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High resolution breath-holding MR imaging of the abdomen with a phased-array multicoil
CMIG 257
Pergamon
Computerized Medical Imaging and Graphics
Computerized Medical Imaging and Graphics 22 (1998) 301–308
High resolution breath-holding MR imaging of the abdomen with a phased-array multicoil Tomohiro Namimoto*, Yasuyuki Yamashita, Hiroaki Yamamoto, Yasuko Abe, Katsuhiko Mitsuzaki, Mutsumasa Takahashi Department of Radiology, Kumamoto University School of Medicine, 1-1-1 Honjo, Kumamoto 860-8551, Japan Received 19 December 1997; revised 18 June 1998
Abstract We prospectively compared standard resolution and high resolution breath-hold T1- and T2-weighted images of the upper abdomen with use of a body phased-array multicoil in 30 patients. The image quality of high resolution T1-weighted FLASH sequence was equal to that of standard resolution sequence, while the quality of high resolution T2-weighted turbo spin-echo sequence was slightly inferior to that of standard resolution sequence. The merit of high resolution image is appreciated especially on a T1-weighted FLASH sequence. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: Liver MR; Pulse sequences; Rapid imaging; High resolution imaging; Comparative study
1. Introduction Various recent modifications of spin-echo (SE) imaging, including turbo spin-echo (TSE), fast low-angle shot (FLASH), and T1-weighted magnetization-prepared gradient-recalled echo such as turbo-FLASH, permit a reduction in imaging time, allowing acquisitions in several sections in one breath-hold. However, a specific limitation of magnetic resonance (MR) imaging relative to computed tomography (CT) for abdominal studies is the lower spatial resolution, with the image matrix of MR images typically being 128 ⫻ 256 as contrasted with 512 ⫻ 512 for CT. In imaging with a small pixel size, the signal-to-noise ratio (S/N) has been significantly reduced with a body coil. Phased-array multicoil systems for volume imaging were recently developed [1–3]. Such coils are composed of a number of surface coils oriented to image a large volume with the improved S/N characteristics of smaller coils. By the application of fast MR imaging with use of a phased-array multicoil, good quality abdominal MR images can be obtained in a single breath-hold [4–7]. However, the use of such coils for the high resolution abdominal MR imaging has not been studied. In the present study, we compared, by quantitative and qualitative measures, the effect of the matrix size in the frequency-encoding direction (160 ⫻ 512 high resolution and 160 ⫻ 256 standard resolution) with FLASH and TSE * Corresponding author. Tel.: +81-96-344-2111; Fax: +81-96-362-4330
0895-6111/98/$19.00 䉷 1998 Elsevier Science Ltd. All rights reserved PII: S0 89 5 -6 1 11 ( 98 ) 00 0 35 - 4
images that were obtained with use of a phased-array multicoil system in one breath-hold in patient with suspected focal abdominal lesions. 2. Materials and methods 2.1. Patients Thirty consecutive patients (16 men and 14 women, 26– 84 years old, mean age 62.9 years) referred for abdominal MR imaging underwent both standard and high resolution MR imaging studies. They had a total of 35 focal lesions: 28 hepatic lesions (12 hepatocellular carcinomas, seven cavernous hemangiomas, six metastases, and three cholangiocarcinomas), five pancreatic lesions (three pancreatic carcinomas, two cystadenomas), one renal cell carcinoma, and one congenital choledochal cyst. Diagnosis of the lesions was based mainly on the histological findings (operation in 14 and biopsy in nine). In the cavernous hemangiomas and choledochal cyst, the diagnoses were based on characteristic morphological findings and signal intensities on MR images. 2.2. Imaging protocol Imaging was performed with a 1.5-T superconductive magnet (Siemens Magnetom Vision). All patients
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underwent both standard resolution (160 [phase] ⫻ 256 [frequency] matrix) and high resolution (160 [phase] ⫻ 512 [frequency] resolution) MR imaging followed by imaging with non-breath-hold spin echo sequences. All patients underwent subsequent dynamic and postcontrast study following administration of 0.1 mmol/kg Gd-DTPA. In the postcontrast study, begun 5 min after injection of GdDTPA, we used the same standard resolution and high resolution T1-weighted FLASH sequence MR imaging that we used for precontrast imaging. For data analysis, the standard resolution images and high resolution images were compared for each of breath-hold precontrast FLASH, breath-hold postcontrast FLASH, and breath-hold TSE sequences. Conventional standard resolution SE T1weighted and T2-weighted images were also analyzed for comparison. Table 1 lists the imaging parameters used in each of the pulse sequences. In order to obtain images in one breathhold, the high resolution imaging was carried out by increasing the resolution along the frequency axis, while the phase encoding steps were necessarily limited, resulting in non-square pixels. For all examinations, a rectangular field of view of 213 [phase] ⫻ 340 [frequency] mm, with a matrix of 160 ⫻ 256 or 160 ⫻ 512, a section thickness of 8–10 mm and a gap of 20% were used in all acquisitions. In the event that the breath-hold sequence did not include the entire liver, the same sequence was repeated. Identical section locations were used in all acquisitions (subject to any inconsistencies in breath hold). The imaging time was 18 s for the standard resolution T1-weighted FLASH sequence, 19 s for the high resolution T1-weighted FLASH sequence, 14 s for the standard resolution T2-weighted TSE sequence, 26 s for the high resolution T2-weighted TSE sequence, 2 min 31 s for the conventional T1-weighted SE images, and 4 min 35 s for the conventional T2-weighted SE images.
2.3. Image analysis For quantitative analysis, an electronic cursor was used for measurements of regions of interest (ROI) in some of the abdominal organs (liver, pancreas and spleen) and lesions. Noise was measured on each image by using an electronic cursor positioned in a space free from artifact outside and lateral to the body, approximately at the level of the tumor. This position was selected because noise varies considerably with position on multicoil-acquired images. The 5/8 rectangular field of view in the anterioposterior direction was adequate for measurement of the noise. For other measurements, the size of the cursor was chosen to include a large representative portion of the organ or lesion. The values of the signal intensities (SI) of the organs were divided by the corresponding standard deviation (SD) of noise to derive the S/N. The contrast-to-noise ratio (C/N) was calculated as C/N ¼ (Lesion SI ¹ Organ SI)/noise SD. Liver-to-spleen contrast (L–S) was calculated as L–S ¼ (Liver SI ¹ Spleen SI)/noise SD were statistically analyzed. The quantitative measurements were analyzed by a matched-pair t-test. Qualitative assessments were made separately by two investigators blind to the imaging parameters. In the event of disagreement, consensus was obtained in conference. A five-rank scale was used in the standard resolution, high resolution, and conventional SE images for the visualization of small anatomic details such as peripheral vasculature structures in the liver, the corticomedullary junction in the kidney, and the common pancreatic duct, and for the lesion conspicuity for cystic and solid lesions. Artifacts, including respiratory ghost, vascular pulsation, susceptibility, and chemical-shift misregistration, and lesion definition were also compared. Scores were defined as follows: fine details considerably ( ¹ 2) or slightly ( ¹ 1) more clearly visualized on the standard resolution images than on the high
Table 1 Pulse sequence parameters T1-weighted SE
Repetition time (ms) Echo time (ms) Flip angle (deg.) No. of acquisitions Frequency oversampling Phase oversampling No. of sections Matrix (phase ⫻ frequency) Matrix size (mm) Bandwidth (Hz per pixel) No. of presaturations Imaging time per section Total imaging time
580 14 90 1* 100% 100% 13 160 ⫻ 256 1.66 ⫻ 1.33 150 2 11.6 s 2 min 31 s
T2-weighted SE
2000 20/80 90 1* 100% 100% 17 160 ⫻ 256 1.77 ⫻ 1.33 20 2 16.2 s 4 min 35 s
T1-weighted FLASH
T2-weighted TSE
Standard resolution High resolution
Standard resolution High resolution (ETL ¼ 15) (ETL ¼ 15)
150 4.8 75 1 100% 0% 12 160 ⫻ 256 2.04 ⫻ 1.03 195 2 2.6 s 18 s
2000 120 90 1 100% 0% 7 160 ⫻ 256 2.04 ⫻ 1.33 130 2 2.0 s 14 s
150 6.0 75 1 100% 0% 7 160 ⫻ 512 2.04 ⫻ 0.66 195 0 2.7 s 19 s
2000 120 90 1 100% 0% 7 160 ⫻ 512 2.04 ⫻ 0.66 130 2 3.7 s 26 s
In all sequences, the field of view was 340 ⫻ 5/8 (rectangular matrix) and the section thickness was 8–10 mm with distance factor of 0.2. *Number of acquisitions was equivalent to two due to phase oversampling. ETL: echo train length.
