AJR 2004; 183:1479-1486
© American Roentgen Ray Society
Musculoskeletal MRI at 3.0 T: Initial Clinical Experience
Garry E. Gold1,
Brian Suh,
Anne Sawyer-Glover and
Christopher Beaulieu
1 All authors: Department of Radiology, Stanford University, 300 Pasteur Dr.,
Rm. S0-68B, Stanford, CA 94305.
Received January 26, 2004;
accepted after revision April 8, 2004.
Address correspondence to G. E. Gold
(gold{at}stanford.edu).
Introduction
MRI of the musculoskeletal system is commonly done at 1.5 T. Higher field
systems, typically 3.0 T, are now available. A higher signal-to-noise ratio
(SNR), which can be used to improve imaging speed or resolution, is a result
of 3.0-T MRI. However, changes in relaxation times at 3.0 T as well as
increased artifacts must be considered. Our initial clinical experience at 3.0
T shows that diagnosis of disease is similar to 1.5 T, with some benefits from
improved resolution and SNR.
High-Field Imaging
In general, the intrinsic SNR experienced during MRI is a function of the
strength of the main magnetic field, the volume of tissue being imaged, and
the radiofrequency coil being used. In theory, if the coil and the subject are
equivalent, imaging at 3.0 T should provide twice the intrinsic SNR of imaging
at 1.5 T [1]. However, changes
to tissue relaxation times, sensitivity to magnetic susceptibility, and the
chemical shift difference between fat and water all influence image quality at
3.0 T. Thus, careful adjustment of the imaging protocols is necessary to
optimize imaging at 3.0 T.
Tissue Contrast
Prior measurements of relaxation times at 4.0 T showed increases in T1
relaxation time of 70–90% and decreases in T2 relaxation time of
10–20% compared with those at 1.5 T
[2]. Recent measurements of
these values in musculoskeletal tissues at 3.0 T show a decrease in T2
relaxation time of about 10% and an increase in T1 relaxation time of about
15–20% [3]. The changes
in these parameters affect the TR and TE appropriate for 3.0 T and ultimately
impact the contrast and SNR of the images produced.
On MRI, tissue contrast is determined by a number of variables, including
the TR and TE chosen by the operator, the T1 and T2 relaxation times of the
tissues being studied, and the use of fat saturation. At 3.0 T, the chosen TR
and TE should reflect the underlying tissues being imaged and the contrast
desired. In most cases, because the T1 relaxation times have increased at 3.0
T, the TR must be longer to achieve the same type of contrast seen at 1.5 T
(Fig. 1A,
1B). Similarly, the TE should
be slightly shorter to account for decreases in T2 relaxation times.
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Fig. 1A. —3.0-T images of knee in 52-year-old woman with popliteal
cyst. In sagittal proton density images (TR/TE, 4,000/15 [A] and
6,000/15 [B]), increased T1 relaxation time of fluid at 3.0 T changes
typical contrast seen. Increased TR brings image contrast back to what is
normally seen at 1.5 T with bright synovial fluid in popliteal cyst
(arrows, A and B).
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Fig. 1B. —3.0-T images of knee in 52-year-old woman with popliteal
cyst. In sagittal proton density images (TR/TE, 4,000/15 [A] and
6,000/15 [B]), increased T1 relaxation time of fluid at 3.0 T changes
typical contrast seen. Increased TR brings image contrast back to what is
normally seen at 1.5 T with bright synovial fluid in popliteal cyst
(arrows, A and B).
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In gradient-echo examinations, the flip angle should be lower to account
for the increased T1 relaxation times. Because T2* effects are doubled at 3.0
T versus 1.5 T [4], TE needs to
be shorter at 3.0 T to produce similar contrast for those sequences (Fig.
2A,
2B,
2C,
2D).
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Fig. 2A. —41-year-old man with anterior cruciate ligament repair.
