been used as a screening examination to identify patients who require angiography or surgery.
• Obscure GI bleed

How is red cell scan performed?

• Blood is taken from the patient. The red blood cells are separated from the rest of the blood sample and then mixed with the radioactive material. The red cells are tagged with 99m Technetium and injected back in the patient
• A second method involves injection of the radioactive material directly in the vein. This attaches to the red cells in the circulation.
• Images are subsequently obtained using the gamma camera.

What are the advantages and disadvantages of red cell scan?
Advantages

• Non-invasive
• More sensitive than angiography (detects bleeding at a rate more than 0.5ml/min). Tc-99m RBC scintigraphy is 93% sensitive and 95% specific for detecting a bleeding site with active arterial or venous bleeding rates as low as 0.04 mL/min (Zuckier LS. Semin Nucl Med 2003) anywhere within the gastrointestinal tract.
• Patients can be scanned up to 24 hrs after injection to detect intermittent bleeding.

Disadvantages

• Anatomical localisation is insufficiently accurate. Peristalsis can cause antegrade or retrograde movement of the radioisotope within the bowel, further increasing the difficulties in localisation.

Discuss the indication for Meckel’s scan?

It is done when GI bleeding is suspected to be due to a Meckel’s diverticulum. Sometimes the lining of the Meckel’s diverticulum contains ‘gastric mucosa’ instead of the normal intestinal lining and this can lead to bleeding by acid induced damage of the adjacent mucosa. When bleeding occurs within a Meckel’s diverticulum, gastric mucosa is nearly always present. A Meckel’s scan is used to look for the presence of gastric mucosa in the intestine.

What is a Meckel’s scan?

• Meckel’s scan is a technetium-99m pertechnetate scintiscan. The technetium-99m pertechnetate is injected in a vein and is taken up by gastric mucosa and hence concentrate in areas where gastric mucosa is present.
• After intravenous injection of the isotope, the gamma camera is used to scan the abdomen. This procedure usually lasts approximately 30 minutes. Gastric mucosa secretes the radioactive isotope; thus, if the diverticulum contains this ectopic tissue, it is recognized as a hot spot.
• Administration of ranitidine enhances the accuracy of the scan. Ranitidine enhances the uptake and blocks the secretion of technetium-99m pertechnetate from ectopic gastric mucosa.

What are the disadvantages of Meckel’s scan?

• Meckel’s scan is specific for gastric mucosa (i.e. in the stomach or ectopic) and not specifically diagnostic of Meckel’s diverticulum, false positive results occur whenever ectopic gastric mucosa is present.
• False negative results can occur when gastric mucosa is very slight or absent in the diverticulum
• Low sensitivity and specificity in adults

What are the indications of PTC?

Indications (PTC is done when ERCP has failed)

• Decompress obstructed biliary tree
• Dilate biliary strictures
• Remove bile duct stones, when ERCP fail
• Divert bile from bile duct leak and stent bile duct defect

Discuss the technique of PTC?

• The patient lies supine and the local area is infiltrated with local anaesthetic.
• Under fluoroscopic control, a needle is introduced into the liver. The stillette is withdrawn from the needle and a syringe containing contrast media attached, contrast media is injected under fluoroscopic control as the needle is slowly withdrawn until a duct is demonstrated. This may require several manipulations of the needle.
• Then the contrast media is injected to fill the ductal system and identify the level of obstruction.
• Following the contrast injection, the radiologist guides a small guide wire through the needle, into the ducts and across the site of blockage while watching the wire and ducts on x-ray.
• Over this wire, a small tube (catheter) is then inserted to allow the bile to be drained from the liver, relieving the jaundice caused by blockage of the duct.
• Three types of drainage procedure can be performed
  • External drainage: a percutaneous catheter is placed into the bile ducts above the lesion. The bile can then drain away into a bag outside the body
  • Internal/external drainage: a percutaneous catheter (with side holes) is placed with the tip in the duodenum
  • Internal drainage: using a totally internal prosthesis (plastic or metal stent)

External drainage is not preferred as the catheters are prone to dislodgement. Catheter dislodgement is a serious complication as this can result in bile leak and biliary peritonitis. As the biliary system is then collapsed, treatment of this complication by percutaneous reinsertion of a second catheter is difficult if the intrahepatic ducts are not dilated and ERCP may not be possible. In this situation bile will continue to leak into the peritoneum and laparotomy may be required.
However, external drainage is sometimes done when the stricture is difficult to stent.

