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Determination of T1- and T2-relaxation times in the spleen of patients with splenomegaly
Magnerrc Resonance Printed in the USA.
Imaging, Vol. 8. pp. 39-42, All rights reserved.
1990 Copyright
0730-725x/90 $3.00 + .oo 0 1990 Pergamon Press plc
l Original Contribution
DETERMINATION OF Tl- AND T2-RELAXATION TIMES IN THE SPLEEN OF PATIENTS WITH SPLENOMEGALY C. THOMSEN, P. JOSEPHSEN, H. KARLE, E. JUHL, P. GRUNDTVIG SBRENSEN, AND 0. HENRIKSEN Departments of Magnetic Resonance, Haematology, and Hepatology, Hvidovre Hospital, University of Copenhagen, Denmark Twenty-nine patients with known splenomegaly and seven healthy volunteers were examined. The TI and T2 relaxation times were read out from a region of interest centrally in the spleen. Even though different mean Z’rand T2 relaxation times were found between the groups, the great scatter and the considerable overlap between the groups makes the contribution of relaxation time measurements to the differential diagnosis of splenomegaly of limited value. .., Keywords:
MRI; Tissue characterization;
T, and T,; Splenomegaly.
scanner (Magnetom H15) operating at 1.5 Tesla. The spleen was imaged with a respiratory triggered double spin echo sequence (TE = 30, 90 ms) with the repetition time (TR) equal to the respiratory period usually about 3-5 s.12 From one of these slices a 15-mm thick slice through the center of the spleen was chosen for relaxation time measurements. The Tr and 7” measurements were based on six combined Carr-Purcell/Carr-Purcell-Meilbum-Gill multi spin-echo sequences, described in Table 1 .8 The slice thickness was 15 mm and the matrix size 128 x 64 yielding a voxel slice of 4 x 8 x 15 mm3. The cummulated acquisition time for the relaxation time measurements was 30 min including image calculations.
INTRODUCTION In vivo measurements of relaxation processes by magnetic resonance imaging (MRI) may be valuable for noninvasive characterization of various pathologies in the spleen. If so, such measurements would be clinically important in the differential diagnosis of the underlying pathology of splenomegaly. So far, very few reports of relaxation time measurements in normal spleen of human beings have been carried out at low to medium magnetic field strength. ‘,2s,7*9 The purpose of the present study was to evaluate whether relaxation time measurements add to the differential diagnosis of splenomegaly. METHODS Twenty-nine patients with known splenomegaly and seven healthy volunteers were studied. The patients were classified into four categories: (i) Splenomegaly due to portal hypertension: mostly liver cirrhosis (15 patients); (ii) Myeloproliferative disorders: polycytemia Vera, myelofibrosis and chronic myelocytic leukemia (seven patients); (iii) Lymphoproliferative disorders: chronic lymphocytic leukemia and non-Hodgkin lymphoma (three patients); (iv) Benign hyperplasia: Felty’s syndrome and hemolytic anemia (four patients). All patients were examined in a Siemens wholebody
RECEIVED 6/ 18/89; ACCEPTED 7/ 12/89. Address correspondence to Carsten Thomsen,
Table 1. The six multi spin-echo sequences used for relaxataion time measurements TR/s 0.20 0.36 0.66 1.20 2.20 4.00
TE/ms 30,60,90.. .120 30,60,90...240 30,60,90...240 30,60,90. ..240 30,60,90. ..240 30,60,90. ..240
Dept. of Magnetic Resonance, Copenhagen, Denmark.
M.D., 39
Acquisitions 4 4 2 1 1 1
Hvidovre
Time/min 1.71 3.07 2.82 2.56 4.69 8.53
Hospital,
DK-2650
40
Magnetic Resonance Imaging 0 Volume 8, Number 1, 1990
T, and T2 were estimated by a three parameter fit to the signal intensities obtained in a region of interest (ROI) located centrally in the spleen.
