It is important to remember that the magnetic field gener-ated by the MRI scanner is on permanently, even when the scanner console is turned off. Ferrous metallic objects taken into the MRI room will be attracted to the magnet and can act as projectiles, posing a significant risk to patients and staff. During MRI scanning the patient must remain still; for most cases general anaesthesia is required and any anaesthetic equipment must be MRI compatible (Figure 4.7) (i.e. contain no ferrous material). Particular risk comes from taking gas cylinders or metal trolleys into MRI rooms because these objects, when moving at speed towards the magnet, can result in fatal injury. Before taking patients into the MRI scanner they should be checked to ensure that collars, harnesses, etc. have been removed, and the clinical records should be checked to ensure the patient has not had a pacemaker implanted nor has poten-tially mobile metallic implants.
(a) Sagittal plane T2W MR images of the stifle of a dog with cranial cruciate disease and (b) a 5-year-old Cavalier ing Charles Spaniel with syringomyelia. The focal image distortion (arrowed) is due to susceptibility artefacts caused by (a) tibial tuberosity
advancement (TTA) implants and (b) a microchip. In these two cases the artefacts did not prevent the studies from being diagnostic.
4.9 (a)
(b)
Veterinary ear defenders are available in a range of sizes and are useful for minimizing the risk of hearing damage to the patient during high-field MRI scanning.
4.10
It is possible for the RF energy generated during MRI to cause heating of wires or cables, which can lead to thermal burns to the patient. When the patient is inside the bore of the magnet care should be taken to ensure that any cables or wires (e.g. for electrocardiography) do not touch the patient and are not formed into loops.
Technique
Image quality in MRI is highly dependent upon the field strength of the scanner, the pulse sequences used and the coil. The amount of signal produced within the patient is proportional to the magnetic field strength of the magnet.
The signal available at 3 T is approximately double that at 1.5 T. Image quality is generally better at higher field strengths (Figure 4.11). Perfectly diagnostic images can be obtained at low field strengths (<0.5 T), although image quality for small anatomical areas (e.g. small dogs’ stifles and cats’ spines) may be inadequate with low-field scanners. Although high-field MRI is often preferred for reasons of image quality, there are practical advantages to low-field MRI in veterinary practice (Figure 4.12).
A 9-year-old Boxer with brain masses (not visible).
(a) igh-field and (b) low-field MR transverse plane T2W images. Although both studies are diagnostic, higher-field MRI allows better image resolution.
4.11 (a)
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For all body regions, but especially spinal imaging, patient positioning is important to allow the area of interest to be positioned correctly within the coil. In spinal MRI it is essential that the spine is straight to allow true sagittal plane images to be acquired. Poor positioning makes image interpretation difficult or impossible (Figure 4.13).
The orientation of the patient within the scanner depends upon the type of MR scanner. In high-field units, patients (except very large dogs) are usually positioned in dorsal recumbency for spinal imaging. In open systems animals are often positioned in lateral recumbency. When imaging extremities, the positioning of the patient is dependent upon the joint and the coil. Joint imaging is normally per-formed with the joint in a neutral or slightly extended pos ition. Excessive flexion should be avoided. In patients being imaged for evaluation of non-neurological tumours (e.g. oral tumours) it is important to include the local and regional lymph nodes on the study to allow for staging of the disease. For head masses this means scanning as far caudal as C2/3, and for pelvic masses the medial iliac and pelvic lymph nodes should be included.
The exact selection of pulse sequences acquired varies with anatomical area, personal preference and type of pathology being imaged but in all cases multiple sequences should be acquired. See the section on MR protocols (below) for suggested minimum sequences to obtain. Images should be acquired in three orthogonal planes for all body regions (Figure 4.14). It is impractical to obtain thin slice images through large areas of the body or
Parameter o field ig field
Type of magnet Permanent Superconducting
Orientation of static magnetic
field Vertical orizontal
Equipment and running costs Low High Space requirements for
scanner Small Large
Scanning times Longer Shorter
Ability to perform spectral fat
saturation Limited Easy
Susceptibility effects Low High
Acoustic noise Low Variable, usually high
Comparison of high-field and low-field MRI.
4.12
Sagittal T2W MR image of an 11-year-old Lurcher with neck pain. The patient has been poorly positioned for the study, hindering evaluation. Because of the failure to position the neck straight it is di cult to assess the size and signal intensity of the caudal cervical spinal cord. Poor patient positioning may result in missed diagnoses.
4.13
(a) Dorsal, (b) sagittal and (c) transverse plane T2W MR images of an 11-year-old Fox Terrier with a right sciatic nerve mass (arrowed) and aortic aneurysm (dashed arrow). In this case the severe atrophy of the right gluteal muscles (arrowheads) secondary to the sciatic nerve mass was not visible on the sagittal plane images and the dilated aorta was not readily identified on the dorsal plane images. The enlarged sciatic nerve was best shown on the transverse plane image.
4.14 (a)
(b)
(c)
spine because the imaging time would be excessively long. As a result, it is not uncommon for significant path-ology to be clearly visible on one imaging plane only, and important information for surgical planning (such as the presence of transitional vertebrae) may be missed if only sagittal plane images are acquired. In all cases, therefore, it is essential that images are acquired in all three orthog-onal planes.
For most patients and body regions the slice thickness needs to be between 3 mm and 4 mm, although this varies with the region being imaged, image plane and patient size. Generally, for sagittal plane images of the brain and spine, slice thickness should be no greater than 3 mm and may need to be less in cats and smaller dogs.
Transverse images should be no greater than 4 mm for the brain in larger dogs or 3 mm in cats and small dogs, and generally for the spine transverse plane images are 3 mm.