Sagittal T2*-weighted gradient-echo images (TR, 600; flip angle, 25°;
matrix, 256 x 192; and slice thickness, 3 mm) show artifact from
interference screw (arrows, A–D) in 1.5-T image (TE =
20) (A), 3.0-T image (TE = 5) (B), 3.0-T image (TE = 10)
(C), and 3.0-T image (TE = 20) (D). T2* decay is increased at
3.0 T. Signal void around screw is larger and signal from bone marrow lower at
3.0 T (D) than at 1.5 T (A). TE of 10 at 3.0 T (C)
produces similar contrast to TE of 20 at 1.5 T (A).
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Fig. 2B. —41-year-old man with anterior cruciate ligament repair.
Sagittal T2*-weighted gradient-echo images (TR, 600; flip angle, 25°;
matrix, 256 x 192; and slice thickness, 3 mm) show artifact from
interference screw (arrows, A–D) in 1.5-T image (TE =
20) (A), 3.0-T image (TE = 5) (B), 3.0-T image (TE = 10)
(C), and 3.0-T image (TE = 20) (D). T2* decay is increased at
3.0 T. Signal void around screw is larger and signal from bone marrow lower at
3.0 T (D) than at 1.5 T (A). TE of 10 at 3.0 T (C)
produces similar contrast to TE of 20 at 1.5 T (A).
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Fig. 2C. —41-year-old man with anterior cruciate ligament repair.
Sagittal T2*-weighted gradient-echo images (TR, 600; flip angle, 25°;
matrix, 256 x 192; and slice thickness, 3 mm) show artifact from
interference screw (arrows, A–D) in 1.5-T image (TE =
20) (A), 3.0-T image (TE = 5) (B), 3.0-T image (TE = 10)
(C), and 3.0-T image (TE = 20) (D). T2* decay is increased at
3.0 T. Signal void around screw is larger and signal from bone marrow lower at
3.0 T (D) than at 1.5 T (A). TE of 10 at 3.0 T (C)
produces similar contrast to TE of 20 at 1.5 T (A).
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Fig. 2D. —41-year-old man with anterior cruciate ligament repair.
Sagittal T2*-weighted gradient-echo images (TR, 600; flip angle, 25°;
matrix, 256 x 192; and slice thickness, 3 mm) show artifact from
interference screw (arrows, A–D) in 1.5-T image (TE =
20) (A), 3.0-T image (TE = 5) (B), 3.0-T image (TE = 10)
(C), and 3.0-T image (TE = 20) (D). T2* decay is increased at
3.0 T. Signal void around screw is larger and signal from bone marrow lower at
3.0 T (D) than at 1.5 T (A). TE of 10 at 3.0 T (C)
produces similar contrast to TE of 20 at 1.5 T (A).
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Artifacts
Because the resonant frequencies of fat and water are twice as far apart at
3.0 T as at 1.5 T, chemical shift of fat pixels in the frequency-encoding
direction will be twice as great at a given imaging bandwidth
[4]. If the receiver bandwidth
is ± 32 kHz at 1.5 T, doubling the receiver bandwidth to ± 64
kHz will result in the same amount of chemical shift artifact at 3.0 T.
Doubling the receiver bandwidth reduces the available SNR by the square root
of 2 because the overall readout window length is shorter at a higher
band-width. However, increased bandwidth permits more slices and shorter TEs.
Careful attention to receiver bandwidth at 3.0 T is important to prevent
chemical shift artifact from obscuring important structures such as the talar
dome cartilage (Fig. 3A,
3B).
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Fig. 3A. —Coronal T1-weighted 3.0-T images of ankle in 44-year-old
woman shows chemical shift. Articular cartilage of talar dome is obscured by
chemical shift artifact in A (± 16-kHz readout band-width,
arrow) but is clearly visible in B (± 64-kHz readout
bandwidth, arrow).
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Fig. 3B. —Coronal T1-weighted 3.0-T images of ankle in 44-year-old
woman shows chemical shift. Articular cartilage of talar dome is obscured by
chemical shift artifact in A (± 16-kHz readout band-width,
arrow) but is clearly visible in B (± 64-kHz readout
bandwidth, arrow).