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Table 2 Signal-to-noise ratio of abdominal organs in each of the eight pulse sequences Liver Precontrast T1-weighted image Standard resolution FLASH High resolution FLASH Standard resolution spin echo Postcontrast T1-weighted image Standard resolution FLASH High resolution FLASH T2-weighted image Standard resolution FLASH High resolution FLASH Standard resolution spin echo
Spleen
Pancreas
67.00 ⫾ 16.2 28.2 ⫾ 5.7* 29.7 ⫾ 11.3*
57.3 ⫾ 15.1 25.0 ⫾ 5.0* 28.3 ⫾ 10.4*
70.2 ⫾ 17.9 28.6 ⫾ 8.5* 32.7 ⫾ 5.5*
82.0 ⫾ 24.3 32.8 ⫾ 7.1*
93.1 ⫾ 23.5 37.2 ⫾ 7.3*
85.6 ⫾ 29.1 36.9 ⫾ 8.7*
14.4 ⫾ 5.7 9.8 ⫾ 3.3 19.5 ⫾ 15.1*
26.2 ⫾ 10.2 22.9 ⫾ 8.4 46.5 ⫾ 31.2
20.3 ⫾ 11.2 14.8 ⫾ 4.6 32.8 ⫾ 21.2
*Value statistically different at p ⬍ 0.01 compared with that of standard resolution FLASH or TSE breath-hold images.
3. Results
significantly lower than those for the conventional SE sequence. Those for the high resolution TSE sequence were comparable to those for the standard resolution TSE and significantly lower than those for the conventional SE sequence. (Table 2). The liver–spleen, lesion–liver and lesion–pancreas C/N values for the high resolution FLASH sequence were significantly smaller than those for the standard resolution FLASH sequence and comparable to those for the conventional SE sequence. The lesion–liver C/N for the high resolution TSE sequence was significantly lower that for the standard resolution TSE or conventional SE sequences. The liver–spleen contrast of the high resolution TSE sequence was comparable to that for the standard resolution TSE sequence (Table 3).
3.1. Quantitative analysis
3.2. Qualitative analysis
The quantitative data for S/N and C/N are summarized in Table 2, Table 3, respectively. The highest S/N values for the liver, spleen and pancreas were obtained with the T1weighted standard resolution FLASH sequence. The S/N values for the high resolution FLASH sequence were comparable to those for the conventional SE sequence. Those for the standard resolution TSE sequence were
The results of the qualitative comparisons among the sequences are summarized in this section.
resolution images; details equivalent on the high resolution and the standard resolution images (0); details slightly (1) or considerably (2) more clearly visualized on the high resolution than on the standard resolution images. The maximum number of lesions evaluated in any one patient was three. Overall image quality was evaluated as poor, fair, good, or excellent, with numerical scores of 0, 1, 2, and 3, respectively, assigned to these terms. Statistical significance of each of the qualitative criteria was determined by the nonparametric Wilcoxon signed-rank test. The presence and type of artifacts were also evaluated.
3.2.1. Overall image quality (Table 4) The overall image quality scores of the images acquired using each of the eight sequences are listed in Table 4. The overall image quality of the high resolution FLASH
Table 3 Contrast-to-noise ratio between lesions and organs in each of the eight pulse sequences Liver–spleen Precontrast T1-weighted image Standard resolution FLASH High resolution FLASH Standard resolution spin echo Postcontrast T1-weighted image Standard resolution FLASH High resolution FLASH T2-weighted image Standard resolution TSE High resolution TSE Standard resolution spin echo
Lesion–liver
Lesion–pancreas
8.6 ⫾ 7.7 3.5 ⫾ 3.7* 2.7 ⫾ 5.5*
¹12.3 ⫾ 22.7 ¹3.5 ⫾ 8.6* ¹7.8 ⫾ 14.2
¹19.7 ⫾ 28.1 ¹5.3 ⫾ 15.6* ¹4.0 ⫾ 7.2*
¹16.4 ⫾ 23.0 ¹4.2 ⫾ 4.8*
2.7 ⫾ 19.2 1.5 ⫾ 7.0
¹13.4 ⫾ 9.1 ¹13.8 ⫾ 7.7 ¹29.0 ⫾ 22.8*
15.4 ⫾ 14.3 9.3 ⫾ 7.3* 19.1 ⫾ 12.2
¹26.9 ⫾ 40.4 ¹12.4 ⫾ 3.5* 14.8 ⫾ 16.8 3.2 ⫾ 9.6* 3.8 ⫾ 33.9*
*Value statistically different at p ⬍ 0.01 compared with that of standard resolution FLASH or TSE breath-hold images.
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Table 4 Qualitative assignment of the image quality of eight sequences Overall image quality
T1-weighted
Postcontrast T1-weighted FLASH
FLASH Standard resolution Excellent ( ¼ 3) Good ( ¼ 2) Fair ( ¼ 1) Poor ( ¼ 0) Average score Standard deviation
2 19 6 3 1.67 0.75
SE High resolution 5 14 7 4 1.67 0.92
TSE Standard resolution
1 0 10 11 1.00* 0.90
T2-weighted SE
High resolution Standard resolution
2 11 9 4 1.42 0.86
3 9 10 4 1.42 0.90
High resolution
2 14 4 5 1.52 0.91
5 5 8 5 1.43 1.07
0 4 10 4 1.00* 0.68
*Value statistically different at p ⬍ 0.01 compared with that of standard resolution FLASH or TSE breath-hold images.
sequence was equivalent to that of the standard resolution FLASH sequence in both precontrast and postcontrast studies. That of the high resolution TSE sequence was slightly inferior to that of the standard resolution TSE sequence. Both standard resolution and high resolution FLASH and TSE sequences were superior to conventional spin echo sequences (p ⬍ 0.01). 3.2.2. Depiction of anatomical details, lesion conspicuity and contrast (Table 5) In T1-weighted sequences, the anatomical details were significantly (p ⬍ 0.05) more clearly depicted on the high resolution FLASH sequence (Fig. 1). The lesion conspicuity and contrast for all lesions were similar for the high resolution and standard resolution of FLASH sequences. On postcontrast imaging, the organ depiction and lesion conspicuity on the high resolution FLASH sequence were significantly superior to those of the standard resolution FLASH sequence (p ⬍ 0.05) (Fig. 2). In T2-weighted sequences, the depiction of anatomic details was similar in all T2-weighted pulse sequences (Figs 1, 3 and 4). The conspicuity of lesions was slightly better on the high resolution TSE sequence than on the standard resolution TSE sequence (Fig. 4). However, the lesion contrast was slightly better on the standard resolution TSE sequence than on the high resolution TSE sequence.
