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DOI:10.2214/AJR.06.1208
AJR 2007; 188:1287-1293
© American Roentgen Ray Society

Original Research

Isotropic MRI of the Knee with 3D Fast Spin-Echo Extended Echo-Train Acquisition (XETA): Initial Experience

Garry E. Gold1, Reed F. Busse2, Carol Beehler2, Eric Han2, Anja C. S. Brau2, Philip J. Beatty3 and Christopher F. Beaulieu1

1 Department of Radiology, Stanford University, Grant Bldg. SO-68B, 300 Pasteur Dr., Stanford, CA 94305-5105.
2 GE Healthcare Global Applied Sciences Laboratory, Menlo Park, CA.
3 Department of Electrical Engineering, Stanford University, Stanford, CA.

Received September 14, 2006; accepted after revision December 6, 2006.

 
Address correspondence to G. E. Gold (gold{at}stanford.edu).

The authors who are not employees of GE Healthcare (G. E. Gold, P. J. Beatty, and C. F. Beaulieu) had control of inclusion of any data and information that might present a conflict of interest for those authors who are employees of GE Healthcare (R. F. Busse, C. Beehler, E. Han, and A. C. S. Brau).


Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of our study was to prospectively compare a recently developed method of isotropic 3D fast spin-echo (FSE) with extended echo-train acquisition (XETA) with 2D FSE and 2D fast recovery FSE (FRFSE) for MRI of the knee.

SUBJECTS AND METHODS. Institutional review board approval, Health Insurance Portability and Accounting Act (HIPAA) compliance, and informed consent were obtained. We studied 10 healthy volunteers and one volunteer with knee pain using 3D FSE XETA, 2D FSE, and 2D FRFSE. Images were obtained both with and without fat suppression. Cartilage and muscle signal-to-noise ratio (SNR) and cartilage-fluid contrast-to-noise ratio (CNR) were compared using a Student's t test. We also compared reformations of 3D FSE XETA with 2D FSE images directly acquired in the axial plane.

RESULTS. Cartilage SNR was higher with 3D FSE XETA (56.8 ± 9 [SD]) compared with the 2D FSE (45.8 ± 8, p < 0.01) and 2D FRFSE (32.5 ± 5.3, p < 0.01). Muscle SNR was significantly higher with 3D FSE XETA (52.1 ± 4.3) than 2D FSE (45.2 ± 9, p < 0.01) and 2D FRFSE (23.6 ± 6.2, p < 0.01). Fluid SNR was significantly higher for 2D FSE (144.9 ± 33) than 3D FSE XETA (104.7 ± 18, p < 0.01). Compared with 2D FSE and 2D FRFSE, 3D FSE XETA had lower cartilage-fluid CNR due to higher cartilage SNR (p < 0.01). Three-dimensional FSE XETA acquired volumetric data sets with isotropic resolution. Reformatted images in the axial plane were similar to axial 2D FSE acquisitions but with thinner slices.

CONCLUSION. Three-dimensional FSE XETA acquires high-resolution (approximately 0.7 mm) isotropic data with intermediate and T2-weighting that may be reformatted in arbitrary planes. Three-dimensional FSE XETA is a promising technique for MRI of the knee.

Keywords: injury • joint • knee • MRI • physics


Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
MRI of the knee is traditionally done with multiple 2D multislice acquisitions. Fast spin-echo (FSE) is commonly used to provide intermediate or T2-weighted images in a reasonable scanning time. These images are useful to look for internal derangements such as meniscal tears [1, 2], ligamentous injury [3], or cartilage damage [4, 5].

Two-dimensional FSE has limitations in examination of the knee. The voxels are not isotropic, with relatively thick slices compared with the in-plane resolution leading to partial volume artifacts. Because of the anisotropic nature of the voxels, these images do not lend themselves to reformations. Blurring of structures that have short T2 relaxation times is common on intermediate-weighted scans. This blurring has led to controversy regarding the utility of this sequence for the diagnosis of meniscal tears [6]. Magnetization transfer due to slice selection can decrease signal in cartilage or muscle [7]. Finally, slice gaps do not permit accurate quantification of structures such as cartilage volume.