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One area in which chemical shift artifacts may affect diagnosis is the
spine, where intervertebral disks may appear larger or smaller depending on
the bandwidth in the frequency direction (Palmer WE et al., presented at the
2003 annual meeting of the American Roentgen Ray Society). Artifacts from
motion or metal in the postoperative patient may present more problems at 3.0
T than at 1.5 T. Susceptibility from small pieces of metal left in and around
the joint will be increased
[4]. Strategies for dealing
with postoperative artifacts at 1.5 T, such as increasing the bandwidth and
minimizing the use of gradient-echo sequences
[5], will also work at 3.0
T.
Radiofrequency Power Deposition
The resonant frequency at 3.0 T (
125 MHz) is twice that at 1.5 T. This
means that the radiofrequency power for excitation at 3.0 T is four times
higher than at 1.5 T [6,
7]. Use of shorter imaging
sequences such as fast spin echo may reduce the radiofrequency power
deposition (Fig. 4A,
4B). A disadvantage of using
short TE fast spin-echo sequences is image blurring due to lower signal echoes
at the edges of k-space. Using a short echo-train length and higher receiver
bandwidth to reduce echo spacing can minimize blurring on short TE fast
spin-echo images. Because the radiofrequency power deposited is a function of
tissue volume excited, this is a greater problem with large body areas such as
the hips (Fig. 5A,
5B) than smaller areas such as
the knee [7].
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Fig. 4A. —Coronal T1-weighted 3.0-T images of knee in 42-year-old
woman. Radiofrequency power deposition considerations at 3.0 T suggest limited
use of fast spinecho imaging for short TR sequences. T1-weighted spin-echo
image (TR = 800) at 3.0 T caused power monitor to reach 66% of average
radiofrequency power limit.
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Fig. 4B. —Coronal T1-weighted 3.0-T images of knee in 42-year-old
woman. Radiofrequency power deposition considerations at 3.0 T suggest limited
use of fast spinecho imaging for short TR sequences. T1-weighted fast
spin-echo image (TR/TE, 800/2) obtained when power monitor reached 33% of
average limit shows slight blurring due to use of two echoes and short TE.
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Fig. 5A. —Images of hip acquired at 3.0 T in healthy 34-year-old man.
Axial proton density image (TR/TE, 3,800/14; matrix, 288 x 224; field of
view, 14 cm; section thickness, 3 mm) (A) and coronal T2-weighted image
(4,200/52; matrix, 288 x 192; field of view, 14 cm; section thickness, 3
mm) (B) were acquired using torso phased-array coil. Acetabular labrum
(arrow) is well seen. Radiofrequency power deposition was high in
these scans because of excitation with body coil.
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Fig. 5B. —Images of hip acquired at 3.0 T in healthy 34-year-old man.
Axial proton density image (TR/TE, 3,800/14; matrix, 288 x 224; field of
view, 14 cm; section thickness, 3 mm) (A) and coronal T2-weighted image
(4,200/52; matrix, 288 x 192; field of view, 14 cm; section thickness, 3
mm) (B) were acquired using torso phased-array coil. Acetabular labrum
(arrow) is well seen. Radiofrequency power deposition was high in
these scans because of excitation with body coil.
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Fat Saturation at 3.0 T
At 3.0 T, the chemical shift between fat and water resonance is twice that
at 1.5 T, or approximately 440 Hz
[1]. This means that fat
saturation at 3.0 T is easier than at 1.5 T in the sense that the peaks are
farther apart. The length of the fat saturation pulses can be shortened from
about 16 to 8 msec. The overhead time per slice spent in fat saturation at 3.0
T during a multislice acquisition is less than at 1.5 T. This means that if
fat saturation is applied, more slices can be acquired at a given TR, slice
thickness, and bandwidth at 3.0 T than at 1.5 T (Fig.
6A,
6B,
6C,
6D).
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Fig. 6A. —Axial and coronal 3.0-T images of 44-year-old woman with
ankle pain. Axial T1-weighted (TR/TE, 800/14) (A) and T2-weighted
(5,400/52) with fat suppression (B) images and coronal T1-weighted
(800/14) (C) and T2-weighted (5,400/52) with fat suppression (D)
images show partial tear in substance of posterior tibial tendon
(arrows, A–D). Fat suppression is uniform at 3.0 T, and
shorter fat-saturation pulses allowed acquisition of more slices at 3.0 T than
at 1.5 T.