3.2.3. Artifacts (Table 6) In T1-weighted FLASH sequences, respiratory ghost due to subcutaneous fat was rarely seen in either the standard resolution or the high resolution sequences. Ringing artifact was occasionally seen on both precontrast and postcontrast standard resolution FLASH sequences. These artifacts were less prominent on the high resolution FLASH sequences. Equivalent susceptibility artifacts associated with bowel gas or metallic clip were observed on the standard resolution and high resolution FLASH sequences. Chemical shift misregistration artifact was more severe on the standard resolution FLASH sequence. In T2-weighted TSE sequences, respiratory ghost was more prominent than on the FLASH sequences, especially on the high resolution TSE sequence. This artifact affected the overall image quality in the majority of cases. However, ringing artifact was less prominent on the high resolution TSE sequence. Susceptibility artifact was not seen in any T2-weighted sequence. Chemical shift artifact was more frequently seen on the conventional T2-weighted SE sequence.
4. Discussion Ideally, the resolution of a MR imaging sequence is maximized while the S/N is maintained near a certain threshold
Table 5 Qualitative comparison of high resolution images with standard resolution images Precontrast T1-weighted FLASH Normal anatomy Lesion conspicuity (overall) Cystic Solid Lesion contrast
0.91 ⫾ 0.29* 0.24 ⫾ 0.94 0.00 ⫾ 1.00 0.23 ⫾ 0.86 ¹0.12 ⫾ 0.70
Postcontrast T1-weighted FLASH 0.76 ⫾ 0.66* 0.66 ⫾ 0.77* 0.57 ⫾ 0.98 0.68 ⫾ 0.72* 0.28 ⫾ 0.70
T2-weighted TSE ¹0.04 ⫾ 0.92 0.10 ⫾ 1.01 0.14 ⫾ 0.90 0.09 ⫾ 1.06 ¹0.24 ⫾ 0.79
Values indicate average score obtained by comparison between high resolution image and standard resolution image as described in the methods section. A positive value indicates that the high resolution images show better image quality, and a negative value that the standard resolution images do. *The high resolution images show significantly (p ⬍ 0.05) better image quality than the standard resolution images.
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Fig. 1. Images of microcystic adenoma of the pancreas: (A) T1-weighted standard resolution FLASH image; (B) T1-weighted high resolution FLASH image; (C) T2-weighted standard resolution TSE image; (D) T2-weighted high resolution TSE image. T1-weighted FLASH images show a cystic tumor in the head of pancreas (arrow). On the high resolution images, septums within the cyst are seen. The main pancreatic duct is also clearly visualized in this sequence (arrowheads).
level in a reasonable imaging time. However, this is rarely achievable, as increasing one factor inevitably reduces one or both of the other two. There are many trade-offs when selecting imaging parameters. With recent advances in MR pulse sequences such as gradient echo or turbo spin echo sequences, the high speed T1-weighted or T2-weighted MR images can be obtained in one breath-hold [4–7]. Decreased scan time permits the acquisition of more data, which can translate into higher spatial resolution, shorter imaging times, or both. However, the decreased voxel size and decreased scan time with the higher resolution sequence usually degrade image quality [8]. With use of a newly developed phased-array multicoil, the S/N is improved so that high resolution images can be obtained in one breathhold. The phased-array multicoil-acquired images allow better lesion detection, higher S/N and C/N, better lesion conspicuity, and better depiction of many intraabdominal structures [9]. The high resolution images have potential advantages other than improved spatial resolution. Chemical shift artifact is diminished in the high resolution images in direct inverse proportion to the resolution of frequency-encode direction [10]. Although we did not find significant
differences, susceptibility artifact is theoretically reduced with the high resolution images due to reduced intravoxel dephasing [11]. Ringing artifact is also reduced due to decreased S/N values on the high resolution images [12], [13]. The major disadvantage of the high resolution imaging is the reduction in S/N, which is proportional to the voxel size. The in-plane resolution of the high resolution images in this study was 2.04 mm (frequency) ⫻ 0.66 mm (phase). This represents a decrease in voxel size of close to a factor of 2 compared with standard resolution images, and a factor of 2.5 and 2.7 compared with conventional T1- and T2weighted SE images. The decrease in S/N in high resolution imaging results in a reduction in C/N. In the quantitative measurements of tissue contrast, the high resolution sequences were significantly inferior to the standard resolution images. The advantages of high resolution images are especially noticeable on the postcontrast T1-weighted FLASH sequence. Fine details such as septums, small vessels and tumor margins were more clearly depicted than on standard resolution images. In T1-weighted FLASH sequences, sufficient S/N was obtained on standard resolution images, resulting in acceptable S/N on high resolution images,
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Fig. 2. Images of hepatoma: (A) postcontrast T1-weighted standard resolution FLASH image; (B) postcontrast T1-weighted high resolution FLASH image. Both images show an invasive tumor involving the majority of the right lobe of the liver. The left lobe is also involved by the tumor. Ringing artifacts are more severe on the standard resolution FLASH image (arrow). Some small lesions can be detected only on the high resolution image (arrowheads).
although C/N for the high resolution FLASH sequence was half that for the standard resolution FLASH sequences. In spite of the decrease in the C/N, lesion conspicuity and depiction of the details of organ structures were improved on the high resolution FLASH sequences. The S/N values in the high resolution T2-weighted TSE sequence were often not sufficiently high. The lesion–liver and lesion–pancreas C/N values for the high resolution TSE sequence were inferior to those for the standard resolution TSE sequence, although the two sequences showed similar lesion conspicuity. Therefore, due to the decreased S/N in the high resolution images, the utility of high resolution images varies among different pulse sequences. The overall image quality of the high resolution sequence was comparable to that of the standard resolution sequence on the T1-weighted FLASH sequence. The image quality of the T2-weighted high resolution TSE sequence was slightly poorer than that of the standard resolution TSE sequence mainly due to the respiratory ghosting artifact. This artifact was more marked on the high resolution TSE sequence because the
Fig. 3. Images of fatty liver and cyst: (A) T2-weighted standard resolution TSE image; (B) T2-weighted high resolution TSE image. Both images clearly show a cystic lesion in the right lobe of the liver. The cyst and peripheral vasculatures in the liver are more clearly visualized on the high resolution TSE sequence due to decrease in the chemical shift and ringing artifact. Respiratory ghost is minimal in both images. The image quality of the high resolution image is superior to that of the standard resolution image.
imaging time for this sequence (26 s) is longer than that for the standard resolution TSE sequence (14 s). Use of multicoil imaging can result in image degradation due to the increased signal from subcutaneous fat immediately adjacent to the surface coils. We conclude that the high resolution FLASH sequence described can provide diagnostically useful T1-weighted images. The sequence has moderate S/N, high spatial resolution, and good image quality, and may therefore be very useful for the detection of small abnormalities as well as the determination of their extent. The high resolution FLASH sequence provides overall image quality similar to that of the standard resolution FLASH sequence and superior to that of the T1-weighted SE sequence. The high resolution T2-weighted TSE sequence results in moderate lesion conspicuity with improved spatial resolution and decreased ringing artifact as compared with the standard resolution TSE sequence. However, the tissue contrast between abdominal organs or between organs and lesions is limited
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5. Summary
Fig. 4. Images of choledochal cyst: (A) T2-weighted standard resolution TSE image; (B) T2-weighted high resolution TSE image. A cystic lesion and peripheral vasculature in the liver are more clearly visualized on the high resolution TSE sequence. A wall structure of cystic duct (arrow) is clearly visualized on the high resolution image. Respiratory ghost is prominent on the standard resolution image.
compared with that in the standard resolution TSE sequence, and respiratory ghost is more prominent on the high resolution TSE sequence. Therefore, the high resolution T2-weighted TSE breath-hold technique can be used as part of a comprehensive breath-hold examination in some patients with small lesions in the abdomen.
