John P. Graham, MVB, MSc, DVR, MRCVS, DACVR, DECVDI
Cross sectional imaging has revolutionized the practice of medicine in the last 30 years. Both CT and MRI generate thin slice images of anatomy without the superimposition of structures which occurs with conventional radiographs. Although spatial resolution of these technologies is poorer than radiographs, images may be manipulated to enhance contrast of specific organs and structures. The result is images of extremely high quality, contrast and resolution.
Alan Carmack, a South African physicist working in the US, developed the theoretical models which are the basis of CT in the 1960s. The first practical CT system was unveiled by G N Hounsfield in 1972 at the meeting of the British Institute of Radiology. The system was capable of scanning heads only but revolutionized the study of intracranial disease. This early system was slow and crude but represented a quantum leap forward. The technology developed rapidly to result in systems which can acquire images of any body part in very short times. In the last few years, new systems which employ multiple arrays of detectors have been developed. Such systems allow an entire human to be imaged in slices less than 1 mm thick in 20-30 seconds. In the early 1990s, many observers thought that CT would be superseded by MRI but the advent of extremely fast, high resolution systems has made CT the preferred technique for many diagnoses.
A testament to the acceptance of CT in veterinary medicine is that all of the facilities which have approved radiology residency programs in North America must have on-site CT equipment. Many referral practices have on-site CT or easy local access. CT has become so widely available because of its versatility (it can be used to image almost any body system), and lower acquisition and maintenance costs in comparison to MRI.
Like a conventional x-ray machine, CT uses rays generated by a high output x-ray tube. Within the gantry, there is a detector assembly opposite the x-ray tube. The tube and detector array rotate around the patient in the most commonly used configuration. A fan-shaped beam of x-rays passes through the patient and the pattern of x-rays which reach the detectors are recorded: a 'projection'. The assembly is then rotated slightly and a new projection is generated. From dozens to hundreds of such projections are made at slightly different angles to completely encompass the patient. A computer uses complex mathematical formulae to analyse all the projection data to create an image or a slice.
The spatial resolution of CT is approximately 0.7 to 1 mm, whereas film can resolve objects of 0.1 mm without difficulty. The advantage of CT is the ability to distinguish different types of soft tissue, such as white and grey matter, or liver and gall bladder. No longer are we confined the five basic opacities of radiographs. CT achieves this degree of contrast by measuring very fine differences in the ability of tissues to stop x-rays. This is expressed as a 'Hounsfield number', for example water = 0, soft tissues = 100-200, dense bone = +500 and air = -1000. CT can distinguish differences of 4 HU, which is sufficient to detect lesions within solid parenchymal organs. CT images are digital and a computer is usually used for viewing, and the gray scale can be adjusted to highlight specific features such as bone or soft tissues. In addition, iodinated contrast agents, the same as used for myelography or intravenous pyelograms, are used by intravenous injection for CT. Lesions with abnormal circulation may show marked contrast enhancement following intravenous injection of relatively small doses of iodinated contrast media.
Disease of the nose, pharynx and ears
Pulmonary and mediastinal disease
Just about anything!
One of the major problems with CT is that image quality is degraded by dense bone such as the petrous temporal bone, which causes streak artifacts. The same problems occurs with metallic objects, such as orthopaedic implants and projectiles. However, new computer programmes have refined image reconstruction and substantially reduced these artifacts. Most CT scanner in veterinary use have minimum scan times of 1 to 2 seconds per slice so general anaesthesia is essential to minimise movement. Even so, respiratory motion is a problem in imaging the thorax and abdomen. Although veterinary radiology has tended to dismiss radiation doses to patients as inconsequential, the doses involved in CT studies can be substantial, especially as finer resolutions are achieved. And finally, although substantially cheaper than MRI, a significant cost is involved. A used CT scanner will typically cost US$100,000-150,000 and an annual service contract will cost US$30,00-60,000.