Two-dimensional fast-recovery FSE (FRFSE) has been developed to increase fluid signal in short-TR imaging [8]. This sequence, similar to driven-equilibrium imaging, tips magnetization back to the z-axis after each TR. Otherwise, this sequence has limitations similar to 2D FSE with respect to anisotropic voxels and magnetization transfer effects.

Many methods of 3D gradient-echo imaging have been applied to the knee [9-16]. An isotropic 3D imaging method based on a radial k-space trajectory has recently been developed [13]. These methods have been shown to be useful in cartilage evaluation [10, 11, 17, 18] and quantification [12, 16] but have not replaced 2D FSE in evaluation of internal derangements.

Three-dimensional FSE has been applied to neuroimaging [19-21] and the abdomen [22]. We have recently developed a fast-recovery 3D FSE method with an extended echo-train acquisition (XETA) that uses variable flip angles to constrain T2 decay over a long echo train, allowing acquisition of T2-weighted images with minimal blurring [23]. We have combined this method with half-Fourier acquisition and an autocalibrating parallel reconstruction technique known as ARC (autocalibrating reconstruction for Cartesian sampling) [24] to reduce TE, reduce echo-train length, and improve scanning efficiency. This allows us to acquire 3D FSE XETA images at isotropic resolutions in reasonable scanning times that are useful for MRI of the knee.

The purpose of our study was to compare 3D FSE XETA with 2D FSE and 2D FRFSE for MRI of the knee. We compared signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) results and the ability to provide accurate reformations from isotropic data.


Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects and Imaging
After institutional review board approval and compliance with the Health Insurance Portability and Accounting Act (HIPAA), informed consent was obtained and MRI was performed on 10 healthy volunteers. We imaged the right knees of 10 healthy volunteers (four men and six women; age range, 26-42 years). The volunteers had no history of knee pain or prior surgery in either knee. All images were acquired on a 1.5-T Signa Excite HDx system (GE Healthcare) with high-performance gradients (maximum gradient strength, 40 mT/m; maximum slew rate, 150 mT/m/s) using a transmit-receive 8-channel knee coil. One additional volunteer with knee pain was imaged at 1.5 T. All images were acquired with and without frequency-selective fat suppression.

Three-dimensional FSE XETA images were acquired in the coronal plane with TR/TEeff, 2,500/38; matrix, 256 x 256; field of view, 16 cm; 0.7-mm sections; and bandwidth, ± 42 kHz, resulting in a nearisotropic resolution of 0.625 x 0.625 x 0.7 mm. Partial Fourier acquisition and ARC parallel imaging reduced scanning time by a factor of 3.4, enabling an echo-train length of 78 to encode a 256 x 256 matrix for one section (slice encode) per shot. To cover the entire knee, 200 sections were acquired in just 8 minutes. When fat suppression was used, the scanning time and anatomic coverage were identical for 3D FSE XETA. Radiofrequency power deposition was within U.S. Food and Drug Administration (FDA) limits. Parallel imaging reconstruction was performed online using host-based prototype software, and reconstructed images were transferred to the scanner database.

Coronal 2D FSE images were acquired with TR/TEeff, 4,000/38; matrix, 256 x 256; field of view, 16 cm; 3-mm slices and 1-mm gap; 2 averages; echo-train length, 8; bandwidth, ± 16 kHz; and scanning time, 4 minutes. When fat suppression was used, the scanning time was kept constant, but fewer slices were imaged. Two-dimensional FSE images were also acquired in the axial plane for comparison with reformations of the 3D data. The TR for this protocol was the same as used in our clinical knee protocol to allow acquisition of enough slices to cover the entire knee anterior to posterior. The acquisition bandwidth was also the same as our clinical protocol.

Coronal 2D FRFSE images were acquired with TR/TEeff, 2,500/38; matrix, 256 x 256; field of view, 16 cm; 3-mm slices and 1-mm gap; 2 averages; echotrain length, 8; bandwidth ± 16 kHz; and scanning time, 3 minutes. When fat suppression was used, the scanning time was kept constant, but fewer slices were imaged. The TR was the same as for the 3D FSE XETA images, and the fast recovery option was used to provide similar contrast. The acquisition bandwidth was the same as our clinical protocol. The TR used did not provide enough slices to cover the entire knee but allowed us to assess contrast changes compared with 3D FSE XETA.

