Mogneric Resonance Imaging, Vol. 10, pp. 579-584, Printed in the USA. All rights reserved.
1992 Copyright 0
0730-725x/92 $5.00 + .w 1992 Pergamon Press Ltd.
l Original Contribution
IN VIVO EVALUATION OF THE REPRODUCIBILITY OF TXAND T2 MEASURED IN THE BRAIN OF PATIENTS WITH MULTIPLE SCLEROSIS H . B. W. LARSSON , * P. CHRISTIANSEN, * I,, ZEEBERG,~
AND 0.
HENRIKSEN*
*Danish Research Center of Magnetic Resonance at Hvidovre, University Hospital, Copenhagen, Denmark TDepartment of Neurology, Vejle Hospital, Denmark The precision (reproducibility) of relaxation times derived from magnetic resonance images of patients with multiple sclerosis (MS) were investigated. Measurements of 10 MS patients were performed at 1.5 T on two occasions within 1 wk. 2’~and T2 was measured using a partial saturation inversion recovery sequence (6 points) and a CarrPurcell-Meiboom-Gill phase alternating-phase shift multiple spin-echo sequence with 32 echoes. Regions of interest (ROI) were placed both in apparently normal white matter and plaques. The precision (k1.96 SD) and the confidence intervals for Tl and T2 for white matter and plaques were calculated. The precision of T, for white matter and plaques was respectively f94 msec and *208 msec. The precision of T2 for white matter and plaques was respectively *18 msec and *26 msec. For all measurements the coefficient of variation was about 9%. Judging from our own study and others as well, a precision better than 10% for Tl and T2 would seem unrealistic at present.
Keywords: MRI; Multiple sclerosis; Relaxation time; Reproducibility.
of imprekision and inaccuracy. The aim of the present study was therefore to evaluate the precision of measured values of Tl and T2 in plaques and in white
Several previous studies have shown that measurements of relaxation times (T, and Tz) in plaques due to multiple sclerosis (MS) may give valuable information concerning edema, demyelination, and gliosis.‘-4 Similarly, investigations of induced experimental allergic encephalomyelitis in animals (a model of MS) suggest that relaxation times may provide valuable information.*-* The relaxation times have also been used to monitor the efficacy of treatments of patients with MS.9**oA number of studies have focused on the accuracy and the precision (reproducibility) of measurements of relaxation times in phantoms2~“-‘6 and in the brain of healthy subjects.‘7*‘8 To our knowledge, the in vivo precision of relaxation times measured in MS plaques has not previously been reported in the literature. The estimation of the in vivo precision can be relevant for evaluation of the usefulness of T, and T2in monitoring the disease during treatment trials. Furthermore, in a previous study of MS patients, we observed a large scatter of T, and T2 of MS plaques, even between plaques in the same patient.2*3 The question is whether this large scatter reflects a true biophysical variation between plaques, or is the result
RECEIVED 6/26/91; ACCEPTED 3/17/92. Address correspondence to Henrik B.W. Larsson,
matter of patients with MS. MATERIAL
AND
METHODS
Patients
Measurements of T, and T2 were performed in ten patients with definite MS. The mean age of the patients was 42 yr (&8 yr) and the mean duration of the disease was 14 yr (+5 yr). All patients had a steady progressive MS form. None had had an acute attack within at least one year prior to the study. All the patients participated in a double blind, placebo-controlled study of the efficacy of a-interferon in slowing down disease progression. Two measurements within 1 wk were performed at the beginning of the placebo period and these results are presented in this study. One measurement was performed at the end of the placebo period, that is, after 3 mo. Two measurements within 1 wk were also performed at the beginning of the non-placebo period and one measurement was per-
Danish Research Center of Magnetic Resonance, Hvidovre University Hospital, DK-2650 Copenhagen, Denmark.
