Introduction

Both CT (computed X-ray tomography) and MRI (magnetic resonance imaging) are used to acquire images that represent thin slices through the body. They are techniques that rely on computers to process electronic signals that can then be displayed as two-dimensional (2-D) images or reconstructed into three-dimensional (3-D) images.

The selection of which imaging modality to use will depend on the questions posed by the clinician. Both CT and MRI provide greater anatomical and pathological detail than conventional radiographs but are both more expensive ways of diagnosing conditions when other modalities such as ultrasonography and radiography can be used.

Things to be considered when deciding which imaging modality to choose are:

 Soft tissue information required

 Bone information required

 Potential value of 3-D reconstructions

 Safety aspects

 Money issues

 Availability of equipment

 Condition of the animal (sedation versus general anaesthesia)

Computed Tomography

CT scanning is a British invention, invented and developed by Godfrey Hounsfield in the early 1970s. CT involves the use of X-rays to produce images. The subject is irradiated using a high-energy, fan-shaped circulating X-ray beam with detectors positioned in a ring around the patient to capture transmitted photons. Grey shades in CT reflect tissue attenuation which is similar to conventional radiography--bone and calcified material appear bright, and gas dark.

The patient lies on a table which advances in small increments between each exposure producing multiple images (slices) of the area of interest. Because of the way the scanner is constructed, only true images can be produced in a plane transverse to the position of the patient lying on the table. Therefore sagittal and dorsal plane images can be produced by reconstruction and reformatting, but the resultant images will have poorer resolution.

CT has a short, wide gantry which allows easy access to the patient which can be useful for scanning critical patients when the patient needs almost constant monitoring.

CT is particularly good for lesions affecting bones, e.g., nasal conditions, thoracic wall conditions, fractures and suspected discospondylitis. Bony changes are seen very well on CT but soft tissue differentiation is not as good as in MR imaging. In particular this applies to the spinal cord, although fat surrounding the spinal cord does provide some contrast.

Magnetic Resonance Imaging

MRI was discovered by Felix Bloch and Edward Purcell in 1952. In the 1970s the technique was modified to allow the production of 2-D, cross-sectional images of the body.

MRI involves placing the subject in a region of high magnetic flux, applying periodic magnetic pulses to change orientation of atomic nuclei within the subject and collecting radio waves that are emitted when the nuclei relax. MRI is particularly good for lesions affecting the brain, spinal cord or intervertebral discs. Grey shades in MRI reflect the relaxation times of tissues, which are influenced by a variety of physiochemical properties.

An MRI scanner is a long, cylindrical magnet into which the patient is moved on a sliding table. Scanning times are much longer than CT so general anaesthesia of veterinary patients is necessary as the patient must be absolutely still.

The magnetic field of the scanner means that no ferrous metallic objects can be brought in to the vicinity of the scanner. Anaesthetic equipment and monitoring equipment must be specially designed to be MRI compatible. Careful consideration of MRI scanning should be applied when dealing with patients who have had pacemakers or implants for fracture repair. Not all metals adversely affect MRI; titanium, for instance, does not.

MRI images can be obtained in transverse, sagittal and dorsal planes with equally good definition and soft tissue detail. This is one advantage of MRI compared to CT. Because of this it has been suggested that MRI is superior to CT.

MRI can replace myelography as cerebrospinal fluid can be seen separate from the spinal cord on MRI images, making it much less invasive and safer than radiography, with the added benefit of better definition and images in other planes. MRI provides better evaluation of extradural, intradural, extramedullary and intradural intramedullary lesions.

Choice of Imaging Technique

To decide whether CT or MRI is likely to be superior for a specific patient, we need to look at the advantages and disadvantages of both. Both can be used to evaluate joints and periarticular tissues.

 What disease process is suspected--e.g., a disease process arising from bone would be better visualised on CT.

 MRI has superior differentiation of soft tissue including cartilage. It has a greater application in imaging intra- and periarticular soft tissues such as ligaments, tendons, menisci and articular cartilage. CT however is usually used for evaluation of mineralised components of joints.

 CT acquires images in a plane parallel to the gantry.

 MRI can acquire images in any plane required.

Practices with a large neurology caseload may therefore consider purchasing an MRI scanner rather than a CT scanner. Conversely practices with a large thoracic or orthopaedic caseload may feel they will benefit more from purchasing a CT scanner.

Conclusion

Both CT and MRI are expensive machines to buy, install and run. Their accessibility therefore may be an issue when deciding which to use for diagnosis. Issues of health and safety should also be considered when using CT (radiation) and MRI (making sure there are no metallic implants or metal within the magnetic fields). Scan time can also vary. CT scan time may be seconds, allowing the use of sedation, whereas MRI scan time is longer resulting in the need for anaesthesia.

MRI and CT, although both providing cross-sectional images, are very different in their acquisition of data and in the images produced. Bearing in mind their individuality it would be difficult to say that one is superior. They each have applications complementary to each other, although there is some overlap. For example, in central nervous system disease MRI would be the primary choice but CT would be fine for large intracranial masses. Although MRI gives better soft tissue differentiation, the speed of CT means intrathoracic/ pulmonary disease is better visualised on a CT scan because movement blur from breathing is eliminated. Therefore a check for metastases would be best on CT. In humans CT is used for an abdominal metastases check and MRI for small complex joints such as the carpus/wrist. Therefore, it must be noted that the decision on which imaging modality to use is not always clear cut and when both are available, the choice will usually depend on the requirements of the patient.