Discuss the complications of PTC?
Sepsis, haemorrhage, Pancreatitis, peritonitis

How is MR enterography performed?

This is performed after oral ingestion of 1.5-2 litres of enteral contrast, which helps to distend the bowel.
MR enterography takes 30-40 minutes of scanning time compared to 10-20 seconds with modern CT scanners.

What are the advantages of MR enterography?

MR enterography has higher sensitivity than CT scan in detecting intestinal and extraintestinal changes in Crohn’s disease.
Other important advantages of MR compared with computed tomography include better tissue contrast and absence of exposure to radiation
The excellent tissue contrast obtained on magnetic resonance imaging can also be used to differentiate between fibrotic and acute inflammatory disease. This differentiation is clinically useful, as fibrotic disease may need surgery, whereas inflammatory disease may benefit from medical treatment.

• This is a non invasive means of imaging the colon. CTC is made possible by the availability of multislice CT scanners that are able to scan the entire abdomen and pelvis with fine slices in a single breath hold.
• CTC uses the patient data acquired from a multislice CT scanner and combines with computer software that post processes the date to generate both 2D and 3D images of the colon for analysis.  More recent software additionally allows a 3-D display with a fly-through view as well as a 360 degree virtual dissection view. There is no consensus on whether a primary review of 3-D images or 2-D-images should be preferred. Studies using the 3-D review with correlation with 2-D images had a higher sensitivity than studies using a primary 2-D approach.
• The total radiation exposure is in the range of 8.8 and 10.2 mSv (approximately similar to barium enema)
• In 1999, it was proposed that the term virtual colonoscopy be replaced by CTC and it is by this term that the technique is now known.

Discuss the need for CTC?
A non invasive way of imaging colon is helpful because:

• Colonoscopy is invasive and not without risks.
• Patient acceptability for colonoscopy is low
• 63.5- 84% of persons undergoing screening colonoscopy do not have neoplasia and therefore would not have required colonoscopy.

Thus a screening method with a high sensitivity for colorectal lesions that could preselect people with neoplasia who would then undergo colonoscopy for removal of these lesions would seem ideal. CT colonography (CTC) is considered an ideal pre-test by some experts.

Discuss the efficacy of CTC?

• Studies have consistently shown that CTC is superior to double-contrast barium enema (DCBE) in the detection of colorectal lesions
• The sensitivity is similar to colonoscopy for colorectal lesions of size 10 mm or more. The sensitivity is variable for lesions less than 10 mm in size.
• A recent meta-analysis of CTC performance showed an overall per-polyp sensitivity of 66%. Sensitivity for polyps >=10 mm was 76% with a lower sensitivity of 59% for polyps 6-9 mm. However, there was a wide variation between different studies. Overall specificity was 83%, with an increase to 92% for polyps >=10 mm.

How is CTC performed?

• Colon is cleansed using bowel preps similar to colonoscopy
• Distension of the colon is necessary to be able to assess the colon surface. This is achieved by insufflating room air or CO2 into the colon. The insufflation can be performed manually, be patient controlled or automated.
• After distension of the colon a CT scan of the abdomen is performed both in the supine and in the prone position (each lasting approximately 13 seconds). The supine and prone positions are to distinguish mobile faecal material from fixed polyps and also to shift residual fluid within the bowel lumen, which can obscure pathology.
• Buscopan may be used to reduce peristalsis during imaging.

What is faecal tagging?

• The imaging characteristic of stool is well established (central air inclusion, shift with dual positioning). However, problems still occur if the stool presents as an immobile structure or a polyp with abutting stool or frankly as a polyp. This may lead to a false positive diagnosis. Faecal tagging offers a solution to this problem.
• Because stool has the same attenuation as the colon mucosa labelling the stool with a contrast agent is required (so-called faecal tagging). This is achieved by oral intake of barium or oral contrast (sodium/meglumine diatrizoate) mostly starting 48 h before CTC. The orally ingested contrast impregnates the residual stool and so appears as hyperdense on CT. This hyperdense stool is in strong contrast to the soft tissue density of the normal colonic structures and tumoural lesions.
• Faecal tagging helps in using a reduced bowel preparation (or even no prep).
• Faecal tagging does not seem to be necessary in the case of a complete bowel preparation and scanning in the prone and supine positions.

What are the contraindications for CTC?
Absolute contraindications: Physical weight restrictions (for scanners), acute abdomen, acute diverticulitis, recent pelvic or abdominal surgery, toxic megacolon,
Relative: Pregnancy, hip joint prosthesis (patients with metallic hip have significant artefact in the pelvis with limited evaluation of colonic segments in this region), claustrophobia, incompetent ileocaecal valve (as colonic distension will be suboptimal).

