In the second part of our series about imaging, John Frank takes you through the basics of non-ionising techniques

Whereas imaging with ionising radiation uses x rays or gamma rays, non-ionising radiation uses either sound waves, in the case of ultrasound, or magnetisation of protons in magnetic resonance imaging (MRI).

Ultrasound

Ultrasound is high frequency sound waves in the megahertz range. These are produced by a special transducer, and pass into the body through what is called an acoustic coupler--a special jelly for abdominal and peripheral work, or a water bath when a structure such as the eye is being examined. The sound waves are reflected back in varying amounts from every anatomical interface and these reflected waves are detected to produce an image. Modern ultrasound machines contain a computer that generates the images and can send them either to film or a picture archiving communication system (fig 1).

Fig 1 A typical modern ultrasound machine. Relatively portable, it can be placed alongside the patient who lies on a couch or bed

Check it--and check it again

Interpreting an ultrasound

Bone and other structures containing calcium, such as gallstones, reflect all incident ultrasound waves and show up densely white with a very black area behind them, called an acoustic shadow, in which no detail can be seen. Ultrasound passes easily through liquid, such as urine in the bladder, and the liquid increases the sound waves passing through it. This acoustic enhancement increases the signal from the structures behind the liquid (fig 2).

Fig 2 An ultrasound image showing the more solid liver tissue with the fluid containing gall bladder. Reflection of ultrasound back from the interfaces in differing amounts enables good separation of the structures and so aids identification of the viscera

The investigation takes place in real time, making the interpretation much easier for the operator. The operator can also view each organ in several planes, see the flow in vessels, and "compress" structures such as the inferior vena cava. These possibilities allow the operator to build up a composite picture of the structures being studied to reach a diagnosis. Modern machines also incorporate advanced techniques, such as Doppler imaging, specifically aimed at studying flow.

The advantages of ultrasound

Ultrasound imaging is relatively easy, and the machines are portable so that they may be taken to wards and intensive care units to examine patients who are ill in bed. It can also help to guide biopsies and facilitate the insertion of drains into abscess cavities thereby avoiding surgery. Radiologists are now doing more and more drainage procedures under ultrasound guidance by the insertion of special drainage catheters through the skin.

Ultrasound has no side effects or contraindications, and patients are extremely accepting of this form of imaging. Usually patients do not need any special preparation. When imaging the pelvis, however, if the patient has a full bladder it pushes the pelvic viscera out of the bony pelvis, making them easier to study.

And the disadvantages

The main disadvantage is that sound waves cannot pass through gas, so if the bowel contains a lot of gas, images are degraded and diagnoses cannot be made. Another important limitation is that the operator can only image the abdomen and limbs. Use in the chest is limited to marking pleural effusions for draining, and looking at the heart and pericardium with special cardiac equipment.

Fig 3 MRI scan of a lateral lumbar spine shows the vertebral bodies and the discs. The central spinal fluid and the discs are white except for the L4-5 disc, which is black because it is degenerate and has prolapsed posteriorly

Main indications for using ultrasound

Ultrasound is now widely used in obstetrics, in abdominal imaging, and in vascular imaging. The technique is good for imaging the abdominal viscera and looking at blood flow and does not cause cellular damage at the intensities used for diagnosis. Obstetricians use ultrasound to monitor the development of the fetus and to detect fetal abnormalities. The main abnormalities relate to the amount of amniotic fluid, the appearances of the kidneys and heart, and structures such as the nuchal fold, which may be altered in conditions such as Down's syndrome. For such detailed and important studies, a full clinical and genetic history must be given, and the obstetrician cooperates in detail with the ultrasonographer.

The types of ultrasound

Apart from the standard two dimensional ultrasound, other forms of ultrasound exist, such as colour Doppler and three dimensional ultrasound. Doctors use colour Doppler to show blood flow in colour against the standard grey scale display of the organ, particularly when they want to know more about the organ vascularity. Three dimensional ultrasound allows accurate determination of volume, especially to visualise small lesions such as breast cancers and to delineate the edges of tumours.

Fig 4 Entrance to an dual-head gamma camera. The patient is placed into the long central tunnel on the couch. The walls are near the patient to ensure an even magnetic field

Magnetic resonance imaging

The physics of MRI is complicated, and you should consult a specialised textbook if you are interested. Basically, MRI uses a strong magnetic field to align all the hydrogen protons in the tissue. The alignment is then disrupted by a specific radio frequency energy called the Larmor frequency (see glossary). As the protons recover their alignment, they emit radio signals and these can be measured and converted by a computer using Fourier transforms to produce an image. Each tissue produces different radio signals which the computer recognises and thus the image is made up of shades of grey.