MD, 519
580
Magnetic Resonance Imaging 0 Volume 10, Number 4, 1992
formed at the end of this period. The results related to the possible effect of a-interferon on the relaxation times will be published separately. Imaging and Repositioning The study was carried out on a whole-body MRscanner (Siemens Magnetom H15), operating at 1.5 T. The brain was imaged in the axial plane in the head coil using a double spin-echo sequence with a repetition time (TR) of 1.8 set, and echo times (TE) of 30 and 90 msec. The slice thickness was 4 mm, and a matrix size of 256 x 256 was used, giving a voxel size of 1.2 x 1.2 x 4 mm3. Twelve slices with an interslice spacing of 4 mm were acquired. Subsequently, the sequence was repeated, imaging the interslice space. A single slice at the level of the lateral ventricular system was then selected with the aim of measuring the relaxation times. Four plastic test tubes with CuS04 solutions at different concentrations had been mounted in the head coil, perpendicular to the axial plane and served as external standard probes for the measurements. The variation of the temperature in the scanner was between 19” and 21”, giving a variation of T, and T2 of less than 6% and 2070,respectively. Repositioning of the patients was achieved by use of sagittal images onto which grids were superimposed. Using these sagittal images and the sagittal images obtained previously, the position of the head was then changed until the angulation was reproduced. The exact position was then determined using the axial images obtained from the double spin-echo sequence actually and formerly. Relaxation Time Measurements Tl was measured by a partial saturation inversion recovery sequence (2). The inversion time (TI) was constant at 150 msec. The TE of the 180” rephasing pulse was 30 msec. The TR varied from 0.24 to 6.0 set (six data points). T2 was measured by a Carr-PurcellMeiboom-Gill (CPMG) phase alternating/phase shift multiple spin-echo sequence with 32 echoes.” The echoes were recorded at 30-msec intervals from 30 to 960 msec. The TR was 4 sec. Cafculation and Statistics Based on the sequential scan images generated, the MR signals were read out from regions of interest (ROIs) in white frontal matter, and MS plaques. The regional areas consisted of 8-20 pixels in the white matter, and of 18-168 pixels in the plaques. Each ROI was carefully placed inside the plaque, leaving the margin of the plaque free and the ROI was reproduced by comparing with the first ROI. In general the patients selected for this study only had few plaques, that
mainly were situated periventricularily. Only one plaque (the largest one) was evaluated in each patient. Finally, ROIs were placed in the center of the external standard probes, covering approximately 10 pixels. Assuming a simple monoexponential behavior of the Tl relaxation process, T, values were calculated from the following formula” using two-parameter least-squares fit:2’ S o: N(N)~-TE&(
1 _ 2e-r’/7i _
e-TR/T,
+ 2e-W-“TE)/Ti
1
where S is the MR signal and N(H) is the proton density. The fitted parameters were [N(N) exp( - TE/T2)] and T, . Prior to the T2 calculation the signals recorded were corrected for background noise, obtained from a large ROI outside the brain. Subsequently T2 was calculated by22 S = keMnTE’fi,
n = 1,2,3,. . _,32 .
where k is a constant. The fitted parameters were k and T2. For a bi-exponential T2 analysis the signal was a sum of two expressions as above. The distinction between a monoexponential and a biexponential relaxation process was carried out as explained elsewhere.2 The difference (d) between the pair of the repeated measurements within one week in the patients was assumed to be normally distributed (Gaussian). If we assumed that the mean difference of d (a) is not significantly different from zero, the standard deviation (SD) of differences between the 10 pairs of repeated measurements of T, and T2 can be calculated as SD = dm. The standard error of d (SEM) is &D%. 95% of the differences will lie between (d - 1.96 SD) and (d + 1.96 SD). These limits represent the precision.23 However, the confidence intervals for these limits should also be calculated to see how precise these estimates are. The standard error of (d + 1.96 SD) and (d - 1.96 SD) is about dm.23 The 95% confidence interval for (d - 1.96 SD) is [(d - 1.96 SD) f tdm)], and the confidence interval for (d + 1.96 SD) is [(d+ 1.96 SD) f tdm], where t is obtained from the Student-t distribution (95%, two-sided). Regression analysis was used to assess if there was any relationship between the numerical difference )dl and the size of the ROIs, that is, the number of pixels. RESULTS Figures 1 and 2 show plots of the difference between the repeated measurements against their mean
T, and Tz of brain of MS patients 0 H.B.W. Difference
581
LARSON ET AL.
between pairsof measurements
L
+2SD
200 1
-
-
-
-.-
_
_ -
-
-
-.-
_._.-.-.