Calculations and Statistics In total 44 signal intensities were read out from an ROI. The signal intensities were fitted using a threeparameter least-square fit to the signal expression for a multi spin-echo sequence. S =pe-“TE’T2
1 +f(td)
- 1 + j(-l)Ng(tp)fN-l(te)j
1 - (-l)N-lfN-l(te) -
g(te)
1 +f(te)
where
f(t)
= eet’q and g(t)
where tp = te/2 and td was the delay from the last apulse to the next a/2 pulse, giving TR = tp + (N 1) te + td. N was the number of echoes four or eight and TE was the echo time 30 ms. The accuracy of relaxation time measurements has been tested on the Eurospin relaxation test object.13 The avarage accuracy was found to be 11% and 13% for the calculated TIC,,and T2,,, times, respectively. By applying a linear calibration TlcOr= 140.1 ms + 0.721 T,_, the accuracy of the T, measurement could be improved to 5%.‘* All the values given are the calculated T, and Tz relaxation times, for reference to other units the accuracy can be improved by applying a linear correction. l2 Differences in T, and T2 relaxation times between groups were tested by a nonparametric Kruskal-Wallis test.” Due to the limited number of patients a 10% significant limit was used.
= 1 - e-“rI. RESULTS
The timing of the CPMG was
multi spin-echo
(1r/2 - tp - rl - te - a2 - te -
sequence
...
-?TN-j - te - ?TN- td)
A
The measured T, and T2 relaxation times are summarized in Fig. 1. It is seen that the Tl and T, times showed a considerable scatter even in the group of healthy volunteers. The mean T, time, standard deviation and mean rank are shown in Table 2. A statis-
T2/ms
B
.
. .
. . ,.
. B.
. : .
I
ii
Fig. 1. Calculated myeloproliferative
TI and T2 relaxation times for the five groups studied. N = normal; disorders; L = lymphoproliferative disorders; B = benign hyperplasia.
P = portal
hypertension;
M =
T,- and T,-relaxation
times in the spleen 0 C.
Table 2. Mean T,, standard deviation, and mean rank for the five groups studied Group
T,/ms
Normal Portal hypertension Myeloproliferative Lymphoproliferative Benign hyperplasia
1183 1148 1385 1484 1079
SD/ms 414 243 370 144 46
THOMSEN
41
ET AL.
T
2/ms
Mean rank
Normd
14.9 16.0 25.1 31.3 13.0
Portal + ++ 8% +*
0
hypertension
l
Lymphopdiferative
0
Myeloprolifemtive
0
Benign hyperplasia
A
0
A +O
Table 3. Mean T2, standard
deviation, and mean rank for the five groups studied
Group Normal Portal hypertension Myeloproliferative Lymphoproliferative Benign hyperplasia
T,/ms
SD/ms
Mean rank
87 90 68 91 70
13 17 21 8 28
20.1 21.8 11.0 23.2 12.9 2000
looo
3000
T
tical significant difference between the groups was found (Kruskal-Wallis test p < 0.05). The mean T, time and mean rank were increased in the groups with lymphoproliferative and myeloproliferative disorders, whereas no difference was found between the other groups. The mean T2 time, standard deviation and mean rank are shown in Table 3. A statistically significant difference between the groups was found (KruskalWallis test p < 0.08). The mean T2 and mean rank were decreased in the group of myeloproliferative disorders and benign hyperplasia. A plot of Tl and T2 relaxation times for the five groups is shown in Fig. 2, a considerable overlap between the groups is seen. In all cases magnetic resonance imaging showed detailed anatomical structures of the spleen, Fig. 3, but no differences were found between the groups.
‘/ms
Fig. 2. Combined T, and T2plot for the five groups studied.
DISCUSSION The main result of the present study was that even in the group of healthy volunteers we observed a considerable variation in the calculated T,and T2values in the spleen. The variation cannot be explained by inaccuracies in the measurements due to factors related to the equipment, because control measurements on the Eurospin relaxation time test object showed an accuracy within lo-15% for Tland T2calculations.‘2,13 Another problem is organ movement due to respiration. As previously reported for the liver,” respiratory gating/triggering did not alter the recorded relaxation
Fig. 3. Respiratory triggered transverse image of the liver and spleen. This patient had liver cirrhosis after a hepatitis B infection. Note the large spleen and the dilated vena lienalis.
curves despite a definite improvement in the image quality. For these reasons we believe that the great scatter of the relaxation values obtained in fact reflects a true biological variation. Among possible factors could be large variations in blood content and different paramagnetic ions concentrations.
