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Wednesday, May 27, 2009

High field Extremity MRI

High Field (1.0 T) Extremity MRI

The dedicated extremity MR scanner at Mass General Imaging West, Waltham, is designed for

scanning the hand, wrist, elbow, foot, ankle, and knee

The diagnostic quality is comparable to 1.5 T conventional whole-body MR scanners

The maximum field of view of the extremity MR is 16 cm, which limits visualization of

longitudinal structures such as the quadriceps muscle and Achilles tendon

The bore size (18 cm) is too small for some applications (e.g. large knees) and is not suitable for

patients with leg or ankle casts or patients with limited flexibility

The 1.0 T MR dedicated extremity scanner (Figure 1),

installed at Mass General West Imaging, Waltham, is

designed to image the bones, joints, and soft tissues of

the hand, wrist (Figure 2), elbow, foot, ankle, and

knee. Because the joint of interest is centered in the

extremity scanner, it is always at the “sweet spot” of

the magnet, optimizing image quality, which is not

possible for elbow, wrist, or hand images in a

conventional whole-body MR scanner.

Images obtained with a 1.0 T MR extremity scanner are

generally regarded as comparable to those obtained in

a standard 1.5 T whole-body scanner, although there

are limited objective studies at this time. A

comparative study of patients with rheumatoid arthritis

showed excellent agreement between 1.5 T whole-body

MR and 1.0 T extremity MR in the scores for erosion,

synovitis, and bone marrow edema. Another study

compared accuracy and test-retest precision of

quantitative cartilage morphology in these two MR

systems and found no systematic bias between the

measurements of the cartilaginous surface of the

medial tibial plateau, the lateral tibial plateau, or the

central medial femoral condoyle. However, the study

revealed a statistically significant (P <>

of about 10% in cartilage volume (VC) and cartilage

thickness (ThCtAB) in the central lateral femoral



Although the large majority of patients can be scanned

with extremity MR, it is not suitable for all patients. The

bore size is 18 cm at its narrowest (Table 1), which

means that the scanner cannot be used for knee

imaging if the knee or distal thigh circumference is

greater than 22 1/4 inches (56.5 cm). Although casts

on wrists or arms are usually small enough to fit into

the magnet, a cast on the leg or ankle will not fit. In

addition, the patient must be able to flex his or her

ankle in order to pass the foot through the center of

F igure 1. The 1.0 T extremity MR scanner.

the bore. Therefore it is not possible to obtain images

of the knee if the patient has a cast on the ankle of the

same leg, and patients with limited ankle mobility may

find it difficult or impossible to insert their leg. Finally,

limited hip mobility can make it uncomfortable for a

patient to separate the legs sufficiently to place one leg

i n the magnet while the other rests on the floor.

The field of view of extremity MR can be no larger than

16 cm, which is smaller than a conventional scanner.

This means that the system truncates the visualization

of structures such as the quadriceps or Achilles tendon.

It is also not possible to view the entire hand or foot in

a single set of images. If the site of pain is localized to,

for example, the calcaneus region or the ball of the

foot, this is not a problem. However, the scanner is not

suitable for diagnosis of patients with extensive tumor

involvement because it is necessary to visualize the

whole foot in these patients.


Patients sit or recline on an ergonomically designed

chair, positioned so that the appropriate limb is

comfortably resting inside the bore of the extremity MR

scanner. Patients find the extremity MR scanner less

intimidating than a conventional MR scan because the

experience is not claustrophobic and the scanner is

relatively quiet. This sense of comfort helps patients

remain still and reduces problems due to motion

artifact. Therefore, it could be excellent for pediatric

patients. Scan duration is 30-35 minutes.



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