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Volume 7 Issue 5 - May 2009
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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



Limitations
Procedure
Scheduling
Further Information
References

T
he 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 < 0.05) variation of about 10% in cartilage volume (VC) and cartilage thickness (ThCtAB) in the central lateral femoral condyle.

Limitations
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 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 in 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.
 
Figure 1. The 1.0 T extremity MR scanner
 
Figure 1. The 1.0 T extremity MR scanner.


Procedure
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.


Table 1. High Field Extremity MRI Specifications
Field Strength 1.0 Tesla
Bore size 18 cm, flared to 20 cm at entrance
Bore circumference 56.5 cm (22 ¼ inches)
Field of view 4-16 mm
Slice thickness 2D: 2–10 mm
3D: 0.5–10 mm
Patient chair weight limit 350 lb
Scan Time 30-35 minutes


 
Figure 2. Images of the wrist obtained with the 1.0 T extremity scanner. (A) Axial image shows tendons and median nerve in the carpal tunnel, (B) and (C) Coronal images showing scaphoid lesion (arrow). (D) Gadolinium contrast-enhanced image excludes osteonecrosis.


Scheduling
Extremity MR images can be ordered through ROE or by calling 617-724-XRAY (9729). In both cases, it is possible to specify the dedicated extremity MR scanner, located at Mass General Imaging West, Waltham, or a conventional scanner. Note that the weight limit of the extremity scanner system is 350 lb.

If a patient has been scheduled for the dedicated extremity MR scanner but the technologist finds that he or she cannot be accommodated, the examination will have to be rescheduled on a conventional scanner. Every effort will be made to make an appointment the same day, although that appointment may be at another imaging site, such as Mass General Imaging, Chelsea, or the main campus.
  Further Information
For further questions on extremity MRI, please contact William E. Palmer, M.D. , Director of Musculoskeletal Imaging and Intervention at the Massachusetts General Hospital, 617-726-7717.

We would like to thank William E. Palmer, M.D., Tyler Martin, Operations Manager at Mass General West Imaging, Waltham, and Debra Miller, MR technologist, for their advice and assistance in preparation of this article.










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References
   

Inglis, D, Pui, M, Ioannidis, G, et al. (2007) Accuracy and test-retest precision of quantitative cartilage morphology on a 1.0 T peripheral magnetic resonance imaging system. Osteoarthritis Cartilage 15: 110-5

Naraghi, AM, White, LM, Patel, C and Keystone, E  (2008). Comparison of 1.0 T extremity MRI (eMRI) and 1.5 T whole body conventional MRI (cMRI) systems in assessment of rheumatoid arthritis (RA) of the hand and wrist. Annual Meeting and Scientific Assembly of the Radiology Society of North America. Chicago, Abstract # SSA14-03.