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Fig. 6B. —Axial and coronal 3.0-T images of 44-year-old woman with
ankle pain. Axial T1-weighted (TR/TE, 800/14) (A) and T2-weighted
(5,400/52) with fat suppression (B) images and coronal T1-weighted
(800/14) (C) and T2-weighted (5,400/52) with fat suppression (D)
images show partial tear in substance of posterior tibial tendon
(arrows, A–D). Fat suppression is uniform at 3.0 T, and
shorter fat-saturation pulses allowed acquisition of more slices at 3.0 T than
at 1.5 T.
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Fig. 6C. —Axial and coronal 3.0-T images of 44-year-old woman with
ankle pain. Axial T1-weighted (TR/TE, 800/14) (A) and T2-weighted
(5,400/52) with fat suppression (B) images and coronal T1-weighted
(800/14) (C) and T2-weighted (5,400/52) with fat suppression (D)
images show partial tear in substance of posterior tibial tendon
(arrows, A–D). Fat suppression is uniform at 3.0 T, and
shorter fat-saturation pulses allowed acquisition of more slices at 3.0 T than
at 1.5 T.
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Fig. 6D. —Axial and coronal 3.0-T images of 44-year-old woman with
ankle pain. Axial T1-weighted (TR/TE, 800/14) (A) and T2-weighted
(5,400/52) with fat suppression (B) images and coronal T1-weighted
(800/14) (C) and T2-weighted (5,400/52) with fat suppression (D)
images show partial tear in substance of posterior tibial tendon
(arrows, A–D). Fat suppression is uniform at 3.0 T, and
shorter fat-saturation pulses allowed acquisition of more slices at 3.0 T than
at 1.5 T.
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Imaging of Disease at 3.0 T
Images of disease at 3.0 T appear similar to those at 1.5 T (Figs.
6A,
6B,
6C,
6D,
7A,
7B,
8A,
8B,
8C,
8D,
9A,
9B,
10A,
10B,
10C,
10D). Fluid continues to be
bright in areas of disease and tears. Shortening of T2 relaxation times may
lead to decreased problems from magic angle effects
[8]. Tears result in increased
T2 relaxation times in tendons (Fig.
6A,
6B,
6C,
6D). Overall, improved
resolution and speed should allow improved diagnostic accuracy or at least
improved diagnostic confidence.
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Fig. 7A. —Axial 3.0-T images of shoulder in 28-year-old man after
skiing injury. Images acquired with proton-density weighting (TR/TE, 5,000/14;
section thickness, 2.5 mm; matrix, 512 x 192; and fat suppression) show
intrasubstance tear in subscapularis tendon (arrow, A). Image
contrast is similar to that seen at 1.5 T, and excellent detail is seen in
glenoid labrum (arrow, B).
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Fig. 7B. —Axial 3.0-T images of shoulder in 28-year-old man after
skiing injury. Images acquired with proton-density weighting (TR/TE, 5,000/14;
section thickness, 2.5 mm; matrix, 512 x 192; and fat suppression) show
intrasubstance tear in subscapularis tendon (arrow, A). Image
contrast is similar to that seen at 1.5 T, and excellent detail is seen in
glenoid labrum (arrow, B).
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Fig. 8A. —Axial 3.0-T images of ankle in 39-year-old man with ankle
pain. Proximal T1-weighted (TR/TE, 800/14) (A) and fat-suppressed
T2-weighted (5,400/52) (B) images and distal T1-weighted (800/14)
(C) and fat-suppressed T2-weighted (5,400/52) (D) images show
contrast between fluid and flexor hallucis longus tendon (arrows) is
similar to that seen at 1.5 T. Additional signal at 3.0 T allowed imaging at
512 matrix over 14-cm field of view on both T1- and T2-weighted images.
