Diagnostic procedure
Equivalent number of chest X-rays
Approximate equivalent period of natural background radiation
Chest X-ray
1
3 days
Hip X-ray
20
2 months
Abdominal X-ray
35
4 months
CT head
100
10 months
CT chest
400
3.6 years
CT abdomen
500
4.5 years
The framework governing the diagnostic use of ionising radiation is the Ionising Radiation Medical Exposure Regulations (IRMER), and there is no upper dose limit, provided the perceived benefit outweighs the risk.
Magnetic Resonance Imaging (MRI)
Magnetic fields, typically over 30,000 times the strength of Earth’s magnetic field, are used to align the hydrogen atoms in the body along the same axis, and images are constructed via algorithms that measure signals when the protons relax. MRI provides unparalleled soft tissue definition of structures beyond accurate reach of ultrasound (US). It has a lower spatial resolution than CT but can resolve adjacent structures of similar density better than CT, where there is a difference in hydrogen atom relaxation.
MRI Safety
the magnetic fields applied are not known to cause harm to biological tissues, including in pregnancy, but may displace or disrupt MRI incompatible devices or foreign bodies. An MRI safety checklist provides guidance on MRI suitability and the information required includes model numbers of the in-situ devices, date of insertion and present position. Magnetic objects brought into proximity of the magnet become lethal projectiles and there are strict rules on entering the safety zone.
Ultrasound (US)
Current is passed through piezoelectric crystals in a transducer, causing vibration and emission of sound waves, which travel through soft tissues. The waves are reflected or refracted to various degrees, and return to the transducer to vibrate the crystals to generate an electric current for conversion into an image. US offers dynamic imaging in any plane, with or without contrast, and measurement of tissue stiffness or compressibility. US may resolve infinitely small structures, provided they have different acoustic impedance to the adjacent soft tissue. US is often used to measure flow within blood vessels.
Ultrasound Safety
risks are practically negligible, although there is slow tissue heating if the probe is held over the same area for a very long time.
Contrast Media
A contrast medium is used to enhance the differences between similar tissues and to characterize abnormalities. The classification slightly varies between study types. For X-rays, contrast media are positive (iodine and barium), or negative (air and carbon dioxide). For MRI, they are paramagnetic or superparamagnetic, but other means are also used to examine the bowel or the vagina. In ultrasound, the contrast is micro-bubbles. For most contrast media, there are various routes and timings of administration, indications and contraindications, and alternatives. The radiology department provides guidance and protocols for each study for diagnostic accuracy and patient safety. Of note, iodine contrast media are nephrotoxic and contraindicated in patients with nephropathy. MRI contrast media are also excreted via the kidneys. However, MRI and US with contrast are alternatives to CT in patients allergic to iodine.
A favourite surgical examination question is regarding the type of contrast media administered enterally for upper and lower GI tract fluoroscopy studies. Broadly speaking, there are barium sulphate and iodine based water-soluble contrast media. Barium provides superior mucosal coating, but causes peritonitis if it extravasates into the peritoneum and should not be used for assessment of a bowel anastomosis or in a patient who may undergo abdominal surgery. Iodine based water soluble contrast media are safe in the peritoneum. On the other hand, iso- or hypoosmolar iodine-based water-soluble media are relatively safe if extravasated into the lung or mediastinum, while hyperosmolar media draw in water and cause pulmonary oedema, and barium may cause pneumonitis.
Core Imaging: Chest X-Ray
Appearance on Film
Black – air/gas
Dark grey – fat
Light grey – soft tissue/fluid
White – bone, calcified structures
Bright white – metal
Interpretation of Film
When confronted with a complicated chest X-ray, it is easy to flounder. Having a system is important as it provides something to fall back on. There is a very easy system to remember when reporting chest X-rays: RIP…ABCDE!
Firstly, is the radiograph of diagnostic quality? This is referred to as the film “adequacy” and is comprised of three parts:
Rotation
Are the heads of the clavicles symmetrical either side of the manubrium?
Inspiration
Ribs: 2 components of the ribs on PA Film are seen.
Posterior ribs (easier to see and orientated horizontally: 9–10 normally visible)
Anterior ribs (harder to see and orientated at a 45 degree angle: 5 normally visible)
Penetration
One should just about be able to define the lateral borders of the vertebral bodies behind the sternum.
At this point also assess for any man-made objects in situ and the appropriateness of their positioning.
ET tube: >4 cm (ideally 5 cm) above the carina, as the ET tube can descend upon neck flexion
Chest drain: note where the tip of the drain lies
NG tube: the tube should remain central throughout its course in the mediastinum, the tip lying below the level of the left hemi diaphragm.
Central lines: tips should be ‘projected’ over the SVC
Valve replacements
Mediastinal drains
Pacemakers: number of leads and any breaks in the leads
Sternotomy wires: any breaks and whether well aligned
Secondly, the ABCDE system can be used to examine all necessary structures on the radiograph.
Airways
Visible on X-ray: trachea, right and left main bronchus.
Remember that the right main bronchus is more vertical and wider than the left, meaning aspirated objects (including NG tubes) often pass into the right.
Tracheal deviation: unequal intra-thoracic pressure between right and left sides.
Deviation away from affected side: pneumothorax, pleural effusion, large mass
Deviation towards affected side: collapse, lobectomy, pulmonary fibrosis
Bones
4 bones easily visualized: ribs, clavicle, sternum and vertebral bodies.
Look for: fractures and increased/decreased density, which may suggest metastatic disease.
Cardiac Silhouette
Cardiothoracic (CT) ratio = max horizontal cardiac width/max horizontal thoracic width.
Cardiomegaly (>50 % CT ratio) may be due to heart failure, pleural effusion, hypertrophy (aortic stenosis, HTN) or dilatation (myocardial damage).
Remember heart size is accurate on PA film, but not AP film. Mediastinum may be widened (>8 cm) due to poor technical quality.
Diaphragm
The right hemi-diaphragm should be higher than the left due to the liver beneath it. However it should not be >5 cm higher. If diaphragmatic paralysis is suspected, phrenic nerve palsy secondary to mediastinal malignancy should be ruled out.
Causes of elevated hemi-diaphragm: diminished lung volume, phrenic nerve palsy and hepato/splenomegaly.
Small pocket of air visible under left hemi-diaphragm represents air in the stomach (‘gastric air bubble’).
Everything Else: “Review Areas”
Look for: meniscus sign (pleural effusion), changes behind the heart, hilar enlargement (malignancy, infection, sarcoidosis, pulmonary HTN), changes below the diaphragms, sternoclavicular joints, rib deposits, Riggler’s sign (see later), shoulder fractures, and mastectomy.
Core Imaging: Abdominal X-Ray
A standard abdominal radiograph (AXR) is a supine projection with the patient lying down on his or her back. In some circumstances, an erect AXR is requested; its advantage over a supine film is the visualization of air-fluid levels.
Interpretation of Film
Again, it is important that the technical details of an AXR are assessed. The date the film was taken and the name, age, and sex of the patient are all worth noting. Next ask what type of AXR it is: supine, erect, or decubitus?
The following system can then be used to examine the radiograph.
Intra-Luminal Gas
Look at the amount and distribution of gas in the bowels (intraluminal gas).
Work backwards: start by looking to see if there is any gas in the rectum and then work proximally to where you would expect to see large bowel and then small bowel.
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