What are the complications of CTC?

CTC is safe but not completely without risk. Two large retrospective reports from the UK and Israel including nearly 29 000 mostly symptomatic patients reported a perforation rate of 0.06% and 0.08%, respectively.

Other complications occasionally reported consisted of nausea and vomiting

What are the disadvantages of CTC?

• Impaired ability to detect flat neoplastic lesions- detection of flat lesions is especially important as they are more likely to contain advanced histology. The accuracy of CTC for flat lesions is unknown.
• Nearly all of the published studies CTC were performed by highly experienced academic radiologists. It is currently unknown if similar results will be achieved outside such centres.
• Radiation exposure:  The exact excess cancer risk due to the exposure to radiation by CTC is unknown. It is estimated that 1.5-2.0% of all cancers in the US might be attributable to the radiation from CT studies in general. The risk depends on age and dose of radiation used. For a 50-year-old individual one CT colonography scan was estimated to increase the life-time risk of cancer by 0.13-0.15% compared to 0.07 for a 70-year-old.
• According to mathematical models CTC currently is less cost-effective than colonoscopy. CTC will further lose effectiveness if the referral rate for colonoscopy is too high (for e.g. for polyps less than 10 mm in size).
• CTC is not suitable for mucosal diseases such as inflammatory bowel disease or angiodysplasia

What are the benefits of CTC?

• Non-invasive
• No sedation
• Short procedure
• Better tolerated by patients than barium enema. There is less movement required during the examination and no messy barium or post procedure constipation.
• Extra colonic structures are routinely assessed during CTC.

What are the current indications for CTC?
Current evidence does not seem to allow recommending widespread CTC screening outside of studies or specialised centres. CTC is the method of choice for evaluating the colon proximal of obstructing lesions and after incomplete colonoscopy.
CTC can also be used for imaging the large bowel in patients thought to be unfit for colonoscopy. In these patients prep less or limited bowel prep may be used with faecal tagging.

Ref:
Pox CP et al. Role of CT colonography in colorectal cancer screening: risks and benefits. Gut 2010 59: 692-700

• Single contrast barium enema uses barium to highlight your large intestine.
• Double contrast barium enema uses barium, but also delivers air into the colon to expand it. This allows for even better images.

DCBE is more sensitive than SCBE for detecting colorectal lesions. However, elderly and debilitated patients may be difficult to examine by the double-contrast method because of the need for more frequent movements with double contrast study. Also, detection of small colonic polyps under 1 cm in size is less crucial in elderly patients

Discuss the technique of barium enema?

• Bowel prep as for colonoscopy
• The patient lay down on back (or left lat position) on the x-ray table
• An enema tube is inserted in the rectum.
• Routine distention of the retention balloon at the tip of the enema tube is not necessary. Encouraging patients to retain the air and barium is usually sufficient. Retention balloons are inflated only in patients who are expelling air and barium from the anal canal and only after a normal distal rectum is demonstrated fluoroscopically.
• The enema tube is connected to a bag that holds the liquid barium sulphate (fine, white, odourless and non-toxic).
• The patient is turned in various positions to facilitate passage of the barium through the colon.
• A large enough volume of barium is required to coat the colon. If about one-third of the luminal diameter of distended colon is filled with barium, then enough barium has been instilled to coat the colon.
• Room air (or CO2) is gently and intermittently insufflated into the colon for DCBE
• For DCBE, the patient may be rolled 360° anywhere from one to four times, usually in partial turns. If a patient is elderly or feeble and has difficulty turning, the study should be converted to a SCBE examination. Most patients can accomplish two to three complete turns on the fluoroscopy table, which is sufficient for adequate colonic coating.
• The enema tube tip may be removed after an adequate amount of air and barium has reached the right side of the colon.
• A barium enema and the process of taking X-ray images of the bowel takes around 15 to 30 minutes to complete.

Discuss the efficacy of barium enema?
In experienced hands, the detection of cancer using DCBE techniques in symptomatic patients shows a sensitivity of 90.2% and specificity of 99.5% (Connolly et al 2002). Large polyp detection is also good with a sensitivity of 82.7% compared to colonoscopy which has detection rates of 89.9% (OTT 2000)
Discuss the complications of barium enema?
Bowel perforation 0.02-0.04% (Williams SR 1991)
Abdominal discomfort and bloating

Ref
Barium enema video
Williams SM 1991