Two additional 3D FSE XETA acquisitions were made outside this comparative framework. One volunteer was imaged at 3.0 T with an 8-channel knee coil to attain higher resolution (0.6-mm isotropic) and SNR, and another volunteer was imaged at 1.5 T with the original parameters except for a reduced TR/TEeff of 800/24. The minimum TEeff in 3D FSE XETA is limited by the minimum refocusing flip angle and gradient crushers used. Two hundred sections were acquired in just 5 minutes with two signal averages. This acquisition was to show the potential of 3D FSE XETA to acquire isotropic images with near-T1 weighting. The TE used was the minimum that could be achieved with the current sequence.

Image Evaluation
Signal from articular cartilage, muscle, synovial fluid, and noise was measured in all patients from a region of interest (ROI) in the medial femoral condylar cartilage, vastus medialis muscle, and fluid in the intracondylar notch on images without fat suppression. The measurements were performed by a single radiologist with 10 years of experience in the interpretation of MRI of the knee who was not blinded to the sequence. The circular ROI for cartilage and fluid measurements was 3 mm in diameter. The circular ROIs for muscle and noise measurements were 9 mm in diameter. The SD of the noise was measured from a single ROI placed in the background. Because no coil intensity correction was applied and the parallel imaging method operates in k-space, noise is uniform through the image. The SNRs for cartilage, muscle, and joint fluid were calculated by dividing the signal level by the SD of the noise. The fluid-cartilage CNR was calculated by subtracting the cartilage SNR from the fluid SNR.

Axial reformations of the isotropic 3D FSE XETA images were produced using Osirix and compared with 2D FSE images acquired in the axial plane. Oblique reformations were also produced to show the course of Wrisberg's ligament in one volunteer.

Statistical Analysis
Statistical analysis was performed using Excel 11.1.1 (Microsoft). The 3D FSE XETA and 2D FSE images were compared pairwise with respect to SNR and cartilage-fluid CNR using a paired sample Student's t test. A p value of < 0.05 was considered significant.


Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Cartilage SNR was statistically higher with 3D FSE XETA (56.8 ± 9 [SD]) compared with 2D FSE (45.8 ± 8, p < 0.01) and 2D FRFSE (32.5 ± 5.3, p < 0.01). Muscle SNR was significantly higher with 3D FSE XETA (52.1 ± 4.3) than with 2D FSE (45.2 ± 9, p < 0.01) and 2D FRFSE (23.6 ± 6.2, p < 0.01). Fluid SNR was significantly higher for 2D FSE (144.9 ± 33) than for 3D FSE XETA (104.7 ± 18, p <0.01) (Fig. 1A, 1B). Fluid SNR was not statistically different for 3D FSE XETA and 2D FRFSE.


Figure 1
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  		Fig. 1A —Bar graphs show signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (light gray bars), 2D FSE (white bars), and 2D fast recovery FSE (FRFSE) (black bars). Asterisks denote significant differences (p < 0.01). Cartilage SNR for 3D FSE XETA (TR/TEeff, 2,500/38), 2D FSE (TR/TEeff, 4,000/38), and 2D FRFSE (TR/TEeff, 2,500/38). Cartilage and muscle SNRs are significantly higher for 3D FSE XETA compared with 2D FSE and 2D FRFSE. Fluid SNR for 2D FSE is higher than 3D FSE XETA. Fluid SNRs for 2D FRFSE and 3D FSE XETA are not statistically different.

 

Figure 2
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  		Fig. 1B —Bar graphs show signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (light gray bars), 2D FSE (white bars), and 2D fast recovery FSE (FRFSE) (black bars). Asterisks denote significant differences (p < 0.01). Fluid-cartilage CNR was higher for 2D FSE and 2D FRFSE than 3D FSE XETA.

