Abdominal Imaging Flashcards
Types of imaging modalities
Plain abdominal x-ray
Computed tomography (CT)
Contrast
Ultrasound
Endoscopy
Laparoscopy
Common uses of plain abdominal x-ray (AXR)
Looking for foreign bodies and dilated loops of bowel (sign of intestinal obstruction)
Ionising radiation plain abdominal x-ray
It exposes the patient to the equivalent of approximately 4 months of background radiation
Plain abdominal x-ray
Limited use as the anatomy is more complex than thoracic anatomy and most abdominal tissues are of similar density so differentiation of different structures is very difficult
Uses of computed tomography CT
Widely used in abdominal imaging for many pathologies
Very often used with a contrast agent
Ionising radiation and CT
Frequently used
much greater dose of ionising radiation
Exposes the patient to the equivalent of approximately 4.5 years of background radiation
Computed tomography CT
Created by directing a very large dose of x-rays at the patient from various angles and reproducing the image digitally producing 3D images
Viewers need to scroll through the series of images to see each part of the abdomen
How to identify small intestine on x-ray
Lines going across the tube - plicae circularis
Width of normal small intestine on x-ray
Up to 3cm
Width of normal large intestine on x-ray
Up to 6cm
How to identify large intestine on x-ray
Haustral folds across tube
Largest part of large intestine
Caecum
Width of normal caecum on x-ray
Up to 9cm
Uses of contrast
Barium swallow
Barium enema
CT abdomen with contrast
CT aortogram
Trauma CT
Examples of contrast agents
Iodine
Barium (for x-rays and CT)
Gadolinium (for MRI)
What contrast is used for x-rays or CT
Barium
Which contrast is used for MRI
Gadolinium
Contrast agents
Radio-opaque liquids that are used to increase differentiation between different tissues
Can be swallowed or inserted into the GI tract via the rectum before x-rays or CT scans to visualise the GI tract
It is not absorbed so is later excreted via the anus
Axial CT image
Standing at feet and looking up patient
Administration of contrast
• Contrast agents can be swallowed or inserted into the GI tract via the rectum before X-rays or CT scans to visualise the GI tract. The contrast is not absorbed so is later excreted via the anus.
• Contrast agents can also be injected into the veins or arteries during X-rays, CTs or MRIs to visualise the blood vessels. This is particularly useful as it can help radiologists to identify inflammation, cancer, blockage of a blood vessel and bleeding, amongst other things. The contrast is then filtered out of the blood by the kidneys and excreted in the urine.
When is contrast injected into vein or arteries
Identification of inflammation, cancer, blockage of a blood vessel and bleeding
Uses of ultrasound
liver, gallbladder, biliary tree, aorta, kidneys, bladder volume, vascular access, free fluid in the abdomen, pregnancy/foetal scans, uterus, ovaries, testicles.
Why are ultrasound not as useful as CT
Produce a 2D image which is less detailed
Ultrasound
uses sound waves of a higher frequency than we can hear which are directed from a probe towards the target structure, reflected off it and received back at the probe. The time taken and amplitude of the rebound wave are detected and used to calculate the densities and depths of the structures. Images are viewed ‘real-time’ on a screen and the probe can be placed wherever necessary to view the desired organ.
Are ultrasounds harmful
non-ionising so theoretically cause no harm to the patient.
Performing an ultrasound
relatively difficult to perform and interpret so specialist training is required, but non-radiologist clinicians can also be trained to use ultrasound such as in the emergency department and critical care.
Endoscopy
• Very frequently used to directly visualise the lumen of hollow organs using a camera.
• Non-ionising because it’s just a camera, but patients may require sedation for the procedure, and there are risks of perforation of stomach or intestine.
Uses of endoscopy
visualise the lumen and walls of the oesophagus, stomach, small and large intestine (e.g. looking for a source of bleeding or cancer, or removing foreign bodies)
Capsule endoscopy
sometimes used where a camera the size of a pill is swallowed. It photographs the GI tract as it passes through and is retrieved after it is excreted.
Laparoscopy
• ‘Keyhole surgery’ - involves making small finger-sized holes in the anterior abdominal wall and inserting cameras to visualise the peritoneal cavity. Can be used just to have a look, but often progresses to actual surgery if something needs removing or cauterising
Advantages of abdominal x-ray (AXR)
• Relatively quick and easy to acquire
• Lower radiation dose than CT
• Can be interpreted by non-radiologists
Disadvantages of abdominal x-ray (AXR)
• Poor differentiation between structures so only useful for a few clinical presentations
• Ionising radiation
Advantages of CT
• Relatively quick and easy to acquire
• Very good differentiation of structures (particularly when used with contrast)
Disadvantages of CT
• High ionising radiation dose
• Patients need to lie still for the scan
• Requires a radiologist to formally interpret images
Advantages of contrast
• Used during other imaging methods to greatly increase differentiation of tissues and assess for other pathology that may not be visible otherwise (see above)
Disadvantages of contrast
• IV contrast is potentially associated with ‘Contrast-Induced Nephropathy’ (damage to the kidneys as they try to filter the contrast out of the blood)
• Some patients can have an allergic reaction to contrast agents
Advantages of ultrasound
• Logistically very easy to acquire images as US machines can be brought to the patient’s bed side.
• Non-ionising
• Can be repeated to assess for change
• Multiple uses (see above)
• Non-radiologists can be trained to perform certain more simplistic scans and interpret images
Disadvantages of ultrasound
• Formal ultrasound scans by trained ultrasonographers are often not available ‘out of hours’ in most hospitals.
• Images are 2-dimensional and only of small areas of the body, so utility of images is limited to answering very specific clinical questions
Advantages of endoscopy
• Non-ionising
• Direct visualisation images
• Ability to take biopsies or perform certain procedures during endoscopy
Disadvantages of endoscopy
• Uncomfortable procedure, so patients may require sedation (which has its own risks)
• Risk of perforation of hollow organs
Advantages of laparoscopy
• Non-ionising
• Direct visualisation images
• Ability to perform surgical procedures during laparoscopy
Disadvantages of laparoscopy
• Patients require general anaesthesia (which has its own significant risks)
• Risk of surgical complications (e.g. bleeding, infection, damage to organs, herniation, wound breakdown etc.)
Basic abdominal anatomy CT
CT scans produce a series of images, not a single image like a plain X-ray, so you may need to scroll through the images and view different planes to see different organs.
Basic abdominal anatomy
• Small bowel is often located in the centre of the abdomen and has numerous plicae circulares which span the full width of the lumen. Large bowel is often located around the outside and has haustra which are further apart and don’t span the full width of the lumen.
• ‘3, 6, 9 rule’ - the maximum ‘normal’ diameter of the small intestine is roughly 3cm, large intestine is 6cm and caecum is 9cm. Any larger than this suggests dilated bowel and possibly obstruction.