-
_._
_._
_
_ -.-
_
. IOO-
+2SD__________-,_____-
l
T
-
.
.
lm n 0
T
.
n
.
l
.
.
.
l
m
-2s~________-m--_______0
-lOO-
A
. .
.
-2
SD
-
-
-
-
-.-
-
-
-
-
-
_ _ _ _ _ _._
_ _ _.-.-
_
.L 1 1
200
400
I
8
600
800
I
1000
I
I
1200
1400
I
1600
I
l800
I
2000
l
Tl
Fig. 1. Repeated measures of T, in patients with MS. Difference against mean values. The bars (t 1 SD) refer to the external standard probes. n : white matter; l : plaque; ------: 2 SD of white matter; -. -. -. -: 2 SD of plaque.
for T, and T,, respectively. The SD of the four external standard probes was calculated in the same way as in patients and is also shown in the figures as a function of the calculated mean of all measurements (20 times for each probe). Unfortunately, T2 of the standard probes was somewhat higher than T2 in brain tissue; especially the fourth standard probe was out of range and was therefore omitted in Fig. 2. In no cases was d significantly different from zero and was set to zero in the following. With regard to the T, measurement, the SD of differences were 48 msec and 106 msec for white matter and plaques, respectively. This means that we expect that 95% of repeated measurements will lie within the interval 94 msec to -94 msec and 208 msec to -208 msec from the first measurement in white matter and plaques, respectively. The 95% confidence intervals for the upper and lower limit for white matter were 35 msec to 151 msec and -35 msec to - 151 msec, respectively. The 95% confidence interval for the upper and lower limit for the plaques was 79 msec to 339 msec and -79 msec to -339 msec, respectively. The mean and SD of the four external standard probes were: 276 msec and 16 msec (SD/mean = 5.8%), 983 msec and 95 msec (9.70/o), 1329 msec and 104 msec (7.8%), and 1913 msec and 179 msec (9.4%), see Fig. 1. These values of the standard probes agree with values obtained by us in a previous study.’ The slope
of the regression line between the numerical value of the differences and the number of pixels included in the ROIs was not significantly different from zero either for the white matter or for the plaques, (p > 0.10). With regard to T2 measurement, the SD of the differences were 9 msec and 13 msec for white matter and plaques, respectively. This means again that 95% of repeated measurements will be within the interval 18 msec to -18 msec and 26 msec to -26 msec from the first measurement in white matter and plaques, respectively. The 95% confidence intervals for the upper and lower limit for white matter were 8 msec to 29 msec and -8 msec to -29 msec, respectively. The 95% confidence interval for the upper and lower limit for the plaques were 10 msec to 43 msec and -10 msec to -43 msec, respectively. The mean and SD of the four external standard probes were: 267 msec and 10 msec (3.7Vo), 853 msec and 24 msec (2.8%), 1014 msec and 59 msec (S.SVo), and 1316 msec and 104 msec (7.90/o), see Fig. 2. The slope of the regression line between the numerical value of the differences and the number of pixels included in the ROIs was not significantly different from zero either for the white matter or for the plaques. Plaques that followed a biexponential T2 relaxation process were not found. This is probably a consequence of the patients selected as having a steady progressive MS form without any acute attack.3
582
Magnetic Resonance
Imaging 0 Volume
10, Number 4, 1992
Diffamncebetwaan pain of measurements
50 40 i
30 * 20 -
+2SD
_.- - _;
_ _ - - - - - -.-
l2~________
.
. lo0.'
: 1 .'
..
l
-10.