42
Magnetic Resonance Imaging 0 Volume 8, Number 1, 1990
We observed a prolonged Ti relaxation time in the patients with myelo- and lymphoproliferative disorders compared to the other groups. Unfortunately a great overlap between the groups was found. The T2relaxation time was marginally reduced in the patients with myeloproliferative disorders and in the patients with benign hyperpalsia, but the overlap between the groups was considerable. Combining the T, and Tz relaxation times increased the separation between the groups (Fig. 2), so the myeloproliferative disorders were centred down to the right. But even taking both Tland T2relaxation times into account no clear separation was found. Only few studies on in vivo relaxation times measurements in human spleens of healthy human beings have been published so far.2*s,7,9 For the Tlvalues a direct comparison between those and our present study is difficult due to differences in the magnetic field strength. Generally we observed larger T,values in agreement with the higher field strength of 1.5 Tesla compared to 0.35-0.50 Tesla. Our T,values are in reasonable agreement with those reported by Ehman et al.’ and Nyman et a1.,9 whereas the compiled T2 data from various human and animal studies reviewed by Bottomley et al.* are somewhat shorter. To our knowledge no studies on T, and T2 relaxation times in enlarged human spleen have been reported so far. In all our patients we found an enlarged spleen, but no focal lesions were seen.3.4 These findings were in accordance with Nyman et a1.,9 where MRI only showed focal lesions in one out of ten spleens with non-Hodgkin’s lymphoma infiltrations. CONCLUSIONS Magnetic resonance imaging of the human spleen gives detailed topographic and macroscopic anatomical information when respiratory gating/triggering is used. Even though different mean T,and T2relaxation times were found between the groups, the great scatter and the considerably overlap between the groups makes the contribution of relaxation time measurements to the differential diagnosis of splenomegaly of limited value. Such measurements may, however, still be clinically valuable in monitoring various forms of treatment.
Improvement of MR-tissue characterization by including information on tissue metabolism based on volume selective spectroscopy may be of clinical value in the differential diagnosis of splenomegaly. REFERENCES 1. Alder, D.D.; Glazer, G.M.; Aisen, A.M. MRI of the spleen: Normal appearance and findings in sickle-cell anemia. AJR 147:843; 1986. 2. Bottomley, P.A.; Foster, T.H.; Argersinger, R.E.; Pfeifer, L.M. A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from l-100 MHz: Dependence on tissue type, NMR frequency, temperature, species, excision, and age. Med. P&s. 11:425; 1984. 3. Hess, C.F.; Griebel, J.; Schmiedl, U.; Kurtz, B.; Koelbel, G.; Jaehde, E. Focal lesions of the spleen: Preliminary results with fast MR imaging at 1.5 T. J. Comput. Assist. Tornogr. 12:569; 1988. 4. Cohen, M.D.; Klatte, E.C.; Smith, J .A., et al. Magnetic resonance imaging of lymphomas in children. Pediatr. Radiol. 15:179; 1985. 5. Ehman, R.L.; Kjos, B.O.; Hricak, H.; Brasch, R.C.; Higgins, C.B. Relative intensity of abdominal organs in MR images. J. Comput. Assist. Tomogr. 138:315; 1985. R.; Stark, D.D.; Saini, S.; 6. Hahn, P.F.; Weissleder, Elizondo, G.; Ferrucci, J.T. MR imaging of focal splenic tumors. AJR 150:823; 1988. J.R.; Reid, A.; Hutchison, I. Smith, F.W.; Mallard, J.M.S. Nuclear magnetic resonance tomographic imaging in liver disease. Lancer, i:963; 1981. se8. Maudsley, A.A. Modified Carr-Purcell-Meibom-Gill quence for NMR Fourier imaging applications. J. Magn. Reson. 69:488; 1986. 9. Nyman R.; Rhen S.; Ericsson A., et al. An attempt to characterize malignant lymphoma in spleen, liver and lymph nodes with magnetic resonance imaging. Acta Radio/. 28:527; 1987. 10. Nyman, R.; Rehn, S.; Glimelius B., et al. Magnetic resonance imaging, chest radiography, computed tomography and ultrasonography in malignant lymphoma. Acta Radiol. 28:l; 1987. statistic for the behavioral sci11. Siegel, S. Nonparametric ences. London: McGraw-Hill; 1956. 12. Thomsen, C.; Henriksen, 0.; Ring, P. In vivo measurements of relaxation process in the human liver by MRI. The role of respiratory gating/triggering. Magn. Reson. Imaging 6:43 1; 1988. 13 Thomsen, C.; Jensen, K.E.; Jensen, M.; Rubaek-Olsen, E.; Henriksen, 0. MR pulse sequences for selective relaxation time measurements. A phantom study. Magn. Reson. Imaging 8:43-50; 1990.