Should I use MRI or computed tomography?

The indications for MRI are similar to computed tomography (CT), but CT is better when looking at bone, and MRI is better at characterising soft tissue, especially in the brain. The images produced by MRI are exquisite, and are almost identical to a living anatomy section. MRI is supreme in neurological imaging and in showing the internal detail of structures such as the knee and spine and also in the abdomen and pelvis.

An essential consideration for any department is the relative costs of MRI and CT machines and also the running costs. A top of the range 16 multislice CT machine, capable of high speed imaging and three dimensional reconstructions, costs around £500 000 ($800 000; €700 000), and an MRI machine some 50% more, at around £800 000. The running costs and the installation problems are also greater for MRI. These machines are expensive, especially where health funding is limited, and so a broad look needs to be taken about the population to be served and the disease spread. It may be that in less developed countries, MRI is limited to the biggest hospital within the country with CT more widely available, especially as less advanced machines such as two multislice are cheaper and can deal with most clinical situations.

A series of MRI images takes much longer to produce than a CT scan, and the different MRI sequences cannot all be acquired at the same time. For this reason, in certain cases--such as chest and heart problems--CT is preferable. Despite the high speed of a multislice CT, you need to allow 15-20 minutes for each patient from start to finish, even though a complete chest scan can be done in 16 seconds. When there is a problem, you should discuss it fully with the imaging department.

Fig 3 shows an MRI image, reproduced as a single view, but a whole series of the area under investigation is produced for the radiologist to view and report on.

Disadvantages of MRI

Although MRI produces superb images, the length of time required to produce the images, the noise, and the claustrophobic atmosphere may make MRI impractical in certain situations (Figs 4 and 5). If a patient moves the image may also be degraded.

Preparing a patient for an MRI scan

There are certain points that you need to stress to the patient before a scan. Because the patient lies near the walls and the tunnel is so long patients may experience severe claustrophobia, which occurs in 15%-20% of cases. The machine is also noisy during operation, and patients may find it disconcerting. Some patients need to be examined under general anaesthetic.

Fig 5 View inside an MRI machine. At the end on the roof of the tunnel is a video camera so that the patient can be observed at all times from the control room

Due to the intense magnetic field, care must be taken that no magnetic objects are in the room, such as credit cards in the patient's pocket. The magnetic field could wipe the magnetic strip on the card. Go to www.studentbmj.com for a checklist. More importantly, radiologists need to know whether the patient has any metal clips in their body, such as clipped cerebral aneurysms, pacemakers, or artificial joints. Non-magnetic metals usually pose no problems, but steel clips, depending on their location, might well be a contraindication to using MRI. If you know of any such clips, then say so on the request form, as sometimes the patient may not be aware of them.

Radiologists also need to know that there are no tiny metal fragments in the eye. These could move under the intense magnetic field, and in extreme cases destroy the eye. Intrauterine contraceptive devices and hip replacements do not usually present a problem, although patients with hip replacements sometimes say that they feel warmth there.

Other questions in the checklist help to prevent any untoward problems arising--for example, as the scan takes a long time, diabetic patients need to know so that they do not have a hypoglycaemic reaction.

Radiologists also need to know the weight of the patient, as the machine has a weight limit, and if the patient is too heavy, then they cannot be scanned. Patient size is also important, since obese patients may not fit in the scanner. Open magnet scanners exist, which look like two hamburger buns above and below the patient, and radiologists can use this type of scanner for obese patients.

Which technique to use

Remember that you can always contact the imaging department for help and advice, and a useful booklet to consult is Making the Best Use of a Department of Clinical Radiology produced by the Royal College of Radiologists. This covers every aspect of imaging and, if adhered to, will ensure that your patients are not subjected to unnecessary radiation. In imaging, as in all specialties, do not investigate just for the sake of investigating. You must have a definite question you need to answer.

Ultrasound--high frequency sound waves emitted from a special probe; the reflections are detected by the probe and a computer then generates images in real time allowing measurements to be made

MRI (magnetic resonance imaging)--an imaging tool using a high strength magnetic field to align protons before subjecting them to tuned radio frequencies (also known as nuclear magnetic resonance imaging)

RF pulses--the portion of the electromagnetic spectrum roughly 100 kHz-300 GHz. When protons are in a magnetic field, the result is a net magnetisation in the direction of the main magnetic field. Exposure of these spins to radiation at the Larmor frequency allows this net magnetisation to be detected

Fourier transform--Fourier theory states that any function can be represented as a series of sines and cosines. Fourier transforms are mathematical techniques for converting between time and frequency domains

PACS (picture archiving communication system)--filmless system in which all images are stored on computer and are available on multiple terminals around the hospital

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