Introduction to imaging:The abdomen

In the fifth part of our series, John Frank takes you through techniques for imaging the abdomen

Endoscopy and radiology in gastrointestinal disease

You can study the gastrointestinal tract in a variety of ways. The bowel may be studied using x rays or endoscopically. A serious disadvantage of endoscopy is that it carries a risk of perforation of the bowel, which must be explained to the patient. You should do endoscopy under sedation, which has attached risks. Also, when studying the colon endoscopically, you may not be able to get the endoscope all the way around to the caecum, so the examination is incomplete.

The main indication for endoscopy of the upper gastrointestinal tract is the suspicion of stomach pathology, which may be benign or malignant, and the procedure allows you to do biopsies or resections. But radiology allows visualisation of the whole small bowel. This may well be useful where small bowel pathology, such as malabsorption, is a problem. Pathology of the lower gastrointestinal tract may manifest as bleeding, pain, or obstruction, and you can visualise the colon either radiologically, using a barium enema, or endoscopically (fig 1).

Fig 1 (above) Image of the whole bowel got by giving the patient barium sulphate liquid to drink, and taking images after varying lengths of time. You can clearly see the stomach and the whole small bowel, with some filling of the caecum and proximal colon

You need to study the colon separately either using colonoscopy or a double contrast barium enema. In this procedure, you put barium into the colon through the rectum, and then puff air in to coat the bowel wall with the barium. You can then take several images, showing the bowel wall and any pathology (fig 2).

Fig 2 (below) Images taken with the patient lying first on one side and then on the other showing the whole of the large bowel in double contrast

Nuclear medicine in gastrointestinal disease

Doctors can use nuclear medicine techniques to image the bowel, either in cases of inflammatory bowel disease, or in cases of blood loss from a suspected Meckel's diverticulum. This is an outpouching from the distal ileum, which may contain functioning gastric mucosa. You can also do gastric emptying studies using labelled food and drink.

In cases of inflammatory bowel disease, the patient's own white cells are taken and labelled with a special compound of technetium-99 in the laboratory and are then reinjected into the patient. Images are taken at one and three hours after reinjection. Where there is active inflammatory bowel disease, the labelled white cells accumulate, and can be imaged (fig 3).

Fig 3 This image of the colon taken using a 99Tc labelled compound shows extensive activity within the transverse and descending colon in a patient with active Crohn's disease. This is of greater value than a barium enema as it shows active bowel disease

In cases of blood loss with lower abdominal pain, if all other more common conditions such as tumours and diverticular disease are excluded, the cause may be a Meckel's diverticulum. If it contains functioning gastric mucosa, it may bleed. However, if the diverticulum does not contain gastric mucosa, then it will not bleed. Gastric mucosa takes up ⁹⁹Tc (as pertechnetate) and so can be imaged (fig 4).

Fig 4 (left) Intense uptake in the stomach, lying across the upper abdomen, and the bladder at the bottom of the image, with a small focus of activity lying some way superior to the bladder. This is a Meckel's diverticulum with functioning gastric mucosa

Gastric emptying studies and studies of colonic transit time are useful in cases of unexplained discomfort and constipation. In gastric emptying studies, the patient eats and drinks food labelled with isotopes, and images of the stomach are taken over a two hour period. Regions of interest are drawn around the stomach, and the computer draws time-activity curves, which are compared with a normal database, and give a value for the time taken for the stomach to empty. It is usually expressed as the half life (t_1/2).

Colonic transit studies involve the patient swallowing radio-opaque shapes on three consecutive days, and x rays of the abdomen are obtained three days after the last ingestion. The number of shapes seen give an indication of the colonic transit time.

Ultrasound in gastrointestinal disease

You can see ascites easily in ultrasound images. The free fluid shows up as black and it surrounds the viscera. Plain abdominal x ray images also show ascites, which appear as an overall greyness. All the landmarks, such as psoas, are obliterated by the fluid within the abdominal cavity which fills in the paracolic gutters, and as the bowel contains gas, it floats on top of the fluid (fig 5).