Magnetic Resonance Imaging
MRI was developed from nuclear magnetic resonance a chemical analysis technique, which used the behaviour of substances placed in a strong magnetic field to determine composition. MRI added the ability to localize components of tissue in addition to determining their chemical composition. Paul Lauterbur and Peter Mansfield working independently, in the US and UK respectively, are credited with developing MRI, for which they received the Nobel Prize in 2003. The first MR images were obtained in 1972 but the technique did not enter clinical practice until the early eighties.
Unlike CT, no ionising radiation is used. MRI uses hydrogen atoms to generate an image. Hydrogen is universally distributed in the body, principally as water atoms. So, at its most crude, MRI generates a map of the body's water content. Hydrogen atoms are basically protons and because these spin and have an electrical charge, each atom acts as a tiny bar magnet. Under normal circumstances these tiny magnets are arranged randomly. MRI uses relatively strong magnetic fields which range from 0.05 Tesla to 3.0 Tesla in clinical use, and up to 14 Tesla for research use. In a strong magnetic field a small majority of the protons will be forced to point in the direction of the field while spinning at a very specific rate. A radio signal pulse, at the same frequency as the spin of the protons, will knock them out of their equilibrium state. As the atoms return to their original state they release energy in the form of a radio signal, effectively an echo of the original pulse used to disturb the protons which is collected by the scanner and processed. Smaller gradient magnetic fields are used to localize signals from specific blocks of tissue
While CT offers good soft tissue detail, the contrast seen with MRI is superb. Different sequences of radio pulses can be used to emphasise different tissue characteristics. Some of the more commonly employed imaging techniques have the following characteristics:
T1 weighted--fluid = black (hypointense), soft tissues = grey, fat = white (hyperintense)
T2 weighted--fluid = white, fat = grey, muscle = black
Bone, ligaments and tendons appears black on all image sequences as they very little water content and therefore very little hydrogen to generate signal.
FLAIR sequences suppress signal from simple fluids such as CSF
STIR sequences suppress signal from fat
Like CT, MRI uses contrast agents which enhance lesion conspicuity. However, in the case of MRI, the agents are based on gadolinium, which alters the local magnetic field and changes signal intensity. Lesions which accumulate gadolinium appear bright (hyperintense). The spatial resolution MRI is comparable to CT at approximately 1 mm, although the excellent contrast resolution creates the impression of much finer detail. Unlike CT, which is limited to images in the plane of the gantry, images can be obtained in any plane, and so slices can be varied infinitely to highlight lesions. In recent years, the use of faster computers has allowed the development of fast dynamic acquisition techniques can be used to evaluate function in addition to anatomy, such as function MRI which can map activity in the cerebral cortex as specific actions are performed. Another interesting development is the use of MR spectroscopy. This technique harks back to the original use of this technique as a chemical analytical tool. The chemical content of a specific volume of tissue can be analysed qualitatively and quantitatively for specific components. Research in humans indicates some tumours have specific chemical signatures and this techniques promises the option of noninvasive but tissue specific diagnoses.
MRI poses no radiation hazard but the strong magnetic fields mean ferromagnetic objects cannot be used in the scan room--oxygen tanks, surgical implants and haemostats may all become projectiles. Pacemakers may be stopped and artificial heart valves may be locked open or closed. The rapidly switching magnetic fields can induce currents in metal such as external fixators and cause serious burns. Credit cards are instantaneously scrubbed. Analogue watches may be stopped or suffer magnetization and become quite inaccurate. MRI uses radio waves which are within the normal spectrum of broadcast and so the scan room must be shielded from all radio waves which is quite expensive. The operating costs of MRI units are very high. Superconducting magnets require liquid helium and nitrogen to maintain a temperature just above absolute zero and minimize electrical resistance. Similar to CT, acquisition costs are high, but a number of companies are offering 'fee per scan' lease contracts which reduce the capital outlay. Lower field strength units (0.15-0.4T) are less expensive to acquire, install and maintain and are becoming more common in veterinary referral clinics. As with CT, anaesthesia is required but we still encounter movement problems as data is collected from block of tissue rather than a slice, and acquisitions may take several minutes.