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Fig. 8B. —Axial 3.0-T images of ankle in 39-year-old man with ankle
pain. Proximal T1-weighted (TR/TE, 800/14) (A) and fat-suppressed
T2-weighted (5,400/52) (B) images and distal T1-weighted (800/14)
(C) and fat-suppressed T2-weighted (5,400/52) (D) images show
contrast between fluid and flexor hallucis longus tendon (arrows) is
similar to that seen at 1.5 T. Additional signal at 3.0 T allowed imaging at
512 matrix over 14-cm field of view on both T1- and T2-weighted images.
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Fig. 8C. —Axial 3.0-T images of ankle in 39-year-old man with ankle
pain. Proximal T1-weighted (TR/TE, 800/14) (A) and fat-suppressed
T2-weighted (5,400/52) (B) images and distal T1-weighted (800/14)
(C) and fat-suppressed T2-weighted (5,400/52) (D) images show
contrast between fluid and flexor hallucis longus tendon (arrows) is
similar to that seen at 1.5 T. Additional signal at 3.0 T allowed imaging at
512 matrix over 14-cm field of view on both T1- and T2-weighted images.
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Fig. 8D. —Axial 3.0-T images of ankle in 39-year-old man with ankle
pain. Proximal T1-weighted (TR/TE, 800/14) (A) and fat-suppressed
T2-weighted (5,400/52) (B) images and distal T1-weighted (800/14)
(C) and fat-suppressed T2-weighted (5,400/52) (D) images show
contrast between fluid and flexor hallucis longus tendon (arrows) is
similar to that seen at 1.5 T. Additional signal at 3.0 T allowed imaging at
512 matrix over 14-cm field of view on both T1- and T2-weighted images.
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Fig. 9A. —Sagittal 3.0-T images of knee in 30-year-old man after skiing
injury. Proton density–weighted image with fat suppression (TR/TE,
5,000/14; matrix, 512 x 192; field of view, 16 cm; section thickness, 3
mm) shows tear in anterior cruciate ligament (arrow).
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Fig. 9B. —Sagittal 3.0-T images of knee in 30-year-old man after skiing
injury. T1-weighted image (800/14; matrix, 512 x 192; field of view, 16
cm; section thickness, 4 mm) shows tear in anterior cruciate ligament
(arrow) that was proven at arthroscopy.
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Fig. 10A. —Images of knee obtained at 3.0 T in 38-year-old woman with
knee pain. Sagittal proton density–weighted (TR/TE, 5,000/14; matrix,
512 x 192; field of view, 16 cm; slice thickness, 3 mm) (A) and
T2-weighted with fat suppression (5,000/14; matrix, 512 x 192; field of
view, 16 cm; slice thickness, 3 mm) (B) images and coronal T1-weighted
(800/14; matrix, 512 x 192; field of view, 16 cm; slice thickness, 3 mm)
(C) and T2-weighted (5,000/52; matrix, 512 x 192; field of view,
16 cm; slice thickness, 3 mm) (D) images show extensive tear in
posterior horn of medial meniscus (arrows). This tear was proven at
arthroscopy.
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Fig. 10B. —Images of knee obtained at 3.0 T in 38-year-old woman with
knee pain. Sagittal proton density–weighted (TR/TE, 5,000/14; matrix,
512 x 192; field of view, 16 cm; slice thickness, 3 mm) (A) and
T2-weighted with fat suppression (5,000/14; matrix, 512 x 192; field of
view, 16 cm; slice thickness, 3 mm) (B) images and coronal T1-weighted
(800/14; matrix, 512 x 192; field of view, 16 cm; slice thickness, 3 mm)
(C) and T2-weighted (5,000/52; matrix, 512 x 192; field of view,
16 cm; slice thickness, 3 mm) (D) images show extensive tear in
posterior horn of medial meniscus (arrows). This tear was proven at
arthroscopy.