Fig 5 Ultrasound image showing the liver with the ascitic fluid above it

Liver, gall bladder, and spleen imaging

You can see these organs best using ultrasound, which has the added advantage of not using ionising radiation (fig 6). You can see any liver pathology such as metastases or abscesses. However, you can also use computed tomography or magnetic resonance imaging for this (fig 7).

Fig 6 Ultrasound image showing several circular white areas--secondary cancer deposits--within the darker speckled liver


Fig 7 (above) Secondary cancer lesions are also well shown on computed tomographs. This example shows a large number of dark circular lesions in all parts of the liver. You can also see the spleen has no deposits within it

The best way to view the gall bladder is by using ultrasound. This shows up as black because it is filled with fluid (fig 8).

Fig 8 (below) Ultrasound scans of a normal and a gall bladder containing a large gall stone

Although you can see the pancreas on ultrasound, the best way to view the pancreatic duct is by using contrast, either injected via an endoscope (endoscopic retrograde choledochopancreatography) after which the pancreas is x rayed to get images or using magnetic resonance cholangiopancreatography (fig 9). This enables you to see any blockages in the ducts.

Fig 9 (left) Specially processed magnetic resonance cholangiopancreatograph, showing the fluid in the gall bladder, the cystic duct, the pancreatic duct, stomach, and duodenum as white

You can see the spleen on ultrasound, but in cases of splenic trauma or after splenectomy, it is often necessary to know if any functioning splenic tissue is still present. If there is, then it may be possible to stop the prophylactic antibiotics given in such cases to protect the patient, because in young patients with no splenic function, risk of septicaemia may be increased. To view splenic tissue, you can obtain nuclear medicine images, which use either denatured red cells (which accumulate in the spleen) labelled with ⁹⁹Tc or with a colloid labelled with ⁹⁹Tc which shows the liver and spleen as the colloid is taken up by the reticuloendothelial system (Kupfer cells) (fig 10).

Fig 10 (below) Image got using 99Tc labelled denatured red blood cells of a young man after splenectomy for a major car crash. The bottom half of the spleen is present and is functioning

Renal disease

The kidneys may become infected or obstructed, and you can investigate them with contrast x ray studies (intravenous excretory urography), computed tomo- graphy, ultrasound, or nuclear medicine (fig 11).

Fig 11 (above) Normal x ray film taken 15 minutes after the injection of iodine containing contrast, showing the kidneys and bladder

A common presentation of renal disease is loin pain and haematuria, and if the symptoms suggest renal colic (intense unremitting pain extending from the loin down to the pudenda with haematuria, either visible or on dipstick testing), then intravenous excretory urography is often requested. This investigation may show the obstruction to flow of contrast, caused by the stone in the ureter (fig 12).

Fig 12 (above) Abdominal x ray images of a patient with renal colic. Taken at 15 minutes after injection (left), showing faint but normal excretion on the left, with a dense renal outline on the right. Taken 1.5 hours (right) after injection, shows a dilated system on the right, to the level of the lower ureter. This is due to obstruction, caused by a small calculus. The bladder is normal, but indented by a bulky uterus, which contains an interuterine contraceptive device

You can also use ultrasound or computed tomography in cases of renal colic. Ultrasound can show the dilated renal pelvis, and if the stone is in the upper ureter, this may also sometimes be visualised (fig 13). Computed tomography without any intravenous contrast is quicker than a conventional intravenous excretory urography and is often better tolerated by the patient, who may be in severe pain.

Fig 13 (below) Ultrasound image showing dilated renal pelvis (P) with normal kidney (K) in a patient who presented with acute right loin pain. The intravenous excretory urograph taken at the same time confirms the dilated renal pelvis, which is accentuated in this image because the patient was given an intravenous diuretic (frusemide) during the urography

The computed tomography intravenous excretory urography starts with a whole body survey, which may or may not show the stone depending on its size. However, the series of images without the use of intravascular contrast, will show the stone most clearly (fig 14).