 
Fluid-cartilage CNR was higher for 2D FSE (99.1 ± 25) and 2D FRFSE (90.1 ± 15.8) than for 3D FSE XETA (48.0 ± 9.1, p < 0.01) (Fig. 1A, 1B). Contrast between fluid and articular cartilage was seen on both methods, with high fluid signal on 2D FSE, 2D FRFSE, and 3D FSE XETA (Fig. 2A, 2B, 2C).


Figure 3
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  		Fig. 2A —25-year-old female volunteer. Coronal 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38) (A), coronal 2D FSE (TR/TEeff, 4,000/38) (B), and coronal 2D FRFSE (TR/TEeff, 2,500/38) (C) images at 1.5 T show fluid signal (arrows) is high compared with articular cartilage. Note that muscle signal and cartilage signal are lower in B and C compared with A. This is likely due to magnetization transfer effects of 2D sequences.

 

Figure 4
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  		Fig. 2B —25-year-old female volunteer. Coronal 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38) (A), coronal 2D FSE (TR/TEeff, 4,000/38) (B), and coronal 2D FRFSE (TR/TEeff, 2,500/38) (C) images at 1.5 T show fluid signal (arrows) is high compared with articular cartilage. Note that muscle signal and cartilage signal are lower in B and C compared with A. This is likely due to magnetization transfer effects of 2D sequences.

 

Figure 5
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  		Fig. 2C —25-year-old female volunteer. Coronal 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38) (A), coronal 2D FSE (TR/TEeff, 4,000/38) (B), and coronal 2D FRFSE (TR/TEeff, 2,500/38) (C) images at 1.5 T show fluid signal (arrows) is high compared with articular cartilage. Note that muscle signal and cartilage signal are lower in B and C compared with A. This is likely due to magnetization transfer effects of 2D sequences.

 
Images from 3D FSE XETA were reformatted in multiple planes (Fig. 3A, 3B, 3C). Imaging at 3.0 T allowed us to improve the resolution of 3D FSE XETA to 0.6-mm isotropic compared with 0.7-mm isotropic for the 1.5-T images [25]. An advantage of 3D FSE XETA compared with 2D imaging is the ability to average slices so that source and reformatted images can be displayed at any multiple of the 0.7-mm slice thickness (Fig. 4A, 4B, 4C, 4D, 4E). Averaged slices showed additional SNR compared with the source slices.


Figure 6
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  		Fig. 3A —35-year-old male volunteer. Three-dimensional fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38) images at 3.0 T. Resolution was 0.6 mm isotropic. Coronal source image, (A), sagittal reformation image, (B), and axial reformation image (C) show excellent depiction of knee anatomy.

 

Figure 7
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  		Fig. 3B —35-year-old male volunteer. Three-dimensional fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38) images at 3.0 T. Resolution was 0.6 mm isotropic. Coronal source image, (A), sagittal reformation image, (B), and axial reformation image (C) show excellent depiction of knee anatomy.

 

Figure 8
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  		Fig. 3C —35-year-old male volunteer. Three-dimensional fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38) images at 3.0 T. Resolution was 0.6 mm isotropic. Coronal source image, (A), sagittal reformation image, (B), and axial reformation image (C) show excellent depiction of knee anatomy.

 

Figure 9
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  		Fig. 4A —Images of 30-year-old male volunteer show comparison of axial reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38 with 2D FSE (TR/TEeff, 4,000/38). All images used fat suppression. Coronal source image of 3D FSE XETA at 1.5 T with 0.7-mm slice thickness.

 

Figure 10
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  		Fig. 4B —Images of 30-year-old male volunteer show comparison of axial reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38 with 2D FSE (TR/TEeff, 4,000/38). All images used fat suppression. Axial 3D FSE XETA reformation with 0.7-mm slice thickness.

 

Figure 11
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  		Fig. 4C —Images of 30-year-old male volunteer show comparison of axial reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38 with 2D FSE (TR/TEeff, 4,000/38). All images used fat suppression. Axial 3D FSE XETA reformation with four slices averaged to a 3-mm slice thickness.

 

Figure 12
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  		Fig. 4D —Images of 30-year-old male volunteer show comparison of axial reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38 with 2D FSE (TR/TEeff, 4,000/38). All images used fat suppression. Axial 2D FSE image with 3-mm slice thickness.