-20 -
- - -
.
n
am
.
_25&________ _2SD_ - -._ - - - - - - - - - - - - -
-30 1
Fig. 2. Repeated measures of T, in patients with MS. Difference against mean values. The bars (+ 1 SD) refer to the external standard probes. B: white matter; l : plaque; ------: 2 SD of white matter; -. -. -. -: 2 SD of plaque.
DISCUSSION In previous studies it has been found that both T, and T2in MS plaques show a large variation. I-3 In the present study, we found a SD of the difference (d) between repeated measurements of 106 msec and 13 msec for Tland T,,respectively, corresponding to coefficients of variation of approximately 9%. This confirms our previous conclusion that the large scatter of relaxation times in plaques reflects true biophysical differences in tissue composition, and is not caused by inaccuracies in the measurements.2*3 Figure 1 indicates that the SD of d is proportional to the measured value of T,and that the SD of in vivo measurements are not different from corresponding measurements in phantoms. From the present study and a previous study, l6 it is seen that the SD of T2 measurements in phantoms is about 2.5 to 10 msec for a range of T2values of 52 msec to 267 msec, and that the coefficient of variation for a range of T2values from 52 msec to 1316 msec is about 2.5% to 10%. Again, a SD of in vivo T2 measurements of 9 msec and 13 msec for respectively white matter and plaques does not seem to differ from the SD obtained in phantoms. This tells us that repositioning of the head was satisfactorily performed. The 95% confidence limits of (d + 1.96 SD) for T, and T2 in MS plaques may seem somewhat
large probably also reflecting the small number of patients investigated. However, even the worst case, that is, limits of +339 msec and k43 msec for precision of, respectively, T,and T2 in plaques, does not contradict our interpretation that the large scatter of T,and T2 reflects true biophysical differences. We h&e previously measured the accuracy and the precision in phantoms. For the actual sequences used we have found the accuracy of T,measurements to,be 3-l 5% in the biological range of 500-2000 msec, and the accuracy of T2 was better than 10% for typical values Our precision in phantoms of T2 (So-500 msec). 2*4p16 did not deviate more than 10% from one time to the next (long-term precision) for Tland T2 for relevant biological values. Others have measured accuracy and precision in phantoms but direct comparison is difficult because of different field strength and the use of other methods of calculation. Johnson et al.” calculated Tland T2 from three sets of images; two spin echo images (SE2000,40,SE2000/120)and one inversion recovery image (IR2000,500,40).They found an accuracy of 7% and not much better than 20% for Tland T2 measurements, respectively, and precision of 6% and 8% for T, and T2 measurements, respectively. Breger et al. ‘* used a multiple saturation-recovery sequence with varying inversion time (150, 300,600, and 1200 msec) for T,calculation, and a Carr-Purcell-
TI and
T2of brain of MS patients l H.B.W.
