During a magnetic resonance imaging (MRI) exam, the technologist will image the region of interest for the referring physician. The scan may be done to visualize anatomy within this region, such as abnormalities related to pain, new conditions or preexisting conditions. In some cases, the region of interest may be localized, which can then be imaged with a smaller field of view (FOV). In others, this may span a large portion of the body, which may be easier to scan with a larger FOV.
What is the field of view?
The field of view is defined by the size of two- or three-dimensional spatial encoding area of the MRI image.1 When a patient is being scanned, the field of view should contain as much of the region of interest as possible. If the FOV is smaller than the region of interest, the technologist may have to acquire the image in stages, imaging one part of the region and then moving to the next part.
As one might expect, a smaller FOV shows less, and a larger FOV shows more of an area. The smaller field of view may offer high-resolution images, but they may have a lower measured signal. A larger field of view can image a larger area of the body with less breaks for positioning.
What determines field of view size?
The FOV may be adjusted prior to imaging. This works similarly to a camera's shutter, which opens and closes to adjust the lighting of a picture. The wider FOV may allow the scanner to acquire data about a greater area of the patient's body. Like a camera's shutter, the technologist cannot change the FOV in the middle of acquisition.
In most cases, the largest FOV option on an MR scanner is about 50 cm long, 50 cm wide, and 50 cm deep, though size may vary from one manufacturer to another. The FOV size can be decreased by the technologist to focus more on a specific body part, tissue structure or organ.
When is a large field of view used?
The area of the body that is being imaged directly affects the size of the FOV. Large regions of the body and imaging of multiple body parts often will result in a large FOV. Some examples include imaging the abdomen, total spine imaging, long bone imaging, whole body imaging, metastatic imaging, and lymphoma imaging.
If the imaging study is being done on the abdomen, the radiologist may want to image the entire torso. Imaging the abdomen can take multiple sequences with either a large or small FOV, which are then fused together to make a larger FOV mage for interpretation. In order to avoid interference from respiratory motion, the scan may be completed over several respiratory cycles, or it may be taken over multiple breath-holds. When this happens, it may be beneficial to image more of the abdomen at one time.
Total spine imaging covers the cervical, thoracic, and lumbar spine. This imaging study is done to evaluate the spine for abnormalities, including checking for multiple sclerosis or metastatic lesions. In particular, total spine imaging may be used to check for drop metastases or other forms of metastatic lesions.
Imaging of certain bones may require the use of a large FOV. This is because these bones are longer than they are wide, which means the desired region of interest is more rectangular than square. One example of this is the femur. Long bone imaging can be useful for the evaluation of metastatic lesions in a patient's bones.
Whole body imaging may be done in patients with cancer. This is due to the chance for the cancer to spread to other areas of the body. In some cases, the metastatic cancer may be far-reaching, meaning it spreads beyond the nearby organs and body parts to body parts that are further away from the original cancer. Whole body MRI is particularly useful in detecting abnormalities throughout the body.
When checking for metastases in cancer cases, it can be important to ensure as much of the body is imaged in as few pulse sequences as possible. This can save the patient time in the scanner overall, while still providing the detailed information necessary for the physician’s assessment. In order to do this, MR departments may open the field of view of the scanner more. Since more of the body is imaged in one position, it can mean a decrease in the number of “stages” or positions of the scan.
Lymphoma, which is cancer that forms in the lymphatic system, can affect a large portion of the body. The lymphatic system includes lymph nodes, the spleen, the thymus gland and bone marrow. Because these body parts span the body, imaging using large FOV may be helpful.
Large FOV imaging may be useful for physicians and radiologists desiring images of large body parts or large sections of the body, especially imaging abnormalities in long bones, cancer patients and bones. In these cases, the FOV of the scanner may be opened to acquire more data about the body prior to imaging. Radiologists may use large FOVs on a regular basis, which may help reduce the number of pulse sequences needed due to the decrease in stages needed for the scan.
- 'Field of View'. MR-Tip.com. https://www.mr-tip.com/serv1.php?type=db1&dbs=Field%20of%20View. Last accessed October 16, 2019.