We prospectively compared standard resolution and high resolution breath-hold T1-weighted fast low-angle shot (FLASH) and T2-weighted turbo spin echo (TSE) imaging with use of a phased-array multicoil system. Thirty consecutive patients referred for abdominal MR imaging underwent both standard and high resolution MR imaging studies. They had a total of 35 focal lesions: 28 hepatic lesions, five pancreatic lesions, one renal cell carcinoma, and one congenital choledochal cyst. Imaging was performed with a 1.5-T superconductive magnet (Siemens Magnetom Vision). All patients underwent both standard resolution (160 [phase] ⫻ 256 [frequency] matrix) and high resolution (160 [phase] ⫻ 512 [frequency] resolution) MR imaging followed by imaging with non-breathhold spin echo sequences. In the postcontrast study after injection of Gd-DTPA, we used the same standard resolution and high resolution T1-weighted FLASH sequence MR imaging that we used for precontrast imaging. For data analysis, the standard resolution images and high resolution images were compared for each of breathhold precontrast FLASH, breath-hold postcontrast FLASH, and breath-hold TSE sequences. For quantitative analysis, the signal-to-noise ratio (S/N), the contrast-tonoise ratio (C/N) and liver-to-spleen contrast (L–S) was calculated. On both T1-weighted and T2-weighted sequences, the standard resolution images showed better S/N than high resolution images. The C/N values for the high resolution FLASH and TSE sequence were significantly smaller than those for the standard resolution FLASH and TSE sequences, respectively. The liver–spleen contrast of the high resolution TSE sequence was comparable to that for the standard resolution TSE sequence. In T1-weighted sequences, the anatomical details were significantly more clearly depicted on the high resolution FLASH sequence (p ⬍ 0.05). The lesion conspicuity and contrast for all lesions were similar for the high resolution and standard resolution of FLASH sequences. On postcontrast imaging, the organ depiction and lesion conspicuity on the high
Table 6 Numbers of artifacts noted for each sequence Artifact
T1-weighted
Postcontrast T1-weighted FLASH
FLASH Standard resolution Respiratory ghost Ringing Pulsation Susceptibility Chemical shift
SE
T2-weighted TSE
High resolution
Standard resolution
High resolution Standard resolution
SE High resolution
1
2
23
3
5
16
21
16
27 6 3 6
8 25 3 2
5 1 2 0
17 6 7 8
7 15 3 2
10 5 0 2
4 9 0 0
3 3 0 9
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resolution FLASH sequence were significantly superior to those of the standard resolution FLASH sequence (p ⬍ 0.05). The overall image quality was slightly better on the high resolution FLASH sequence than on the standard resolution FLASH sequence. In T2-weighted sequences, the depiction of anatomic details was similar in all T2-weighted pulse sequences. The conspicuity of lesions was slightly better on the high resolution TSE sequence than on the standard resolution TSE sequence. However, the lesion contrast was slightly better on the standard resolution TSE sequence than on the high resolution TSE sequence. The overall image quality of the high resolution TSE sequence was slightly inferior to that of the standard resolution TSE sequence. The high resolution FLASH sequence described can provide diagnostically useful T1-weighted images, and can be used in routine examinations of the upper abdomen. The high resolution T2-weighted TSE breath-hold technique is not recommended for the routine protocol, rather this technique may be advantageous in instances where better depiction of a lesion is needed.
[10]
[11]
[12]
[13]
Ehman RL, Riederer SJ. MR imaging of the abdomen with a phased-array multicoil: prospective clinical evaluation. Radiology, 1995;195:769–776. Smith AL, Weinstein MA, Hurst GC, DeRemer DR, Cole RA, Duchesneau PM. Intracranial chemical-shift artifacts on MR images of the brain: observations and relation to sampling bandwidth. AJR, 1990;154:1275–1283. Young IR. The benefits of increasing spatial resolution as a means of reducing artifacts due to field inhomogeneities. Magn Reson Imaging, 1988;6:585–590. Semelka RC, Shoenut JP, Kroeker MA et al. Focal liver disease: comparison of dynamic contrast-enhanced CT and T2-weighted fat suppressed, FLASH, and dynamic gadolinium-enhanced MR imaging at 1.5T. Radiology, 1992;184:687–694. Constable RT, Henkelman RM. Data extrapolation for truncation artifact removal. Magn Reson Med, 1991;17:108–118.
Tomohiro Namimoto, M.D., graduated and received his M.D. degree from Kagawa Medical Collage, Japan in 1992. He was a resident in radiology at Kumamoto Rousai Hospital in Yatsushiro, Kumamoto from 1993 to 1995. He is currently a researcher at Kumamoto University. His research interest is the abdominal and genito-urinary MR imaging.
Yasuyuki Yamashita, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1981. He is currently an Associated Professor of Radiology at Kumamoto University, Japan.
References [1] Roemer PB, Edelstein WA, Hays CE, Souza SP, Mueller OM. The NMR phased array. Magn Reson Med, 1990;16:192–205. [2] Hayes CE, Hattes N, Roemer PB. Volume imaging with MR phased arrays. Magn Reson Med, 1991;18:181–191. [3] Hayes CE, Dietz MJ, King BF, Ehman RL. Pelvic imaging with phased-array coils: quantitative assessment of signal-to-noise ratio improvement. JMRI, 1992;3:323–326. [4] Semelka RC, Shoenut JP, Kroeker RM. T2-weighted MR imaging of focal hepatic lesions: comparison of various RARE and fat-suppressed spin-echo sequences. JMRI, 1993;3:323–327. [5] Semelka RC, Simm FC, Recht M, Deimling M, Lenz G, Laub GA. T1weighted sequence for MR imaging of the liver: comparison of three techniques for single-breath, whole volume acquisition at 1.0 and 1.5 T. Radiology, 1991;180:629–635. [6] Rydberg JN, Lomas DJ, Coakley KJ, Hough DM, Ehman R, Riederer SJ. Comparison of breath-hold fast spin-echo and conventional spinecho pulse sequences for T2-weighted MR imaging of liver lesions. Radiology, 1995;194:431–437. [7] Low RN, Francis IR, Sigeti JS, Foo TK. Abdominal MR imaging: comparison of T2-weighted fast and conventional spin-echo, and contrast-enhanced fast multiplanar spoiled gradient-recalled imaging. Radiology, 1993;186:803–811. [8] Smith RC, Reinhold C, McCauley TR et al. Multicoil high-resolution fast spin-echo MR imaging of the female pelvis. Radiology, 1992;184:671–675. [9] Campeau NG, Johnson CD, Felmlee JP, Rydberg FN, Butts RK,
Hiroaki Yamamoto, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1990. He is currently a Director in radiology at Izumi City Hospital in Izumi, Kagoshima since 1995. His research interest is abdominal MR imaging.
Yasuko Abe, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1993. She was a resident in radiology at Arao City Hospital in Arao, Kumamoto from 1994 to 1995. She is currently a researcher at Kumamoto University. Her research interest is the abdominal MR imaging.