Meiboom-Gill multiple spin-echo sequence with four echoes (25, 50, 75, and 100 msec) for T2calculation. They found that the precision in phantoms was 5-14% and 2-10% for T, and T2measurements, respectively. These values do not seem to be very different from what we found in phantoms. In a similar phantom study of Breger et al., l3 the coefficient of variation of T, was found to be about 5%) and 6-9% for T2. However, every measurement used for calculation of the coefficient of variation was in fact a mean of three consecutive measurements, and is not a realistic approach with regard to investigation of patients. In appearance, Kjos et al. ” have also evaluated the reproducibility of relaxation times in a clinical study of CNS. The method they used is not clearly described. Their study is, however, merely an evaluation of relaxation times between patients, and no repeated studies of the individual subject have been performed. There are many sources of errors contributing to inaccuracies and imprecision. Our estimated values of accuracy and precision must necessarily include most of these errors. In conclusion, the present study indicates, that only a change of more than 208 msec for T1 and a change of more than 26 msec for T2may be considered a statistically significant change during a treatment trial or‘ when the time evolution of the plaque is studied. Judging from our own study and others as well, a precision better than 10% for Tland T2would seem unrealistic at present. Furthermore, the large dispersal of T, and T2 we have observed previously in MS plaques’ can only be explained by biophysical differences in tissue composition, and the rapid change of relaxation times observed in acute MS plaques3 is a true reflection of changes in tissue composition. Thus, relaxation times can provide information on MS plaques in vivo, and can be used to monitor the disease during treatment trials. REFERENCES Ormerod, I.E.C.; Bronstein, A.; Rudge, P.; Johnson, G.; MacManus, D.; Halliday, A.M.; Barret, H.; Du Boulay, E.P.G.H.; Kendall, B.E.; Moseley, I.F.; Jones, S.J.; Kriss, A.; Peringer, E. Magnetic resonance imaging in clinically isolated lesions of the brain stem. J. Neurol. Neurosurg. Psychiatry 49:737-743; 1986. Larsson, H.B.W.; Frederiksen, J.; Kjazr, L.; Henriksen, 0.; Olesen, J. In vivo determination of T, and T2 in the brain of patients with severe but stable multiple sclerosis. Magn. Reson. Med. 7~43-55; 1988. Larsson, H.B.W.; Frederiksen, J.; Petersen, J.; Nordenbo, A.; Zeeberg, I.; Henriksen, 0.; Olesen, J. Assessment of demyelination, edema, and gliosisby in vivo determination of T, and T2 in the brain of patients with
LARSSONET AL.
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acute attack of multiple sclerosis. Magn. Reson. Med. 11:337-348; 1989. 4. MacDonald, H.L.; Bell, B.A.; Smith, M.A.; Kean, D.M; Tocher, J.L.; Douglas, R.H.B.; Miller, J.D.; Best, J.J.K. Correlation of human NMR T, values measured in vivo and brain water content. Br. J. Radiol. 59:355357; 1986. 5. Barnes, D.; McDonald, W.I.; Johnson, G.; Tofts, P.S.; Landon, D.N. Quantitative nuclear magnetic resonance imaging: characterisation of experimental cerebral oedema. J. Neural. Neurosurg. Psychiatry 50:125-133; 1987. 6. Karlik, S. J.; Strejan, G.; Gilbert, J. J.; Noseworthy, J.H. NMR studies in experimental allergic encephalomyelitis (EAE): Normalization of T, and T2 with parenchymal cellular infiltration. Neurology 36: 1112-l 114; 1986. 7. Bederson, J.B.; Bartkowzki, H.M.; Moon, K.; HalksMiller, M.; Nishimura, M.C.; Brant-Zawadzki, M.; Pitts, L.H. Nuclear magnetic resonance imaging and spectroscopy in experimental brain edema in a rat model. J. Neurosurg. 64:795-802; 1986. 8. O’Brian, J.T.; Noseworthy, J.H.; Gilbert, J.J.; Karlik, S. J. NMR changes in experimental allergic encephalomyelitis: NMR changes precede clinical and pathological events. Magn. Res. Med. 5:109-117; 1987. 9. Kesselring, J.; Miller, D.H.; MacManus, D.G.; Johnson, G.; Milligan, N.M.; Scolding, N.; Composton, D.A.S.; McDonald, W.I. Quantitative magnetic resonance imaging in multiple sclerosis: the effect of high dose intravenous methylprednisolone. J. Neurol. Neurosurg. Psychiatry 52:14-17; 1989. 