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Fig. 10C. —Images of knee obtained at 3.0 T in 38-year-old woman with
knee pain. Sagittal proton density–weighted (TR/TE, 5,000/14; matrix,
512 x 192; field of view, 16 cm; slice thickness, 3 mm) (A) and
T2-weighted with fat suppression (5,000/14; matrix, 512 x 192; field of
view, 16 cm; slice thickness, 3 mm) (B) images and coronal T1-weighted
(800/14; matrix, 512 x 192; field of view, 16 cm; slice thickness, 3 mm)
(C) and T2-weighted (5,000/52; matrix, 512 x 192; field of view,
16 cm; slice thickness, 3 mm) (D) images show extensive tear in
posterior horn of medial meniscus (arrows). This tear was proven at
arthroscopy.
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Fig. 10D. —Images of knee obtained at 3.0 T in 38-year-old woman with
knee pain. Sagittal proton density–weighted (TR/TE, 5,000/14; matrix,
512 x 192; field of view, 16 cm; slice thickness, 3 mm) (A) and
T2-weighted with fat suppression (5,000/14; matrix, 512 x 192; field of
view, 16 cm; slice thickness, 3 mm) (B) images and coronal T1-weighted
(800/14; matrix, 512 x 192; field of view, 16 cm; slice thickness, 3 mm)
(C) and T2-weighted (5,000/52; matrix, 512 x 192; field of view,
16 cm; slice thickness, 3 mm) (D) images show extensive tear in
posterior horn of medial meniscus (arrows). This tear was proven at
arthroscopy.
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Improvements in Speed
As mentioned earlier, the SNR at 3.0 T is approximately double that at 1.5
T. Because SNR is proportional to the square of the scanning time, it is
possible to go up to four times faster at 3.0 T than at 1.5 T with equivalent
SNR. This is only true if the scanning at 1.5 T is performed with multiple
averages for increased SNR and the relaxation time changes at 3.0 T do not
significantly impact the SNR. In practice, it may be possible to go twice as
fast at 3.0 T (Fig. 11A,
11B,
11C,
11D,
11E).
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Fig. 11A. —Rapid knee imaging protocol for 3.0 T in 52-year-old woman
with knee pain. Scanning times for each sequence are indicated in brackets.
Total acquisition time for this knee examination was 9 min 30 sec.
Degenerative changes are seen in medial compartment. Coronal T2-weighted fast
spin-echo image [1 min 18 sec], (TR/TE, 8,400/60, matrix, 512 x 192;
field of view, 16 cm; section thickness, 4 mm; echo-train length, 8) shows
torn and displaced medical meniscus (arrow).
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Fig. 11B. —Rapid knee imaging protocol for 3.0 T in 52-year-old woman
with knee pain. Scanning times for each sequence are indicated in brackets.
Total acquisition time for this knee examination was 9 min 30 sec.
Degenerative changes are seen in medial compartment. Coronal T1-weighted fast
spin-echo image [1 min 19 sec] (800/12, matrix, 512 x 192; field of
view, 16 cm; section thickness, 4 mm; echo-train length, 2) shows medical
compartment osteophyte (arrow).
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Fig. 11C. —Rapid knee imaging protocol for 3.0 T in 52-year-old woman
with knee pain. Scanning times for each sequence are indicated in brackets.
Total acquisition time for this knee examination was 9 min 30 sec.
Degenerative changes are seen in medial compartment. Sagittal proton density
fast spin-echo image [2 min 30 sec] (6,000/15, matrix, 512 x 192; field
of view, 16 cm; section thickness, 3 mm; echo-train length, 8).
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Fig. 11D. —Rapid knee imaging protocol for 3.0 T in 52-year-old woman
with knee pain. Scanning times for each sequence are indicated in brackets.
Total acquisition time for this knee examination was 9 min 30 sec.
Degenerative changes are seen in medial compartment. Sagittal fat-suppressed
T2-weighted fast spin-echo image [1 min 49 sec] (8,400/60; matrix, 512 x
192; field of view, 16 cm; section thickness, 3 mm; echo-train length,
12).
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Fig. 11E. —Rapid knee imaging protocol for 3.0 T in 52-year-old woman
with knee pain. Scanning times for each sequence are indicated in brackets.
Total acquisition time for this knee examination was 9 min 30 sec.
