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Critical evaluation of the quality of synthetic MR images
Mugnertc Resonance Printed in the USA.
Imaging. Vol. 4, pp. 435-436. All righr\ reserved.
1986 Copyright
0
0730-725X/86 1986 Pergamon
$3.00 + .OO Journals Ltd.
l Original Contribution
CRITICAL
EVALUATION
OF THE QUALITY
OF SYNTHETIC
MR IMAGES
ROBERT E. SCH~PFLIN, PETER B~SIGER AND DIETER MEIER Institute of Biomedical Engineering of the University and ETH Zurich, Moussonstrasse CH-8044 Zurich, Switzerland
18,
and the delay time To constant over all segments and to produce the same number of echoes within each segment. The variation of the phase encoding gradient is performed only after data acquisition of an entire set of segments. Consequentially, moving artifacts can be essentially reduced. For a careful evaluation of the results obtained with these new pulse sequences, they were applied to specially designed phantoms in which body conditions are simulated. Bottles in the phantoms are filled with solutions that cover the entire physiological and pathophysiological range of T, and T, values of human body tissue. The “true” T, and T, values were measured by standard MR procedures on a conventional laboratory spectrometer. To achieve high accuracy of the Tl and T2 values, different methods of fitting the data to the exponential decay had to be evaluated. Table 1 shows the comparison between T2 values obtained with this imaging sequence and the corresponding “true” values. Based on the results obtained by executing each segment individually, we expect the new pulse sequence to yield results of the same accuracy also for the Tl values. The final test for the accuracy of the intrinsic MR
of MR images mainly depends on the three MR tissue parameters spin density, Ti and T,, as well as on the pulse sequence applied for data acquisition. The contrast can be varied over a wide range and can be optimized with respect to different diagnostic applications. Synthetic imaging is a technique which permits the computer simulation of images produced by arbitrarily selected pulse sequences. The basic information utilized there are previously measured spin density T, and T2-images. Some advantages and applications of these computer simulations are The tissue contrast
The calculation of an image is faster than its measurement. Contrast can be optimized after patient examination. Test of new pulse sequences is easy. Images can be generated for pulse sequences with parameter settings that are unattainable because of system limitations. The possibility to use synthetic images for educational purposes. The quality of a synthetic image highly depends on the quality of the determination of the tissue parameters (intrinsic parameters). Determination of the tissue parameters with the aid of usually implemented pulse sequences does not yield optimal results. Therefore a new pulse sequence was designed. It allows measurement of T, and T2 simultaneously and mainly improves T, measurements. It consists of one spin echo and a variable number of inversion recovery segments with different delay times. For the parameter determination, it is necessary to keep the difference between the repetition time TR
Table 1. Comparison Measured (ms) 33 * 2 59 + 2 88 + 3 116+4 144 * 5 435
of T2 values “True” (ms) 30 58 86 117 146
436
Magnetic Resonance Imaging 0 Volume 4, Number 5, 1986
values consists in the comparison of in vivo synthetic images with actual image data acquired using the corresponding pulse sequence. We found that in a T, weighted sequence of measured and simulated images with increasing echo time a range of at least six times the original echo time can be covered with good agreement between the measured and the calculated image.
The noise in the synthetic image is reduced in comparison with the noise in the measured image. This fact agrees with theoretical predictions since the calculated image contains the information from more than one measurement. Some smaller details visible in the measured image disappear in the corresponding synthetic image. These artifacts in the synthetic image are caused at least in part by partial volume effects.
Imaging. Vol. 4, pp. 435-436. All righr\ reserved.
1986 Copyright
0
0730-725X/86 1986 Pergamon
$3.00 + .OO Journals Ltd.
l Original Contribution
CRITICAL
EVALUATION
OF THE QUALITY
OF SYNTHETIC
MR IMAGES
ROBERT E. SCH~PFLIN, PETER B~SIGER AND DIETER MEIER Institute of Biomedical Engineering of the University and ETH Zurich, Moussonstrasse CH-8044 Zurich, Switzerland
18,
and the delay time To constant over all segments and to produce the same number of echoes within each segment. The variation of the phase encoding gradient is performed only after data acquisition of an entire set of segments. Consequentially, moving artifacts can be essentially reduced. For a careful evaluation of the results obtained with these new pulse sequences, they were applied to specially designed phantoms in which body conditions are simulated. Bottles in the phantoms are filled with solutions that cover the entire physiological and pathophysiological range of T, and T, values of human body tissue. The “true” T, and T, values were measured by standard MR procedures on a conventional laboratory spectrometer. To achieve high accuracy of the Tl and T2 values, different methods of fitting the data to the exponential decay had to be evaluated. Table 1 shows the comparison between T2 values obtained with this imaging sequence and the corresponding “true” values. Based on the results obtained by executing each segment individually, we expect the new pulse sequence to yield results of the same accuracy also for the Tl values. The final test for the accuracy of the intrinsic MR
of MR images mainly depends on the three MR tissue parameters spin density, Ti and T,, as well as on the pulse sequence applied for data acquisition. The contrast can be varied over a wide range and can be optimized with respect to different diagnostic applications. Synthetic imaging is a technique which permits the computer simulation of images produced by arbitrarily selected pulse sequences. The basic information utilized there are previously measured spin density T, and T2-images. Some advantages and applications of these computer simulations are The tissue contrast
The calculation of an image is faster than its measurement. Contrast can be optimized after patient examination. Test of new pulse sequences is easy. Images can be generated for pulse sequences with parameter settings that are unattainable because of system limitations. The possibility to use synthetic images for educational purposes. The quality of a synthetic image highly depends on the quality of the determination of the tissue parameters (intrinsic parameters). Determination of the tissue parameters with the aid of usually implemented pulse sequences does not yield optimal results. Therefore a new pulse sequence was designed. It allows measurement of T, and T2 simultaneously and mainly improves T, measurements. It consists of one spin echo and a variable number of inversion recovery segments with different delay times. For the parameter determination, it is necessary to keep the difference between the repetition time TR
Table 1. Comparison Measured (ms) 33 * 2 59 + 2 88 + 3 116+4 144 * 5 435
of T2 values “True” (ms) 30 58 86 117 146
436
Magnetic Resonance Imaging 0 Volume 4, Number 5, 1986
values consists in the comparison of in vivo synthetic images with actual image data acquired using the corresponding pulse sequence. We found that in a T, weighted sequence of measured and simulated images with increasing echo time a range of at least six times the original echo time can be covered with good agreement between the measured and the calculated image.
The noise in the synthetic image is reduced in comparison with the noise in the measured image. This fact agrees with theoretical predictions since the calculated image contains the information from more than one measurement. Some smaller details visible in the measured image disappear in the corresponding synthetic image. These artifacts in the synthetic image are caused at least in part by partial volume effects.