10. Brainin, M.; Neuhold, A.; Reisner, T.; Maida, E.; Lang, S.; Deecke, L. Changes within the “normal” cerebral white matter of multiple sclerosis patients during acute attacks and during high-dose cortisone therapy assessed by means of quantitative MRI. J. Neurol. Neurosurg. Psychiatry 52:1355-1359; 1989. 11. Kjos, B.O.; Ehman, R.L.; Brant-Zawadzki, M. Reproducibility of TI and T2 relaxation times calculated from routine MR-imaging sequences: Phantom study. AJR 144:1157-l 163; 1985. 12. Breger, R.K.; Wehrli, F.W.; Charles, H.C.; MacFall, J-R.; Haughton, V.M. Reproducibility of relaxation and spin-density parameters in phantoms and the human brain measured by MR imaging at 1.5 T. Magn. Res. Med. 3:649-662; 1986. 13. Breger, R.K.; Rimm, A.A.; Fisher, M.E.; Papke, R.A.; Haughton, V.M. T, and T2 measurements on a 1.5 T commercial MR imager. Radiology 171:273-276; 1989. 14. Kjzr, L.; Thomsen, C.; Henriksen, 0.; Ring, P.; Stubgaard, M.; Pedersen, E.J. Evaluation of relaxation time measurements by magnetic resonance imaging. Acta Radiologica 28:345-351; 1987. 15. Kjzr, L.; Thornsen, C.; Larsson, HBW.; Henriksen. 0.;
Ring, P. Evaluation of biexponential relaxation processes by magnetic resonance imaging. A phantom study. Acta Radiologica 29:473-479; 1988. 16. Thomsen, C.; Jensen, K.E.; Jensen, M.; Olesen, E.R.; Henriksen,
0. MR pulse sequences for selective relax-
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ation time measurements: A phantom study. Magn. Reson. Imaging 8:43-50; 1990. 17. Kjos, B.O.; Ehman, R.L.; Brant-Zawadzki, M.; Kelly, W.M.; Normann, D.; Newton, T.H. Reproducibility of relaxation times and spin density calculated from routine MR imaging sequences: Clinical study of the CNS. AJR 144:1165-1170; 1985. 18. Johnson, G.; Ormerod, I.E.C.; Branes, D.; Tofts, P.S., and McManus, D. Accuracy and precision in the measurement of relaxation times from nuclear magnetic resonance images. Br. .I. Radiol. 60:143-153; 1987. 19. Graumann, R.; Oppelt, A.; Stetter, E. Multiple spinecho imaging with a 2D Fourier method. Mugn. Res. Med. 3:707-721; 1986.
20. Hendrick, R.E.; Nelson, T.R.; Hendee, W.R. Optimizing tissue contrast in magnetic resonance imaging. Magn. Reson. Imaging 2: 193-200; 1984. 21. Kirkegaard, P. A Fortran IV Version of the Sum-of-Exponential Least-Squares Code Exposum. Rise, Denmark: The Danish Atomic Energy Commission; 1970. 22. Sperber, G.; Ericsson, A.; Hemmingsson, A.; Jung, B.; Thuomas, K-A. Improved formulae for signal amplitudes in repeated NMR sequences. Applications in NMR imaging. Magn. Res. Med. 3:685-698; 1986. 23. Bland, J.M.; Altman, D.G. Statistical methods for assessing agreement between two methods of clinical measurement. Lancer 8:307-310; 1986.
1992 Copyright 0
0730-725x/92 $5.00 + .w 1992 Pergamon Press Ltd.
l Original Contribution
IN VIVO EVALUATION OF THE REPRODUCIBILITY OF TXAND T2 MEASURED IN THE BRAIN OF PATIENTS WITH MULTIPLE SCLEROSIS H . B. W. LARSSON , * P. CHRISTIANSEN, * I,, ZEEBERG,~
AND 0.
HENRIKSEN*
*Danish Research Center of Magnetic Resonance at Hvidovre, University Hospital, Copenhagen, Denmark TDepartment of Neurology, Vejle Hospital, Denmark The precision (reproducibility) of relaxation times derived from magnetic resonance images of patients with multiple sclerosis (MS) were investigated. Measurements of 10 MS patients were performed at 1.5 T on two occasions within 1 wk. 2’~and T2 was measured using a partial saturation inversion recovery sequence (6 points) and a CarrPurcell-Meiboom-Gill phase alternating-phase shift multiple spin-echo sequence with 32 echoes. Regions of interest (ROI) were placed both in apparently normal white matter and plaques. The precision (k1.96 SD) and the confidence intervals for Tl and T2 for white matter and plaques were calculated. The precision of T, for white matter and plaques was respectively f94 msec and *208 msec. The precision of T2 for white matter and plaques was respectively *18 msec and *26 msec. For all measurements the coefficient of variation was about 9%. Judging from our own study and others as well, a precision better than 10% for Tl and T2 would seem unrealistic at present.