Katsuhiko Mitsuzaki, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1991. He was a resident in radiology at Kumamoto Rousai Hospital in Yatsushiro, Kumamoto from 1992 to 1994. He is currently a researcher at Kumamoto University. His research interest is abdominal MR imaging and interventional radiology.
Mutsumasa Takahashi, M.D., graduated and received his M.D. degree from Kyushu University, Japan in 1960. He was a Professor of Radiology at Akita University, Japan from 1973 to 1980. He is currently a Professor of Radiology at Kumamoto University, Japan.
Pergamon
Computerized Medical Imaging and Graphics
Computerized Medical Imaging and Graphics 22 (1998) 301–308
High resolution breath-holding MR imaging of the abdomen with a phased-array multicoil Tomohiro Namimoto*, Yasuyuki Yamashita, Hiroaki Yamamoto, Yasuko Abe, Katsuhiko Mitsuzaki, Mutsumasa Takahashi Department of Radiology, Kumamoto University School of Medicine, 1-1-1 Honjo, Kumamoto 860-8551, Japan Received 19 December 1997; revised 18 June 1998
Abstract We prospectively compared standard resolution and high resolution breath-hold T1- and T2-weighted images of the upper abdomen with use of a body phased-array multicoil in 30 patients. The image quality of high resolution T1-weighted FLASH sequence was equal to that of standard resolution sequence, while the quality of high resolution T2-weighted turbo spin-echo sequence was slightly inferior to that of standard resolution sequence. The merit of high resolution image is appreciated especially on a T1-weighted FLASH sequence. 䉷 1998 Elsevier Science Ltd. All rights reserved Keywords: Liver MR; Pulse sequences; Rapid imaging; High resolution imaging; Comparative study
1. Introduction Various recent modifications of spin-echo (SE) imaging, including turbo spin-echo (TSE), fast low-angle shot (FLASH), and T1-weighted magnetization-prepared gradient-recalled echo such as turbo-FLASH, permit a reduction in imaging time, allowing acquisitions in several sections in one breath-hold. However, a specific limitation of magnetic resonance (MR) imaging relative to computed tomography (CT) for abdominal studies is the lower spatial resolution, with the image matrix of MR images typically being 128 ⫻ 256 as contrasted with 512 ⫻ 512 for CT. In imaging with a small pixel size, the signal-to-noise ratio (S/N) has been significantly reduced with a body coil. Phased-array multicoil systems for volume imaging were recently developed [1–3]. Such coils are composed of a number of surface coils oriented to image a large volume with the improved S/N characteristics of smaller coils. By the application of fast MR imaging with use of a phased-array multicoil, good quality abdominal MR images can be obtained in a single breath-hold [4–7]. However, the use of such coils for the high resolution abdominal MR imaging has not been studied. In the present study, we compared, by quantitative and qualitative measures, the effect of the matrix size in the frequency-encoding direction (160 ⫻ 512 high resolution and 160 ⫻ 256 standard resolution) with FLASH and TSE * Corresponding author. Tel.: +81-96-344-2111; Fax: +81-96-362-4330
0895-6111/98/$19.00 䉷 1998 Elsevier Science Ltd. All rights reserved PII: S0 89 5 -6 1 11 ( 98 ) 00 0 35 - 4
images that were obtained with use of a phased-array multicoil system in one breath-hold in patient with suspected focal abdominal lesions. 2. Materials and methods 2.1. Patients Thirty consecutive patients (16 men and 14 women, 26– 84 years old, mean age 62.9 years) referred for abdominal MR imaging underwent both standard and high resolution MR imaging studies. They had a total of 35 focal lesions: 28 hepatic lesions (12 hepatocellular carcinomas, seven cavernous hemangiomas, six metastases, and three cholangiocarcinomas), five pancreatic lesions (three pancreatic carcinomas, two cystadenomas), one renal cell carcinoma, and one congenital choledochal cyst. Diagnosis of the lesions was based mainly on the histological findings (operation in 14 and biopsy in nine). In the cavernous hemangiomas and choledochal cyst, the diagnoses were based on characteristic morphological findings and signal intensities on MR images. 2.2. Imaging protocol Imaging was performed with a 1.5-T superconductive magnet (Siemens Magnetom Vision). All patients
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underwent both standard resolution (160 [phase] ⫻ 256 [frequency] matrix) and high resolution (160 [phase] ⫻ 512 [frequency] resolution) MR imaging followed by imaging with non-breath-hold spin echo sequences. All patients underwent subsequent dynamic and postcontrast study following administration of 0.1 mmol/kg Gd-DTPA. In the postcontrast study, begun 5 min after injection of GdDTPA, we used the same standard resolution and high resolution T1-weighted FLASH sequence MR imaging that we used for precontrast imaging. For data analysis, the standard resolution images and high resolution images were compared for each of breath-hold precontrast FLASH, breath-hold postcontrast FLASH, and breath-hold TSE sequences. Conventional standard resolution SE T1weighted and T2-weighted images were also analyzed for comparison. Table 1 lists the imaging parameters used in each of the pulse sequences. In order to obtain images in one breathhold, the high resolution imaging was carried out by increasing the resolution along the frequency axis, while the phase encoding steps were necessarily limited, resulting in non-square pixels. For all examinations, a rectangular field of view of 213 [phase] ⫻ 340 [frequency] mm, with a matrix of 160 ⫻ 256 or 160 ⫻ 512, a section thickness of 8–10 mm and a gap of 20% were used in all acquisitions. In the event that the breath-hold sequence did not include the entire liver, the same sequence was repeated. Identical section locations were used in all acquisitions (subject to any inconsistencies in breath hold). The imaging time was 18 s for the standard resolution T1-weighted FLASH sequence, 19 s for the high resolution T1-weighted FLASH sequence, 14 s for the standard resolution T2-weighted TSE sequence, 26 s for the high resolution T2-weighted TSE sequence, 2 min 31 s for the conventional T1-weighted SE images, and 4 min 35 s for the conventional T2-weighted SE images.
2.3. Image analysis For quantitative analysis, an electronic cursor was used for measurements of regions of interest (ROI) in some of the abdominal organs (liver, pancreas and spleen) and lesions. Noise was measured on each image by using an electronic cursor positioned in a space free from artifact outside and lateral to the body, approximately at the level of the tumor. This position was selected because noise varies considerably with position on multicoil-acquired images. The 5/8 rectangular field of view in the anterioposterior direction was adequate for measurement of the noise. For other measurements, the size of the cursor was chosen to include a large representative portion of the organ or lesion. The values of the signal intensities (SI) of the organs were divided by the corresponding standard deviation (SD) of noise to derive the S/N. The contrast-to-noise ratio (C/N) was calculated as C/N ¼ (Lesion SI ¹ Organ SI)/noise SD. Liver-to-spleen contrast (L–S) was calculated as L–S ¼ (Liver SI ¹ Spleen SI)/noise SD were statistically analyzed. The quantitative measurements were analyzed by a matched-pair t-test. Qualitative assessments were made separately by two investigators blind to the imaging parameters. In the event of disagreement, consensus was obtained in conference. A five-rank scale was used in the standard resolution, high resolution, and conventional SE images for the visualization of small anatomic details such as peripheral vasculature structures in the liver, the corticomedullary junction in the kidney, and the common pancreatic duct, and for the lesion conspicuity for cystic and solid lesions. Artifacts, including respiratory ghost, vascular pulsation, susceptibility, and chemical-shift misregistration, and lesion definition were also compared. Scores were defined as follows: fine details considerably ( ¹ 2) or slightly ( ¹ 1) more clearly visualized on the standard resolution images than on the high
Table 1 Pulse sequence parameters T1-weighted SE
Repetition time (ms) Echo time (ms) Flip angle (deg.) No. of acquisitions Frequency oversampling Phase oversampling No. of sections Matrix (phase ⫻ frequency) Matrix size (mm) Bandwidth (Hz per pixel) No. of presaturations Imaging time per section Total imaging time
580 14 90 1* 100% 100% 13 160 ⫻ 256 1.66 ⫻ 1.33 150 2 11.6 s 2 min 31 s
T2-weighted SE
2000 20/80 90 1* 100% 100% 17 160 ⫻ 256 1.77 ⫻ 1.33 20 2 16.2 s 4 min 35 s
T1-weighted FLASH
T2-weighted TSE
Standard resolution High resolution
Standard resolution High resolution (ETL ¼ 15) (ETL ¼ 15)
150 4.8 75 1 100% 0% 12 160 ⫻ 256 2.04 ⫻ 1.03 195 2 2.6 s 18 s
2000 120 90 1 100% 0% 7 160 ⫻ 256 2.04 ⫻ 1.33 130 2 2.0 s 14 s
150 6.0 75 1 100% 0% 7 160 ⫻ 512 2.04 ⫻ 0.66 195 0 2.7 s 19 s
2000 120 90 1 100% 0% 7 160 ⫻ 512 2.04 ⫻ 0.66 130 2 3.7 s 26 s
In all sequences, the field of view was 340 ⫻ 5/8 (rectangular matrix) and the section thickness was 8–10 mm with distance factor of 0.2. *Number of acquisitions was equivalent to two due to phase oversampling. ETL: echo train length.