 

Figure 13
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  		Fig. 4E —Images of 30-year-old male volunteer show comparison of axial reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38 with 2D FSE (TR/TEeff, 4,000/38). All images used fat suppression. Axial reformation from coronal 2D FSE acquisition shows poor image quality due to relatively thick slices and slice gaps.

 
Images from our volunteer with knee pain revealed a meniscal tear in the posterior horn of the medial meniscus (Fig. 5A, 5B, 5C). This was well shown on both the 2D FSE and 3D FSE XETA images. In the coronal plane, the tear was visible on two images from the 2D FSE acquisition but in 12 images from the 3D FSE XETA acquisition. The meniscal tear was also visible on the sagittal and axial reformations of the 3D FSE XETA images but not on the axial 2D FSE images because of the relatively thick slices.


Figure 14
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  		Fig. 5A —27-year-old man with meniscal tear. Coronal 2D fast spin-echo (FSE) image (TR/TEeff, 4,000/38) shows tear (arrow).

 

Figure 15
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  		Fig. 5B —27-year-old man with meniscal tear. Coronal 3D FSE extended echo-train acquisition (XETA) image (TR/TEeff, 2,500/38). Tear (arrow) was visible on two images of 2D FSE acquisition and 12 of coronal 3D FSE XETA images.

 

Figure 16
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  		Fig. 5C —27-year-old man with meniscal tear. Sagittal reformation of 3D FSE XETA data set shows tear (arrow).

 

Multiple and oblique planar reformations are possible, showing relevant anatomy such as the attachment of Wrisberg's ligament to the posterior horn of the lateral meniscus [26, 27] (Fig. 6A, 6B, 6C). Finally, in one volunteer we acquired 3D FSE XETA at a short TR of 1.5 T to acquire T1-weighted images (Fig. 7A, 7B). The short TR allowed us to use two signal averages on this acquisition.


Figure 17
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  		Fig. 6A —31-year-old male volunteer. Images show use of isotropic resolution to define anatomy. Axial (A) and oblique sagittal (B) reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38). Red line in A shows plane of oblique sagittal reformation in B.

 

Figure 18
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  		Fig. 6B —31-year-old male volunteer. Images show use of isotropic resolution to define anatomy. Axial (A) and oblique sagittal (B) reformations of 3D fast spin-echo (FSE) extended echo-train acquisition (XETA) (TR/TEeff, 2,500/38). Red line in A shows plane of oblique sagittal reformation in B.

 

Figure 19
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  		Fig. 6C —31-year-old male volunteer. Images show use of isotropic resolution to define anatomy. Oblique coronal reformation, plane of which is shown by blue lines in A and B. This image shows attachment of Wrisberg's ligament to posterior horn of lateral meniscus (arrow).

 

Figure 20
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  		Fig. 7A —T1-weighted images of knee in 30-year-old male volunteer using 3D fast spin-echo (FSE) extended echotrain acquisition (XETA) (TR/TEeff, 800/24) at 1.5 T. Coronal source image with 0.7 mm isotropic resolution (A) and sagittal reformation (B) show relative T1-weighting.

 

Figure 21
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  		Fig. 7B —T1-weighted images of knee in 30-year-old male volunteer using 3D fast spin-echo (FSE) extended echotrain acquisition (XETA) (TR/TEeff, 800/24) at 1.5 T. Coronal source image with 0.7 mm isotropic resolution (A) and sagittal reformation (B) show relative T1-weighting.

 


Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Three-dimensional XETA with half-Fourier acquisition and ARC parallel imaging provides high-quality isotropic images of the knee with similar contrast to 2D FSE. Isotropic data from 3D FSE XETA allow for reformations in arbitrary planes, making multiple 2D acquisitions unnecessary. The in-plane resolution of 3D FSE XETA (625 µ) is low compared with current clinical protocols (350-625 µ), but overall voxel size is quite small (0.273 mm3). Cartilage and muscle SNR were lower in both 2D FRFSE and 2D FSE compared with 3D FSE XETA. One explanation is the inherent higher SNR in 3D acquisitions. Another explanation is the magnetization transfer effect, which lowers the signal in cartilage and muscle in 2D FSE and 2D FRFSE acquisitions due to irradiation of out-of-slice spins with slice selection pulses [7, 28]. Fluid signal was higher for 2D FSE than 3D FSE XETA, likely due to the longer TR of 2D FSE.