Imaging, Vol. 8. pp. 39-42, All rights reserved.
1990 Copyright
0730-725x/90 $3.00 + .oo 0 1990 Pergamon Press plc
l Original Contribution
DETERMINATION OF Tl- AND T2-RELAXATION TIMES IN THE SPLEEN OF PATIENTS WITH SPLENOMEGALY C. THOMSEN, P. JOSEPHSEN, H. KARLE, E. JUHL, P. GRUNDTVIG SBRENSEN, AND 0. HENRIKSEN Departments of Magnetic Resonance, Haematology, and Hepatology, Hvidovre Hospital, University of Copenhagen, Denmark Twenty-nine patients with known splenomegaly and seven healthy volunteers were examined. The TI and T2 relaxation times were read out from a region of interest centrally in the spleen. Even though different mean Z’rand T2 relaxation times were found between the groups, the great scatter and the considerable overlap between the groups makes the contribution of relaxation time measurements to the differential diagnosis of splenomegaly of limited value. .., Keywords:
MRI; Tissue characterization;
T, and T,; Splenomegaly.
scanner (Magnetom H15) operating at 1.5 Tesla. The spleen was imaged with a respiratory triggered double spin echo sequence (TE = 30, 90 ms) with the repetition time (TR) equal to the respiratory period usually about 3-5 s.12 From one of these slices a 15-mm thick slice through the center of the spleen was chosen for relaxation time measurements. The Tr and 7” measurements were based on six combined Carr-Purcell/Carr-Purcell-Meilbum-Gill multi spin-echo sequences, described in Table 1 .8 The slice thickness was 15 mm and the matrix size 128 x 64 yielding a voxel slice of 4 x 8 x 15 mm3. The cummulated acquisition time for the relaxation time measurements was 30 min including image calculations.
INTRODUCTION In vivo measurements of relaxation processes by magnetic resonance imaging (MRI) may be valuable for noninvasive characterization of various pathologies in the spleen. If so, such measurements would be clinically important in the differential diagnosis of the underlying pathology of splenomegaly. So far, very few reports of relaxation time measurements in normal spleen of human beings have been carried out at low to medium magnetic field strength. ‘,2s,7*9 The purpose of the present study was to evaluate whether relaxation time measurements add to the differential diagnosis of splenomegaly. METHODS Twenty-nine patients with known splenomegaly and seven healthy volunteers were studied. The patients were classified into four categories: (i) Splenomegaly due to portal hypertension: mostly liver cirrhosis (15 patients); (ii) Myeloproliferative disorders: polycytemia Vera, myelofibrosis and chronic myelocytic leukemia (seven patients); (iii) Lymphoproliferative disorders: chronic lymphocytic leukemia and non-Hodgkin lymphoma (three patients); (iv) Benign hyperplasia: Felty’s syndrome and hemolytic anemia (four patients). All patients were examined in a Siemens wholebody
RECEIVED 6/ 18/89; ACCEPTED 7/ 12/89. Address correspondence to Carsten Thomsen,
Table 1. The six multi spin-echo sequences used for relaxataion time measurements TR/s 0.20 0.36 0.66 1.20 2.20 4.00
TE/ms 30,60,90.. .120 30,60,90...240 30,60,90...240 30,60,90. ..240 30,60,90. ..240 30,60,90. ..240
Dept. of Magnetic Resonance, Copenhagen, Denmark.
M.D., 39
Acquisitions 4 4 2 1 1 1
Hvidovre
Time/min 1.71 3.07 2.82 2.56 4.69 8.53
Hospital,
DK-2650
40
Magnetic Resonance Imaging 0 Volume 8, Number 1, 1990
T, and T2 were estimated by a three parameter fit to the signal intensities obtained in a region of interest (ROI) located centrally in the spleen.
