Factors Affecting MRI Image Quality

Magnetic resonance imaging (MRI) has become increasingly available for equine patients and is no longer limited to the referral hospital setting. Low-field systems which allow image acquisition under standing sedation are particularly popular and MRI has often become the advanced imaging modality of choice for veterinarians and owners.

How useful an imaging modality is depends very much on the quality of the images and the way sequences have been aligned to identify pathologies. MRI artefacts are well described in the literature [1] and not only do they reduce the quality of our examinations, but can also be misinterpreted as a pathological lesion. Particular to equine MRI, there are patient and environmental factors that influence the level of image quality which is readily achievable.

Broadly, factors which affect MRI quality can be split into two groups:

• Intrinsic factors: those directly related to the hardware and software of the MRI machine

• Extrinsic factors: a diverse group of variables relating to the patient, room, operator and horse handler.

Intrinsic factors

Consideration of intrinsic factors requires an understanding of the physics involved in creation of an MR image. Here, speed of acquisition, resolution and contrast are affected by the magnetic field strength, hardware and software processing techniques, and these factors in combination with the desire for lesion detection vs. anatomical information, must be taken into account when developing an optimal examination. Rarely is there a perfect solution and instead a balance must be met. For example, as slice thickness increases, the degree of volume averaging increases to result in more blurring [2]. Conversely, thinner slices are possible with increased field strength, and/or increased acquisition time, resulting in less volume averaging and potential for greater accuracy in image interpretation, but at the cost of a decreased signal to noise ratio and possibly motion.

Movement correction software and specific fast sequences can help to counteract the limitations of some acquisition factors. Sequence selection, order of image acquisition and decision-making in the face of artefacts can have a huge impact on image quality of a study. Artefacts such as flow- related artefacts, chemical shift, fat/water cancellation, fat saturation artefact, susceptibility, aliasing, truncation, zipper and magic angle artefacts have been described in both low and high field MRI and all must be taken into account and monitored during image acquisition.

Extrinsic factors

In terms of extrinsic factors that affect MRI quality, patient compliance and patient movement during image acquisition are probably the most important considerations. A settling in phase after arriving at the place of examination is strongly recommended to allow the horse to overcome the excitement of new, unfamiliar surroundings and to provide an opportunity for urination, defaecation and drinking prior to image acquisition.

The areas of the horse to be imaged should be clean and free of metallic material to minimise artefacts. For feet, radiographic screening to reduce the risk of magnetic susceptibility artefacts is recommended. Ideally the MRI room itself should be located in a quiet position to avoid horses becoming unsettled by noise during the examination. Choosing a suitable sedation protocol and potentially managing pain which may interfere with weightbearing is essential.

The horse handler plays a vital, but unfortunately often overlooked, role both before and during examination. Standing the horse square and in a comfortable position will improve the ease and quality of the examination; wedging or supporting the limbs and head to optimise positioning and reduce motion, or rotating the magnet to align the limb can be indicated. Motion, in the form of gross patient movement, gentle swaying, respiratory or cardiac cycle-related motion can all significantly limit the quality of the image obtained and this is why appropriate preparation, sedation and handling are so important. In standing MRI, motion correction is an invaluable tool to achieving diagnostic images, while for high field MRI under general anaesthesia, respiratory gating or other software processing may be helpful.

The MR operator also plays a key role; they must be aware of the potential for artefacts and the science behind their development, in order to avoid or reduce them. Additionally, finding the perfect position for the limb within the magnet, as well as the ideal standing position for each individual animal, is crucial. Correct sequence alignment for different anatomical structures is important; therefore accurate anatomical knowledge and 3-D thinking are helpful. Positioning of pilot images in relation to the area of interest is important to minimise artefacts and optimise interpretation. In some situations only a limited time window for examination may be available due to factors related to the patient. In these situations, sequences and planes selected can greatly impact your ability to make an accurate interpretation, therefore decisions have to be made quickly and it is necessary to identify lesions during the examination in order to tailor sequences and alignments.

MRI examinations under general anaesthesia may appear, on first look, simpler with regard to ensuring image quality but it bears its own, unique challenges. Heavy respiration and strong heartbeats may cause motion artefacts and operating tables need to be well cushioned, with anaesthetic times set to a minimum, in an attempt to avoid neural or muscular damage.

Overall the challenges we face in MRI acquisition are often related to time and motion, with a compromise being the solution in most cases. The value of a well trained and experienced team of operators and handlers that strive for perfection should not be underestimated in order to achieve an accurate diagnosis and realistic prognosis for the patient.

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