Degenerative changes are seen in medial compartment. Axial proton density
fat-saturated fast spin-echo image [2 min 30 sec] (6,000/15; matrix, 512
x 192; field of view, 14 cm; section thickness, 4 mm; echo-train length,
8).
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The principles behind rapid protocol design at 3.0 T are as follows: First,
minimize the use of signal averages. Second, increase the TR to account for
longer relaxation times and improve the SNR. Third, double the receiver
bandwidth compared with 1.5 T on non–fat-saturated sequences. Finally,
fast spin-echo imaging with small echo spacing is useful for all sequences,
including T1-weighted images. The echo-train length must be kept short on the
short TE images to avoid blurring but can be longer on the T2-weighted
images.
Improvements in Resolution
The increased SNR available at 3.0 T may also be used to improve the
resolution of the images acquired. In theory, the resolution in one direction
can be doubled at 3.0 T and generate the equivalent SNR as a 1.5 T image. In
practice, because of changes in relaxation times, the best strategy for using
the increased SNR at 3.0 T for improving resolution may be to acquire more,
thinner slices. An increase in TR for multislice acquisitions allows more
slices to be acquired and offsets the effects of increased T1 relaxation
times. Slice thickness may be halved from 4.0 to 2.0 mm and twice as many
slices acquired (Fig. 12A,
12B). If only one signal
average is used, the total scanninng time is equivalent and SNR is comparable
to that at 1.5 T.
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Fig. 12A. —Coronal 3.0-T images of knee in 34-year-old man with knee
pain. T1-weighted image (TR/TE, 1,000/14; slice thickness, 2 mm) (A)
and zoom MR image (B) of medial meniscus show subtle meniscal tear
(arrow) that was not seen in any other plane. Improved
signal-to-noise ratio at 3.0 T allowed use of thinner slices than we routinely
use at 1.5 T, which may enhance detection of disease.
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Fig. 12B. —Coronal 3.0-T images of knee in 34-year-old man with knee
pain. T1-weighted image (TR/TE, 1,000/14; slice thickness, 2 mm) (A)
and zoom MR image (B) of medial meniscus show subtle meniscal tear
(arrow) that was not seen in any other plane. Improved
signal-to-noise ratio at 3.0 T allowed use of thinner slices than we routinely
use at 1.5 T, which may enhance detection of disease.
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Increased resolution may be helpful in several problem areas of
musculoskeletal imaging [9].
These include the labrum in the shoulder (Fig.
7A,
7B), the talar dome cartilage
(Fig. 4A,
4B), the acetabular labrum
(Fig. 5A,
5B), and the articular
cartilage (Fig. 13A,
13B). Imaging of these areas
at very high resolution may require multiple signal averages for either or
both SNRs, to avoid phase wrap. If imaging is done with fat suppression,
lowering the imaging bandwidth will improve the overall SNR. If T2-weighted
imaging is used, increasing the echo-train length for additional speed is
acceptable. If T1-weighted or proton density (short TE) imaging is performed,
however, a short echo-train length may be preferable to avoid blurring
[10].
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Fig. 13A. —Images of knee in 29-year-old man with knee pain. Axial
proton-density image (TR/TE, 5,000/14; matrix, 512 x 384; slice
thickness, 2 mm) acquired at 1.5 T shows fraying of lateral patella cartilage
(arrow).
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Fig. 13B. —Images of knee in 29-year-old man with knee pain. Axial
proton density image acquired at 3.0 T with identical parameters shows
improved signal-to-noise ratio (SNR). Improved depiction of fraying of lateral
patella cartilage (arrow) is seen at 3.0 T. Overall SNR at 3.0 T
allows imaging at higher resolution.
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Conclusion
MRI provides a powerful tool for imaging and understanding the
musculoskeletal system. The fundamental trade-off between image resolution and
SNR still limits our ability to image in vivo at 1.5 T with high-resolution in
an efficient manner. A 3.0 T system may permit fast routine imaging or
higher-resolution studies. Faster imaging will result in less patient motion,
increased comfort, and better throughput. Increased resolution may result in
more accurate diagnosis but will require prospective validation.
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Top
Introduction
High-Field Imaging