Keywords: MRI; Multiple sclerosis; Relaxation time; Reproducibility.
of imprekision and inaccuracy. The aim of the present study was therefore to evaluate the precision of measured values of Tl and T2 in plaques and in white
Several previous studies have shown that measurements of relaxation times (T, and Tz) in plaques due to multiple sclerosis (MS) may give valuable information concerning edema, demyelination, and gliosis.‘-4 Similarly, investigations of induced experimental allergic encephalomyelitis in animals (a model of MS) suggest that relaxation times may provide valuable information.*-* The relaxation times have also been used to monitor the efficacy of treatments of patients with MS.9**oA number of studies have focused on the accuracy and the precision (reproducibility) of measurements of relaxation times in phantoms2~“-‘6 and in the brain of healthy subjects.‘7*‘8 To our knowledge, the in vivo precision of relaxation times measured in MS plaques has not previously been reported in the literature. The estimation of the in vivo precision can be relevant for evaluation of the usefulness of T, and T2in monitoring the disease during treatment trials. Furthermore, in a previous study of MS patients, we observed a large scatter of T, and T2 of MS plaques, even between plaques in the same patient.2*3 The question is whether this large scatter reflects a true biophysical variation between plaques, or is the result
RECEIVED 6/26/91; ACCEPTED 3/17/92. Address correspondence to Henrik B.W. Larsson,
matter of patients with MS. MATERIAL
AND
METHODS
Patients
Measurements of T, and T2 were performed in ten patients with definite MS. The mean age of the patients was 42 yr (&8 yr) and the mean duration of the disease was 14 yr (+5 yr). All patients had a steady progressive MS form. None had had an acute attack within at least one year prior to the study. All the patients participated in a double blind, placebo-controlled study of the efficacy of a-interferon in slowing down disease progression. Two measurements within 1 wk were performed at the beginning of the placebo period and these results are presented in this study. One measurement was performed at the end of the placebo period, that is, after 3 mo. Two measurements within 1 wk were also performed at the beginning of the non-placebo period and one measurement was per-
Danish Research Center of Magnetic Resonance, Hvidovre University Hospital, DK-2650 Copenhagen, Denmark.
MD, 519
580
Magnetic Resonance Imaging 0 Volume 10, Number 4, 1992
formed at the end of this period. The results related to the possible effect of a-interferon on the relaxation times will be published separately. Imaging and Repositioning The study was carried out on a whole-body MRscanner (Siemens Magnetom H15), operating at 1.5 T. The brain was imaged in the axial plane in the head coil using a double spin-echo sequence with a repetition time (TR) of 1.8 set, and echo times (TE) of 30 and 90 msec. The slice thickness was 4 mm, and a matrix size of 256 x 256 was used, giving a voxel size of 1.2 x 1.2 x 4 mm3. Twelve slices with an interslice spacing of 4 mm were acquired. Subsequently, the sequence was repeated, imaging the interslice space. A single slice at the level of the lateral ventricular system was then selected with the aim of measuring the relaxation times. Four plastic test tubes with CuS04 solutions at different concentrations had been mounted in the head coil, perpendicular to the axial plane and served as external standard probes for the measurements. The variation of the temperature in the scanner was between 19” and 21”, giving a variation of T, and T2 of less than 6% and 2070,respectively. Repositioning of the patients was achieved by use of sagittal images onto which grids were superimposed. Using these sagittal images and the sagittal images obtained previously, the position of the head was then changed until the angulation was reproduced. The exact position was then determined using the axial images obtained from the double spin-echo sequence actually and formerly. Relaxation Time Measurements Tl was measured by a partial saturation inversion recovery sequence (2). The inversion time (TI) was constant at 150 msec. The TE of the 180” rephasing pulse was 30 msec. The TR varied from 0.24 to 6.0 set (six data points). T2 was measured by a Carr-PurcellMeiboom-Gill (CPMG) phase alternating/phase shift multiple spin-echo sequence with 32 echoes.” The echoes were recorded at 30-msec intervals from 30 to 960 msec. The TR was 4 sec. Cafculation and Statistics Based on the sequential scan images generated, the MR signals were read out from regions of interest (ROIs) in white frontal matter, and MS plaques. The regional areas consisted of 8-20 pixels in the white matter, and of 18-168 pixels in the plaques. Each ROI was carefully placed inside the plaque, leaving the margin of the plaque free and the ROI was reproduced by comparing with the first ROI. In general the patients selected for this study only had few plaques, that