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Table 2 Signal-to-noise ratio of abdominal organs in each of the eight pulse sequences Liver Precontrast T1-weighted image Standard resolution FLASH High resolution FLASH Standard resolution spin echo Postcontrast T1-weighted image Standard resolution FLASH High resolution FLASH T2-weighted image Standard resolution FLASH High resolution FLASH Standard resolution spin echo
Spleen
Pancreas
67.00 ⫾ 16.2 28.2 ⫾ 5.7* 29.7 ⫾ 11.3*
57.3 ⫾ 15.1 25.0 ⫾ 5.0* 28.3 ⫾ 10.4*
70.2 ⫾ 17.9 28.6 ⫾ 8.5* 32.7 ⫾ 5.5*
82.0 ⫾ 24.3 32.8 ⫾ 7.1*
93.1 ⫾ 23.5 37.2 ⫾ 7.3*
85.6 ⫾ 29.1 36.9 ⫾ 8.7*
14.4 ⫾ 5.7 9.8 ⫾ 3.3 19.5 ⫾ 15.1*
26.2 ⫾ 10.2 22.9 ⫾ 8.4 46.5 ⫾ 31.2
20.3 ⫾ 11.2 14.8 ⫾ 4.6 32.8 ⫾ 21.2
*Value statistically different at p ⬍ 0.01 compared with that of standard resolution FLASH or TSE breath-hold images.
3. Results
significantly lower than those for the conventional SE sequence. Those for the high resolution TSE sequence were comparable to those for the standard resolution TSE and significantly lower than those for the conventional SE sequence. (Table 2). The liver–spleen, lesion–liver and lesion–pancreas C/N values for the high resolution FLASH sequence were significantly smaller than those for the standard resolution FLASH sequence and comparable to those for the conventional SE sequence. The lesion–liver C/N for the high resolution TSE sequence was significantly lower that for the standard resolution TSE or conventional SE sequences. The liver–spleen contrast of the high resolution TSE sequence was comparable to that for the standard resolution TSE sequence (Table 3).
3.1. Quantitative analysis
3.2. Qualitative analysis
The quantitative data for S/N and C/N are summarized in Table 2, Table 3, respectively. The highest S/N values for the liver, spleen and pancreas were obtained with the T1weighted standard resolution FLASH sequence. The S/N values for the high resolution FLASH sequence were comparable to those for the conventional SE sequence. Those for the standard resolution TSE sequence were
The results of the qualitative comparisons among the sequences are summarized in this section.
resolution images; details equivalent on the high resolution and the standard resolution images (0); details slightly (1) or considerably (2) more clearly visualized on the high resolution than on the standard resolution images. The maximum number of lesions evaluated in any one patient was three. Overall image quality was evaluated as poor, fair, good, or excellent, with numerical scores of 0, 1, 2, and 3, respectively, assigned to these terms. Statistical significance of each of the qualitative criteria was determined by the nonparametric Wilcoxon signed-rank test. The presence and type of artifacts were also evaluated.
3.2.1. Overall image quality (Table 4) The overall image quality scores of the images acquired using each of the eight sequences are listed in Table 4. The overall image quality of the high resolution FLASH
Table 3 Contrast-to-noise ratio between lesions and organs in each of the eight pulse sequences Liver–spleen Precontrast T1-weighted image Standard resolution FLASH High resolution FLASH Standard resolution spin echo Postcontrast T1-weighted image Standard resolution FLASH High resolution FLASH T2-weighted image Standard resolution TSE High resolution TSE Standard resolution spin echo
Lesion–liver
Lesion–pancreas
8.6 ⫾ 7.7 3.5 ⫾ 3.7* 2.7 ⫾ 5.5*
¹12.3 ⫾ 22.7 ¹3.5 ⫾ 8.6* ¹7.8 ⫾ 14.2
¹19.7 ⫾ 28.1 ¹5.3 ⫾ 15.6* ¹4.0 ⫾ 7.2*
¹16.4 ⫾ 23.0 ¹4.2 ⫾ 4.8*
2.7 ⫾ 19.2 1.5 ⫾ 7.0
¹13.4 ⫾ 9.1 ¹13.8 ⫾ 7.7 ¹29.0 ⫾ 22.8*
15.4 ⫾ 14.3 9.3 ⫾ 7.3* 19.1 ⫾ 12.2
¹26.9 ⫾ 40.4 ¹12.4 ⫾ 3.5* 14.8 ⫾ 16.8 3.2 ⫾ 9.6* 3.8 ⫾ 33.9*
*Value statistically different at p ⬍ 0.01 compared with that of standard resolution FLASH or TSE breath-hold images.
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Table 4 Qualitative assignment of the image quality of eight sequences Overall image quality
T1-weighted
Postcontrast T1-weighted FLASH
FLASH Standard resolution Excellent ( ¼ 3) Good ( ¼ 2) Fair ( ¼ 1) Poor ( ¼ 0) Average score Standard deviation
2 19 6 3 1.67 0.75
SE High resolution 5 14 7 4 1.67 0.92
TSE Standard resolution
1 0 10 11 1.00* 0.90
T2-weighted SE
High resolution Standard resolution
2 11 9 4 1.42 0.86
3 9 10 4 1.42 0.90
High resolution
2 14 4 5 1.52 0.91
5 5 8 5 1.43 1.07
0 4 10 4 1.00* 0.68
*Value statistically different at p ⬍ 0.01 compared with that of standard resolution FLASH or TSE breath-hold images.
sequence was equivalent to that of the standard resolution FLASH sequence in both precontrast and postcontrast studies. That of the high resolution TSE sequence was slightly inferior to that of the standard resolution TSE sequence. Both standard resolution and high resolution FLASH and TSE sequences were superior to conventional spin echo sequences (p ⬍ 0.01). 3.2.2. Depiction of anatomical details, lesion conspicuity and contrast (Table 5) In T1-weighted sequences, the anatomical details were significantly (p ⬍ 0.05) more clearly depicted on the high resolution FLASH sequence (Fig. 1). The lesion conspicuity and contrast for all lesions were similar for the high resolution and standard resolution of FLASH sequences. On postcontrast imaging, the organ depiction and lesion conspicuity on the high resolution FLASH sequence were significantly superior to those of the standard resolution FLASH sequence (p ⬍ 0.05) (Fig. 2). In T2-weighted sequences, the depiction of anatomic details was similar in all T2-weighted pulse sequences (Figs 1, 3 and 4). The conspicuity of lesions was slightly better on the high resolution TSE sequence than on the standard resolution TSE sequence (Fig. 4). However, the lesion contrast was slightly better on the standard resolution TSE sequence than on the high resolution TSE sequence.