Calculations and Statistics In total 44 signal intensities were read out from an ROI. The signal intensities were fitted using a threeparameter least-square fit to the signal expression for a multi spin-echo sequence. S =pe-“TE’T2
1 +f(td)
- 1 + j(-l)Ng(tp)fN-l(te)j
1 - (-l)N-lfN-l(te) -
g(te)
1 +f(te)
where
f(t)
= eet’q and g(t)
where tp = te/2 and td was the delay from the last apulse to the next a/2 pulse, giving TR = tp + (N 1) te + td. N was the number of echoes four or eight and TE was the echo time 30 ms. The accuracy of relaxation time measurements has been tested on the Eurospin relaxation test object.13 The avarage accuracy was found to be 11% and 13% for the calculated TIC,,and T2,,, times, respectively. By applying a linear calibration TlcOr= 140.1 ms + 0.721 T,_, the accuracy of the T, measurement could be improved to 5%.‘* All the values given are the calculated T, and Tz relaxation times, for reference to other units the accuracy can be improved by applying a linear correction. l2 Differences in T, and T2 relaxation times between groups were tested by a nonparametric Kruskal-Wallis test.” Due to the limited number of patients a 10% significant limit was used.
= 1 - e-“rI. RESULTS
The timing of the CPMG was
multi spin-echo
(1r/2 - tp - rl - te - a2 - te -
sequence
...
-?TN-j - te - ?TN- td)
A
The measured T, and T2 relaxation times are summarized in Fig. 1. It is seen that the Tl and T, times showed a considerable scatter even in the group of healthy volunteers. The mean T, time, standard deviation and mean rank are shown in Table 2. A statis-
T2/ms
B
.
. .
. . ,.
. B.
. : .
I
ii
Fig. 1. Calculated myeloproliferative
TI and T2 relaxation times for the five groups studied. N = normal; disorders; L = lymphoproliferative disorders; B = benign hyperplasia.
P = portal
hypertension;
M =
T,- and T,-relaxation
times in the spleen 0 C.
Table 2. Mean T,, standard deviation, and mean rank for the five groups studied Group
T,/ms
Normal Portal hypertension Myeloproliferative Lymphoproliferative Benign hyperplasia
1183 1148 1385 1484 1079
SD/ms 414 243 370 144 46
THOMSEN
41
ET AL.
T
2/ms
Mean rank
Normd
14.9 16.0 25.1 31.3 13.0
Portal + ++ 8% +*
0
hypertension
l
Lymphopdiferative
0
Myeloprolifemtive
0
Benign hyperplasia
A
0
A +O
Table 3. Mean T2, standard
deviation, and mean rank for the five groups studied
Group Normal Portal hypertension Myeloproliferative Lymphoproliferative Benign hyperplasia
T,/ms
SD/ms
Mean rank
87 90 68 91 70
13 17 21 8 28
20.1 21.8 11.0 23.2 12.9 2000
looo
3000
T
tical significant difference between the groups was found (Kruskal-Wallis test p < 0.05). The mean T, time and mean rank were increased in the groups with lymphoproliferative and myeloproliferative disorders, whereas no difference was found between the other groups. The mean T2 time, standard deviation and mean rank are shown in Table 3. A statistically significant difference between the groups was found (KruskalWallis test p < 0.08). The mean T2 and mean rank were decreased in the group of myeloproliferative disorders and benign hyperplasia. A plot of Tl and T2 relaxation times for the five groups is shown in Fig. 2, a considerable overlap between the groups is seen. In all cases magnetic resonance imaging showed detailed anatomical structures of the spleen, Fig. 3, but no differences were found between the groups.
‘/ms
Fig. 2. Combined T, and T2plot for the five groups studied.
DISCUSSION The main result of the present study was that even in the group of healthy volunteers we observed a considerable variation in the calculated T,and T2values in the spleen. The variation cannot be explained by inaccuracies in the measurements due to factors related to the equipment, because control measurements on the Eurospin relaxation time test object showed an accuracy within lo-15% for Tland T2calculations.‘2,13 Another problem is organ movement due to respiration. As previously reported for the liver,” respiratory gating/triggering did not alter the recorded relaxation
Fig. 3. Respiratory triggered transverse image of the liver and spleen. This patient had liver cirrhosis after a hepatitis B infection. Note the large spleen and the dilated vena lienalis.
curves despite a definite improvement in the image quality. For these reasons we believe that the great scatter of the relaxation values obtained in fact reflects a true biological variation. Among possible factors could be large variations in blood content and different paramagnetic ions concentrations.
