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 < 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
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
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.
suitable for diagnosis of patients with extensive tumor
involvement because it is necessary to visualize the
whole foot in these patients.
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.

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 < 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

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

http://www.mghradrounds.org/clientuploads/may_2009/may_2009.pdf?PHPSESSID=532207e8d6c2f6cd9d54d62476893009


suitable for diagnosis of patients with extensive tumor

involvement because it is necessary to visualize the

whole foot in these patients.

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.

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