3.2.3. Artifacts (Table 6) In T1-weighted FLASH sequences, respiratory ghost due to subcutaneous fat was rarely seen in either the standard resolution or the high resolution sequences. Ringing artifact was occasionally seen on both precontrast and postcontrast standard resolution FLASH sequences. These artifacts were less prominent on the high resolution FLASH sequences. Equivalent susceptibility artifacts associated with bowel gas or metallic clip were observed on the standard resolution and high resolution FLASH sequences. Chemical shift misregistration artifact was more severe on the standard resolution FLASH sequence. In T2-weighted TSE sequences, respiratory ghost was more prominent than on the FLASH sequences, especially on the high resolution TSE sequence. This artifact affected the overall image quality in the majority of cases. However, ringing artifact was less prominent on the high resolution TSE sequence. Susceptibility artifact was not seen in any T2-weighted sequence. Chemical shift artifact was more frequently seen on the conventional T2-weighted SE sequence.
4. Discussion Ideally, the resolution of a MR imaging sequence is maximized while the S/N is maintained near a certain threshold
Table 5 Qualitative comparison of high resolution images with standard resolution images Precontrast T1-weighted FLASH Normal anatomy Lesion conspicuity (overall) Cystic Solid Lesion contrast
0.91 ⫾ 0.29* 0.24 ⫾ 0.94 0.00 ⫾ 1.00 0.23 ⫾ 0.86 ¹0.12 ⫾ 0.70
Postcontrast T1-weighted FLASH 0.76 ⫾ 0.66* 0.66 ⫾ 0.77* 0.57 ⫾ 0.98 0.68 ⫾ 0.72* 0.28 ⫾ 0.70
T2-weighted TSE ¹0.04 ⫾ 0.92 0.10 ⫾ 1.01 0.14 ⫾ 0.90 0.09 ⫾ 1.06 ¹0.24 ⫾ 0.79
Values indicate average score obtained by comparison between high resolution image and standard resolution image as described in the methods section. A positive value indicates that the high resolution images show better image quality, and a negative value that the standard resolution images do. *The high resolution images show significantly (p ⬍ 0.05) better image quality than the standard resolution images.
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Fig. 1. Images of microcystic adenoma of the pancreas: (A) T1-weighted standard resolution FLASH image; (B) T1-weighted high resolution FLASH image; (C) T2-weighted standard resolution TSE image; (D) T2-weighted high resolution TSE image. T1-weighted FLASH images show a cystic tumor in the head of pancreas (arrow). On the high resolution images, septums within the cyst are seen. The main pancreatic duct is also clearly visualized in this sequence (arrowheads).
level in a reasonable imaging time. However, this is rarely achievable, as increasing one factor inevitably reduces one or both of the other two. There are many trade-offs when selecting imaging parameters. With recent advances in MR pulse sequences such as gradient echo or turbo spin echo sequences, the high speed T1-weighted or T2-weighted MR images can be obtained in one breath-hold [4–7]. Decreased scan time permits the acquisition of more data, which can translate into higher spatial resolution, shorter imaging times, or both. However, the decreased voxel size and decreased scan time with the higher resolution sequence usually degrade image quality [8]. With use of a newly developed phased-array multicoil, the S/N is improved so that high resolution images can be obtained in one breathhold. The phased-array multicoil-acquired images allow better lesion detection, higher S/N and C/N, better lesion conspicuity, and better depiction of many intraabdominal structures [9]. The high resolution images have potential advantages other than improved spatial resolution. Chemical shift artifact is diminished in the high resolution images in direct inverse proportion to the resolution of frequency-encode direction [10]. Although we did not find significant
differences, susceptibility artifact is theoretically reduced with the high resolution images due to reduced intravoxel dephasing [11]. Ringing artifact is also reduced due to decreased S/N values on the high resolution images [12], [13]. The major disadvantage of the high resolution imaging is the reduction in S/N, which is proportional to the voxel size. The in-plane resolution of the high resolution images in this study was 2.04 mm (frequency) ⫻ 0.66 mm (phase). This represents a decrease in voxel size of close to a factor of 2 compared with standard resolution images, and a factor of 2.5 and 2.7 compared with conventional T1- and T2weighted SE images. The decrease in S/N in high resolution imaging results in a reduction in C/N. In the quantitative measurements of tissue contrast, the high resolution sequences were significantly inferior to the standard resolution images. The advantages of high resolution images are especially noticeable on the postcontrast T1-weighted FLASH sequence. Fine details such as septums, small vessels and tumor margins were more clearly depicted than on standard resolution images. In T1-weighted FLASH sequences, sufficient S/N was obtained on standard resolution images, resulting in acceptable S/N on high resolution images,
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Fig. 2. Images of hepatoma: (A) postcontrast T1-weighted standard resolution FLASH image; (B) postcontrast T1-weighted high resolution FLASH image. Both images show an invasive tumor involving the majority of the right lobe of the liver. The left lobe is also involved by the tumor. Ringing artifacts are more severe on the standard resolution FLASH image (arrow). Some small lesions can be detected only on the high resolution image (arrowheads).
although C/N for the high resolution FLASH sequence was half that for the standard resolution FLASH sequences. In spite of the decrease in the C/N, lesion conspicuity and depiction of the details of organ structures were improved on the high resolution FLASH sequences. The S/N values in the high resolution T2-weighted TSE sequence were often not sufficiently high. The lesion–liver and lesion–pancreas C/N values for the high resolution TSE sequence were inferior to those for the standard resolution TSE sequence, although the two sequences showed similar lesion conspicuity. Therefore, due to the decreased S/N in the high resolution images, the utility of high resolution images varies among different pulse sequences. The overall image quality of the high resolution sequence was comparable to that of the standard resolution sequence on the T1-weighted FLASH sequence. The image quality of the T2-weighted high resolution TSE sequence was slightly poorer than that of the standard resolution TSE sequence mainly due to the respiratory ghosting artifact. This artifact was more marked on the high resolution TSE sequence because the
Fig. 3. Images of fatty liver and cyst: (A) T2-weighted standard resolution TSE image; (B) T2-weighted high resolution TSE image. Both images clearly show a cystic lesion in the right lobe of the liver. The cyst and peripheral vasculatures in the liver are more clearly visualized on the high resolution TSE sequence due to decrease in the chemical shift and ringing artifact. Respiratory ghost is minimal in both images. The image quality of the high resolution image is superior to that of the standard resolution image.
imaging time for this sequence (26 s) is longer than that for the standard resolution TSE sequence (14 s). Use of multicoil imaging can result in image degradation due to the increased signal from subcutaneous fat immediately adjacent to the surface coils. We conclude that the high resolution FLASH sequence described can provide diagnostically useful T1-weighted images. The sequence has moderate S/N, high spatial resolution, and good image quality, and may therefore be very useful for the detection of small abnormalities as well as the determination of their extent. The high resolution FLASH sequence provides overall image quality similar to that of the standard resolution FLASH sequence and superior to that of the T1-weighted SE sequence. The high resolution T2-weighted TSE sequence results in moderate lesion conspicuity with improved spatial resolution and decreased ringing artifact as compared with the standard resolution TSE sequence. However, the tissue contrast between abdominal organs or between organs and lesions is limited
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5. Summary
Fig. 4. Images of choledochal cyst: (A) T2-weighted standard resolution TSE image; (B) T2-weighted high resolution TSE image. A cystic lesion and peripheral vasculature in the liver are more clearly visualized on the high resolution TSE sequence. A wall structure of cystic duct (arrow) is clearly visualized on the high resolution image. Respiratory ghost is prominent on the standard resolution image.
compared with that in the standard resolution TSE sequence, and respiratory ghost is more prominent on the high resolution TSE sequence. Therefore, the high resolution T2-weighted TSE breath-hold technique can be used as part of a comprehensive breath-hold examination in some patients with small lesions in the abdomen.