Fluid-cartilage CNR was higher for 2D FSE and 2D FRFSE than for 3D FSE XETA. This is likely due to a combination of the longer TR used in 2D FSE and the lower cartilage SNR in 2D FSE and 2D FRFSE compared with 3D FSE XETA. Cartilage signal in 2D FSE and 2D FRFSE is likely lower due to magnetization transfer effects from the 2D acquisition [7, 28]. The lower magnetization transfer effect with 3D FSE XETA may result in less conspicuity for surface articular cartilage lesions [28]. The lower cartilage CNR with 3D FSE XETA may also result in more difficulty with articular cartilage segmentation. The decreased slice thickness with 3D FSE XETA and ability to reformat the images may overcome these disadvantages. Further study is needed to assess the sensitivity of 3D FSE XETA to cartilage lesions and its ability to provide accurate segmentation.

One advantage of 3D FSE XETA is the ability to reformat images into arbitrary planes. Compared with images acquired directly in the reformation plane with 2D FSE, the reformatted 3D FSE XETA images were similar in depiction of anatomy, with much thinner slices. Reformations in multiple planes could increase the time required to examine the images, although images could be reformatted automatically in the reconstruction or in post-processing before being presented to the radiologist. Reformations of the 2D FSE images are not routinely obtained owing to the relatively thick slices with interslice gaps.

The use of fat suppression with 3D FSE XETA does not change the imaging time for this sequence or affect the anatomic coverage. This is because the majority of time per TR is spent in signal recovery rather than acquiring data from adjacent slices. In 2D FSE and 2D FRFSE, the addition of fat suppression resulted in fewer slices and less anatomic coverage.

Our study has several limitations. We used normal volunteers rather than subjects with internal derangements of the knee. Only one subject with an abnormality was studied, and the meniscal tear was well seen on both 2D FSE and 3D FSE XETA images. However, the tear was seen on many more slices with 3D FSE XETA and was well shown in sagittal and axial reformations. The minimum TEeff of 3D FSE XETA may reduce sensitivity to some meniscal tears, but the increased spatial resolution in the slice direction may make meniscal tears more visible. The minimum TEeff may be lower in future studies with improvements in gradient hardware or software.

This study shows the potential for using 3D FSE XETA in normal volunteers. Future studies are required to show the diagnostic accuracy of 3D FSE XETA compared with routine sequences for internal derangements. The robustness and flexibility of 3D FSE XETA in showing a variety of clinical conditions is also important for clinical use.

The scanning time for 3D FSE XETA was longer than those for 2D FSE and 2D FRFSE, but this may be offset by the ability to do reformations in multiple planes, eliminating the need for multiple 2D acquisitions. We compared a 3D sequence with parallel imaging to two 2D techniques without parallel imaging, so it may be possible to further reduce the scanning times on the 2D methods with parallel imaging at the expense of SNR. No motion artifacts were noted on the 3D FSE XETA images, but the longer scanning times make 3D FSE XETA more susceptible to motion artifacts than 2D FSE. Finally, the current implementation of 3D FSE XETA does not have a method for preventing phase wrap in the slice and phase-encoding directions. This may be improved in the future using special radiofrequency pulses or no phase-wrap techniques [29]. For this study, we acquired the entire volume in the phase-encoding directions (anterior to posterior and left to right) to prevent phase wrap.

Three-dimensional FSE XETA may enable rapid isotropic imaging of the knee with volumetric data for diagnosis of any relevant derangements. This would likely include coregistered volumes with fat-suppressed T2 weighting (8 minutes) and T1 weighting (5 minutes) for a total imaging time of 13 minutes. This could improve clinical efficiency compared with our present 19-minute knee protocol using multiple planes of 2D FSE. Furthermore, the ability to view images at arbitrary slice thicknesses and in oblique and curved planes may improve depiction of anatomy and diagnosis of abnormality.


References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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