Wk5 Clinical Imaging

Question 1

Radiography

Answer 1

• x-rays are part of the electromagnetic spectrum
• produced by bombarding a tungsten anode with electrons under high voltage. This produces heat and radiation
• In medical radiography, the tungsten anode is suspended such that the resulting beam of X-rays passes through the patient: those that emerge are picked up by a suitable detector
• historically, photographic film was used but other detectors linked to computer graphics are now used
• The film is exposed to an extent dependent upon the number of X-rays hitting the detector - this depends upon absorption of the rays by the tissues
• Techniques producing a ‘real time’ image allow moving organs to be studied
• Soft tissues are difficult to distinguish using conventional radiography
• These contrast media are usually pastes based on inorganic barium salts (often ingested orally or rectally), or organic substances containing iodine for administration into narrow bore tubes (such as blood vessels) using a catheter

Question 2

What is an angiography?

Answer 2

A very fine tube, a catheter, is inserted into an accessible vessel, and if necessary manoeuvred into another vessel, where the contrast medium is injected

an arteriogram is produced when arteries are visualised, a venogram when veins are viewed.

Bone and to a lesser extent soft tissues as well as the contrast-bearing vessels will show on a conventional arteriogram. Computer elimination of non-contrast-containing surrounding tissue using digital subtraction angiography provides a very well-defined image of the vessels
which usually appear black

Question 3

Computed tomography

Answer 3

• images are obtained from a series of angular X-ray projections. The data is processed by computer and a ‘slice’ produced of specified thickness can be acquired at less than 1 second per ‘slice’
• By convention, bone is shown at the white end of the grey-scale spectrum. All the views per slice can be viewed simultaneously, or less often used to create a three- dimensional representation on a computer screen
• The clarity of the images obtained with this technique allows discrimination between soft tissues that would not be possible using conventional radiography
• studies of the abdomen and chest usually require ingestion or injection of contrast media. Similarly, vascular structures can be identified by a thorough knowledge of anatomy, but intravenous injection of iodine containing contrast medium may be required.

Question 4

Ultrasound

Answer 4

• Sound waves travelling through a medium are partly reflected when they hit a medium of a different consistency, producing an echo. Ultrasound imaging detects and analyses these echoes.
• The time taken for the echo to return to the source of the sound is indicative of the distance the detector is from the reflecting boundary. Sophisticated computer analysis converts this pulse-echo system into a real-time two-dimensional image representing a ‘slice’ of the body sectioned in the plane of the ultrasound beam.
• High frequency sound waves (3.5 - 10 MHz) are used in medical imaging. As the beam travels through the tissues, two effects determine image production: attenuation and reflection. Attenuation is caused by the loss of energy from the system: the greater the attenuation, the lower the resultant signal intensity received by the detector.
• major advantage, however, is that there are no damaging side- effects documented at the sound energies used and is therefore used routinely to monitor the developing foetus.
• When a wave motion is radiated from a moving source, there is a change in frequency of the wave - the Doppler effect. This principle can be applied to studying moving structures - such as blood flowing through superficial vessels. Doppler probes emit ultrasound of a given frequency, and receive signals echoed back from both stationary sources and moving structures - the Doppler shifted frequencies.

Question 5

MRI

Answer 5

• relatively new, and expensive, technique based upon radio signals emitted from resonating atoms within the body
• This is used to determine the distribution and behaviour of hydrogen ions (protons) in water. The nuclear magnetic resonance effect relies on the fact that protons carry a charge and can therefore be considered as small magnets. In the absence of an external magnetic field, the magnetic moments of all these protons are randomly arranged in 3 planes, x, y and z.
• If a large magnetic field is applied across a tissue - or patient - these magnetic moments align either with or against the field lines of the magnetic field. In this way, a net magnetic vector is established. An MRI machine creates a static magnetic field around the patient who enters a huge electromagnet. A second energy field is generated (using radiofrequency energy, RF) perpendicular to the static field of the MRI machine. This flips, or rotates, the protons away from their alignment within the static field - the extent of realignment is dependent upon the quantity of RF energy absorbed. When this second field is switched off, the protons return to their static field alignment - this process is called relaxation – and, in doing so, emit the RF energy they acquired. This energy is detected, digitised, amplified and spatially encoded producing an image – this technique requires very powerful processing computers. The behaviour of the resonating protons depends upon the atoms surrounding them giving an indication as to the biochemical composition of the area investigated - fat, bone, muscle etc. Functional MRI, fMRI, is used to determine for instance blood flow through a specific region of the brain.

Question 6

Endoscopy

Answer 6

• The oesophagus, stomach and duodenum can be viewed directly by passing a flexible fibre-optic probe - an endoscope - into the upper gastrointestinal tract via the mouth.
• The mucosa can be visualised, aiding the location of a gastric or duodenal ulcer for example, and normal function viewed in situ. Similarly, the large bowel can be examined directly if the endoscope is introduced via the anus
• A related technique is that of bronchoscopy where the fibre-optic instrument enables examination of the trachea and bronchi. This method might be used to view an object inadvertently inhaled into the bronchi.

Question 7

Nuclear Imaging

Answer 7

• the emission of gamma rays by radioactive isotopes - often termed radionuclides - is assessed, producing an image which reflects the uptake of the radionuclide by a given organ system. This technique therefore produces images that reflect function as well as morphology and is very useful in diagnosis of disease
• Various radionuclides are used - most commonly technetium-99m, but also iodine-123, gallium-67 and thallium-201. These radionuclides are then coupled to various compounds in order to target them to the organ of interest. For instance, iodine- 123 injected as sodium iodide will be targeted to the thyroid gland, and technetium-99m as a phosphate will be preferentially deposited in bones.
• When the emission of radionuclides is analysed by a moving detector, the techniques is termed emission computer tomography (ECT).
• ECT is very similar to CT except that the emission of gamma rays is assessed rather than the transmission of X-rays. The detection system used converts gamma rays into light, and as technetium-99m and most of the radionuclides used give off single packets (photons) of gamma rays, the technique is termed single-photon emission (computed) tomography, SPE(C)T.