42
Magnetic Resonance Imaging 0 Volume 8, Number 1, 1990
We observed a prolonged Ti relaxation time in the patients with myelo- and lymphoproliferative disorders compared to the other groups. Unfortunately a great overlap between the groups was found. The T2relaxation time was marginally reduced in the patients with myeloproliferative disorders and in the patients with benign hyperpalsia, but the overlap between the groups was considerable. Combining the T, and Tz relaxation times increased the separation between the groups (Fig. 2), so the myeloproliferative disorders were centred down to the right. But even taking both Tland T2relaxation times into account no clear separation was found. Only few studies on in vivo relaxation times measurements in human spleens of healthy human beings have been published so far.2*s,7,9 For the Tlvalues a direct comparison between those and our present study is difficult due to differences in the magnetic field strength. Generally we observed larger T,values in agreement with the higher field strength of 1.5 Tesla compared to 0.35-0.50 Tesla. Our T,values are in reasonable agreement with those reported by Ehman et al.’ and Nyman et a1.,9 whereas the compiled T2 data from various human and animal studies reviewed by Bottomley et al.* are somewhat shorter. To our knowledge no studies on T, and T2 relaxation times in enlarged human spleen have been reported so far. In all our patients we found an enlarged spleen, but no focal lesions were seen.3.4 These findings were in accordance with Nyman et a1.,9 where MRI only showed focal lesions in one out of ten spleens with non-Hodgkin’s lymphoma infiltrations. CONCLUSIONS Magnetic resonance imaging of the human spleen gives detailed topographic and macroscopic anatomical information when respiratory gating/triggering is used. Even though different mean T,and T2relaxation times were found between the groups, the great scatter and the considerably overlap between the groups makes the contribution of relaxation time measurements to the differential diagnosis of splenomegaly of limited value. Such measurements may, however, still be clinically valuable in monitoring various forms of treatment.
Improvement of MR-tissue characterization by including information on tissue metabolism based on volume selective spectroscopy may be of clinical value in the differential diagnosis of splenomegaly. REFERENCES 1. Alder, D.D.; Glazer, G.M.; Aisen, A.M. MRI of the spleen: Normal appearance and findings in sickle-cell anemia. AJR 147:843; 1986. 2. Bottomley, P.A.; Foster, T.H.; Argersinger, R.E.; Pfeifer, L.M. A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from l-100 MHz: Dependence on tissue type, NMR frequency, temperature, species, excision, and age. Med. P&s. 11:425; 1984. 3. Hess, C.F.; Griebel, J.; Schmiedl, U.; Kurtz, B.; Koelbel, G.; Jaehde, E. Focal lesions of the spleen: Preliminary results with fast MR imaging at 1.5 T. J. Comput. Assist. Tornogr. 12:569; 1988. 4. Cohen, M.D.; Klatte, E.C.; Smith, J .A., et al. Magnetic resonance imaging of lymphomas in children. Pediatr. Radiol. 15:179; 1985. 5. Ehman, R.L.; Kjos, B.O.; Hricak, H.; Brasch, R.C.; Higgins, C.B. Relative intensity of abdominal organs in MR images. J. Comput. Assist. Tomogr. 138:315; 1985. R.; Stark, D.D.; Saini, S.; 6. Hahn, P.F.; Weissleder, Elizondo, G.; Ferrucci, J.T. MR imaging of focal splenic tumors. AJR 150:823; 1988. J.R.; Reid, A.; Hutchison, I. Smith, F.W.; Mallard, J.M.S. Nuclear magnetic resonance tomographic imaging in liver disease. Lancer, i:963; 1981. se8. Maudsley, A.A. Modified Carr-Purcell-Meibom-Gill quence for NMR Fourier imaging applications. J. Magn. Reson. 69:488; 1986. 9. Nyman R.; Rhen S.; Ericsson A., et al. An attempt to characterize malignant lymphoma in spleen, liver and lymph nodes with magnetic resonance imaging. Acta Radio/. 28:527; 1987. 10. Nyman, R.; Rehn, S.; Glimelius B., et al. Magnetic resonance imaging, chest radiography, computed tomography and ultrasonography in malignant lymphoma. Acta Radiol. 28:l; 1987. statistic for the behavioral sci11. Siegel, S. Nonparametric ences. London: McGraw-Hill; 1956. 12. Thomsen, C.; Henriksen, 0.; Ring, P. In vivo measurements of relaxation process in the human liver by MRI. The role of respiratory gating/triggering. Magn. Reson. Imaging 6:43 1; 1988. 13 Thomsen, C.; Jensen, K.E.; Jensen, M.; Rubaek-Olsen, E.; Henriksen, 0. MR pulse sequences for selective relaxation time measurements. A phantom study. Magn. Reson. Imaging 8:43-50; 1990.