Fig 14 (left) The stone shows in the left kidney as a dense white opacity (arrowed)

There are two types of nuclear medicine renogram--static and dynamic. The static scan is called a DMSA (2,3-dimercaptosuccinic acid) scan, is done using DMSA labelled with ⁹⁹Tc. The images are taken one and a half to two hours after the injection of the radiopharmaceutical, and single images are got. These show where there is functioning renal tissue, and so the technique is mainly used to show renal scars, especially in children after urinary tract infections. This is important as scarring in paediatric renal disease is associated with the development of hypertension later in adult life. However, the scan is really only helpful if it is done at least six months after the last documented infection, so the child needs to be on low dose antibiotics the whole time (fig 15).

Fig 15 (right) Normal uptake in the right kidney with obvious defects in the left

A dynamic scan, usually done with ⁹⁹Tc labelled MAG3 (mercapto-acetyl-glycyl- glycyl-glycine), is analogous to intravenous excretory urography. To do this type of scan, the patient either sits or lies in front of a gamma camera, and with the acquisition program running, the radiopharmaceutical is injected. The program collects the data in a series of frames, which are then processed by the computer to draw time-activity graphs of excretion by the kidneys, and so show obstruction (fig 16).

Fig 16 (below) Normal set of renogram curves. Flow of isotope through the heart and into the kidneys (above) and excretion by the kidneys (below). In a case of obstruction, the curves are quite different--you can see the normal curve on the right, but the continuously rising curve on the left indicates obstruction

Vascular system

You can visualise the aorta and visceral vessels by arteriography. The doctor inserts a catheter into the aorta by direct femoral puncture and injects contrast to outline the chosen vessels (fig 17). This is a specialised part of radiology, as doctors can obtain both diagnostic information and do therapeutic procedures. These must be discussed with the interventional radiologist early in the management of the case.

Fig 17 (left) Catheter in the aorta, the aorta, and the vessels arising from it

Intra-abdominal malignancy

Doctors often image generalised malignancy with computed tomography or magnetic resonance imaging, and a useful nuclear medicine test which complements these is positron emission tomography. The computed tomograph or magnetic resonance image can show enlarged nodes or other abnormalities, but the positron emission tomograph will show whether these are metabolically active (fig 18).

Fig 18 (right) Positron emission tomograph showing a lesion in the right lung, with areas of abnormal uptake in the right hilum, the right iliac region, and the right groin. These are due to a primary lung cancer with metastatic spread. The advantage of positron emission tomography is that you can image the whole body. This often shows disease in areas where no disease was suspected

The most modern scanners for positron emission tomography are combined with computed tomography to give a fused image. In this technique, the computer superimposes the positron emission tomograph on the computed tomograph. This allows the radiologist to see which area of abnormal tissue on the computed tomograph is metabolically active, which often indicates tumour (fig 19).

Fig 19 (below) Computed tomography slice of the pelvis (left) shows a mass posterolateral to the bladder on the right (arrow). Fused positron emission tomograph-computed tomograph (right) shows that the lesion is yellow and therefore metabolically active

Conclusion

The abdominal contents can be imaged in many ways. Before embarking on a long and expensive set of investigations, discuss the problems with the imaging department so that the correct tests can be done in the correct order. As before, I cannot stress too much the importance of the booklet Making the Best Use of a Department of Clinical Radiology.¹

John Frank consultant in nuclear medicine and radiology, Charing Cross Hospital, London W6 8RF
Email: jfrank@hhnt.org

I thank the Alliance Medical Imaging Centre, London, for doing the positron emission tomography-computed tomograph, Dave Towey for help in reproducing the images for this article, and Bill Svensson for reviewing this article.

studentBMJ 2004;12:45-88 February ISSN 0966-6494

1. Making the best use of a department of clinical radiology. 5th ed. London: Royal College of Radiologists, 2003.