We prospectively compared standard resolution and high resolution breath-hold T1-weighted fast low-angle shot (FLASH) and T2-weighted turbo spin echo (TSE) imaging with use of a phased-array multicoil system. Thirty consecutive patients referred for abdominal MR imaging underwent both standard and high resolution MR imaging studies. They had a total of 35 focal lesions: 28 hepatic lesions, five pancreatic lesions, one renal cell carcinoma, and one congenital choledochal cyst. Imaging was performed with a 1.5-T superconductive magnet (Siemens Magnetom Vision). All patients underwent both standard resolution (160 [phase] ⫻ 256 [frequency] matrix) and high resolution (160 [phase] ⫻ 512 [frequency] resolution) MR imaging followed by imaging with non-breathhold spin echo sequences. In the postcontrast study after injection of Gd-DTPA, we used the same standard resolution and high resolution T1-weighted FLASH sequence MR imaging that we used for precontrast imaging. For data analysis, the standard resolution images and high resolution images were compared for each of breathhold precontrast FLASH, breath-hold postcontrast FLASH, and breath-hold TSE sequences. For quantitative analysis, the signal-to-noise ratio (S/N), the contrast-tonoise ratio (C/N) and liver-to-spleen contrast (L–S) was calculated. On both T1-weighted and T2-weighted sequences, the standard resolution images showed better S/N than high resolution images. The C/N values for the high resolution FLASH and TSE sequence were significantly smaller than those for the standard resolution FLASH and TSE sequences, respectively. The liver–spleen contrast of the high resolution TSE sequence was comparable to that for the standard resolution TSE sequence. In T1-weighted sequences, the anatomical details were significantly more clearly depicted on the high resolution FLASH sequence (p ⬍ 0.05). The lesion conspicuity and contrast for all lesions were similar for the high resolution and standard resolution of FLASH sequences. On postcontrast imaging, the organ depiction and lesion conspicuity on the high
Table 6 Numbers of artifacts noted for each sequence Artifact
T1-weighted
Postcontrast T1-weighted FLASH
FLASH Standard resolution Respiratory ghost Ringing Pulsation Susceptibility Chemical shift
SE
T2-weighted TSE
High resolution
Standard resolution
High resolution Standard resolution
SE High resolution
1
2
23
3
5
16
21
16
27 6 3 6
8 25 3 2
5 1 2 0
17 6 7 8
7 15 3 2
10 5 0 2
4 9 0 0
3 3 0 9
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resolution FLASH sequence were significantly superior to those of the standard resolution FLASH sequence (p ⬍ 0.05). The overall image quality was slightly better on the high resolution FLASH sequence than on the standard resolution FLASH sequence. In T2-weighted sequences, the depiction of anatomic details was similar in all T2-weighted pulse sequences. The conspicuity of lesions was slightly better on the high resolution TSE sequence than on the standard resolution TSE sequence. However, the lesion contrast was slightly better on the standard resolution TSE sequence than on the high resolution TSE sequence. The overall image quality of the high resolution TSE sequence was slightly inferior to that of the standard resolution TSE sequence. The high resolution FLASH sequence described can provide diagnostically useful T1-weighted images, and can be used in routine examinations of the upper abdomen. The high resolution T2-weighted TSE breath-hold technique is not recommended for the routine protocol, rather this technique may be advantageous in instances where better depiction of a lesion is needed.
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Tomohiro Namimoto, M.D., graduated and received his M.D. degree from Kagawa Medical Collage, Japan in 1992. He was a resident in radiology at Kumamoto Rousai Hospital in Yatsushiro, Kumamoto from 1993 to 1995. He is currently a researcher at Kumamoto University. His research interest is the abdominal and genito-urinary MR imaging.
Yasuyuki Yamashita, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1981. He is currently an Associated Professor of Radiology at Kumamoto University, Japan.
References [1] Roemer PB, Edelstein WA, Hays CE, Souza SP, Mueller OM. The NMR phased array. Magn Reson Med, 1990;16:192–205. [2] Hayes CE, Hattes N, Roemer PB. Volume imaging with MR phased arrays. Magn Reson Med, 1991;18:181–191. [3] Hayes CE, Dietz MJ, King BF, Ehman RL. Pelvic imaging with phased-array coils: quantitative assessment of signal-to-noise ratio improvement. JMRI, 1992;3:323–326. [4] Semelka RC, Shoenut JP, Kroeker RM. T2-weighted MR imaging of focal hepatic lesions: comparison of various RARE and fat-suppressed spin-echo sequences. JMRI, 1993;3:323–327. [5] Semelka RC, Simm FC, Recht M, Deimling M, Lenz G, Laub GA. T1weighted sequence for MR imaging of the liver: comparison of three techniques for single-breath, whole volume acquisition at 1.0 and 1.5 T. Radiology, 1991;180:629–635. [6] Rydberg JN, Lomas DJ, Coakley KJ, Hough DM, Ehman R, Riederer SJ. Comparison of breath-hold fast spin-echo and conventional spinecho pulse sequences for T2-weighted MR imaging of liver lesions. Radiology, 1995;194:431–437. [7] Low RN, Francis IR, Sigeti JS, Foo TK. Abdominal MR imaging: comparison of T2-weighted fast and conventional spin-echo, and contrast-enhanced fast multiplanar spoiled gradient-recalled imaging. Radiology, 1993;186:803–811. [8] Smith RC, Reinhold C, McCauley TR et al. Multicoil high-resolution fast spin-echo MR imaging of the female pelvis. Radiology, 1992;184:671–675. [9] Campeau NG, Johnson CD, Felmlee JP, Rydberg FN, Butts RK,
Hiroaki Yamamoto, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1990. He is currently a Director in radiology at Izumi City Hospital in Izumi, Kagoshima since 1995. His research interest is abdominal MR imaging.
Yasuko Abe, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1993. She was a resident in radiology at Arao City Hospital in Arao, Kumamoto from 1994 to 1995. She is currently a researcher at Kumamoto University. Her research interest is the abdominal MR imaging.
Katsuhiko Mitsuzaki, M.D., graduated and received his M.D. degree from Kumamoto University, Japan in 1991. He was a resident in radiology at Kumamoto Rousai Hospital in Yatsushiro, Kumamoto from 1992 to 1994. He is currently a researcher at Kumamoto University. His research interest is abdominal MR imaging and interventional radiology.
Mutsumasa Takahashi, M.D., graduated and received his M.D. degree from Kyushu University, Japan in 1960. He was a Professor of Radiology at Akita University, Japan from 1973 to 1980. He is currently a Professor of Radiology at Kumamoto University, Japan.