Check name and date
Identify bowel regions: small bowel tends to lie centrally and has valvulae conniventes (lines cross from one wall to the other); large bowel lies peripherally with haustrae (only partially cross the diameter of the bowel).
Look at the gas pattern:
• Central gas pattern: ascites
• Dilated bowel: distal obstruction or pseudo-obstruction
• Extraluminal gas: produces a double-contrast pattern in which the bowel wall is seen clearly because it is highlighted by gas on either side (Rigler’s sign)
Look for calcification:
• Renal or ureteric (best seen on KUB [kidney, ureter, bladder] film), vascular calcification, faecoliths
4.2 Contrast studies
Contrast studies are performed by instilling a radio-opaque compound into a hollow viscus or the bloodstream. Intravenous contrast is commonly used together with CT and MRI and is discussed later.
Occasionally patients can have true allergies to intravenous contrast (approximately 1 in a 1000) and intravenous contrast should be used cautiously in patients with renal impairment (see below). Contrast instilled into the lumen of an organ is not nephrotoxic.
Upper GI tract contrast studies: oral contrast agents such as barium and Gastrografin may be used as a
‘swallow’ to demonstrate tumours, anastomotic leaks, or obstruction in the oesophagus, stomach and small bowel
Bowel contrast studies: barium and Gastrografin may also be used for enema studies to identify tumours, anastomotic leaks, fistulae (‘a fistulogram’) or obstruction
Cholangiography: contrast is used during ERCP or on-table to demonstrate stones, strictures and tumours of the bile ducts and head of pancreas
Renal contrast studies: intravenous contrast is excreted by the kidneys and thus used to highlight the anatomy of the collecting ducts, ureters and bladder as an intravenous urogram (IVU). This has now largely been superseded by the use of CT. Contrast may also be instilled in a retrograde manner into the bladder to demonstrate urethral rupture after trauma
4.3 Screening studies
X-ray screening involves the production of a continuous image by an image intensifier. Bombardment with X-rays causes fluorescence of phosphor crystals within the machine translated into an image on screen.
This allows a procedure to be guided by visualising the needle tip or contrast flow in real time.
Rather than transient exposure to X-rays, screening therefore delivers a higher radiation dose to the patient and surrounding personnel must wear shielding.
4.4 Ultrasonography
In a nutshell ...
How does ultrasonography work?
Ultrasonography is the use of pulsed high-frequency sound waves that are differentially reflected by tissues of different densities.
Uses of ultrasonography
Ultrasonography can be combined with Doppler as duplex scanning (to assess blood flow) or with computer technology to form a three-dimensional image. Ultrasonography is commonly used for imaging of the abdomen, pelvis, cardiac anatomy and thorax, and vasculature. It is used for:
Diagnosis, eg liver metastasis, renal pathology, fluid collections, breast disease Monitoring, eg obstetrics, vascular graft patency
Treatment, eg guiding percutaneous procedures such as drain insertion, radiofrequency ablation
Ultrasonography does not involve exposure to radiation. It works by using pulsed high-frequency sound waves (1–5 MHz) emitted from the ultrasound probe. The sound is generated by vibration of a
piezoelectric crystal when a transient electrical field is applied to it. These pulses of sound are
differentially reflected by the planes between different tissues (eg tissue and fluid or fluid and air). The higher the density of the object, the more sound is reflected. If all the sound is reflected (eg off a calcified object) then an acoustic shadow appears beyond that object. Reflected waves are identified by the same
probe and are analysed by the machine into distance and intensity. This is then displayed as a two-dimensional image on a screen.
The incorporation of the Doppler effect with ultrasonography (called duplex scanning) has allowed assessment of blood flow in peripheral and visceral vessels. The moving blood changes the frequency of the echo reflected to the probe – creating a higher frequency if it is moving towards the probe and a lower frequency if it is moving away from the probe. How much the frequency is changed depends upon how fast the object is moving.
Recent developments in ultrasonography include three-dimensional imaging. Several two-dimensional images are acquired by moving the probes across the body surface or by rotating inserted probes. The two-dimensional scans are then combined by computer software to form three-dimensional images.
Contrast-enhanced ultrasonography uses microbubble-based contrast agents to improve the echogenicity of blood flow and allow better visualisation of vascularity.
Advantages of ultrasonography No radiation
Good visualisation of soft tissues, fluids and calculi Duplex scanning can be used to assess blood flow
Disadvantages of ultrasonography
Limited by dense structures that deflect sound waves (eg bone) Limited by body habitus (obesity)
Difficult to accurately visualise the retroperitoneum Operator-dependent
4.5 Computed tomography
In a nutshell ...
How does CT work?
CT images are created from the integration of X-ray images as the X-ray tube travels in a circle around the patient. The density of different tissues causes differential X-ray attenuation, which is recorded as an image with different levels of grey.
Image enhancement: scanning may be performed with or without IV contrast to demonstrate vessels and enhance vascular lesions
Radiation dose: CT scans represent multiple X-rays, so the cumulative radiation dose is high
CT images are created from the integration of X-ray images. Images are displayed as stacked slices of the whole (similar to slices in a loaf of bread). The X-rays are directed through the slice from multiple
orientations as the X-ray tube and detectors travel in a circle around the patient. X-rays are differentially scattered or absorbed due to the density of the different tissues. This X-ray attenuation is recorded by the X-ray detector. A specialised algorithm is then used by the computer to display the X-ray attenuation
levels on the screen as different levels of grey. These densities are different for gas, fluid and tissues and can be measured in Hounsfield units. The patient platform then moves an automated distance ready for the next image or slice.
To maximise their effectiveness in differentiating tissues while minimising patient exposure, CT scanners use a limited dose of relatively low-energy X-rays. They acquire data rapidly to minimise artefact created by movement of the patient during scanning (eg breathing, voluntary movement). In order to do this they use high-output X-ray sources and large, sensitive detectors.