mainly were situated periventricularily. Only one plaque (the largest one) was evaluated in each patient. Finally, ROIs were placed in the center of the external standard probes, covering approximately 10 pixels. Assuming a simple monoexponential behavior of the Tl relaxation process, T, values were calculated from the following formula” using two-parameter least-squares fit:2’ S o: N(N)~-TE&(
1 _ 2e-r’/7i _
e-TR/T,
+ 2e-W-“TE)/Ti
1
where S is the MR signal and N(H) is the proton density. The fitted parameters were [N(N) exp( - TE/T2)] and T, . Prior to the T2 calculation the signals recorded were corrected for background noise, obtained from a large ROI outside the brain. Subsequently T2 was calculated by22 S = keMnTE’fi,
n = 1,2,3,. . _,32 .
where k is a constant. The fitted parameters were k and T2. For a bi-exponential T2 analysis the signal was a sum of two expressions as above. The distinction between a monoexponential and a biexponential relaxation process was carried out as explained elsewhere.2 The difference (d) between the pair of the repeated measurements within one week in the patients was assumed to be normally distributed (Gaussian). If we assumed that the mean difference of d (a) is not significantly different from zero, the standard deviation (SD) of differences between the 10 pairs of repeated measurements of T, and T2 can be calculated as SD = dm. The standard error of d (SEM) is &D%. 95% of the differences will lie between (d - 1.96 SD) and (d + 1.96 SD). These limits represent the precision.23 However, the confidence intervals for these limits should also be calculated to see how precise these estimates are. The standard error of (d + 1.96 SD) and (d - 1.96 SD) is about dm.23 The 95% confidence interval for (d - 1.96 SD) is [(d - 1.96 SD) f tdm)], and the confidence interval for (d + 1.96 SD) is [(d+ 1.96 SD) f tdm], where t is obtained from the Student-t distribution (95%, two-sided). Regression analysis was used to assess if there was any relationship between the numerical difference )dl and the size of the ROIs, that is, the number of pixels. RESULTS Figures 1 and 2 show plots of the difference between the repeated measurements against their mean
T, and Tz of brain of MS patients 0 H.B.W. Difference
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between pairsof measurements
L
+2SD
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-
-
-
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-
-
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.
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400
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8
600
800
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1000
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I
1200
1400
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1600
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Fig. 1. Repeated measures of T, in patients with MS. Difference against mean values. The bars (t 1 SD) refer to the external standard probes. n : white matter; l : plaque; ------: 2 SD of white matter; -. -. -. -: 2 SD of plaque.
for T, and T,, respectively. The SD of the four external standard probes was calculated in the same way as in patients and is also shown in the figures as a function of the calculated mean of all measurements (20 times for each probe). Unfortunately, T2 of the standard probes was somewhat higher than T2 in brain tissue; especially the fourth standard probe was out of range and was therefore omitted in Fig. 2. In no cases was d significantly different from zero and was set to zero in the following. With regard to the T, measurement, the SD of differences were 48 msec and 106 msec for white matter and plaques, respectively. This means that we expect that 95% of repeated measurements will lie within the interval 94 msec to -94 msec and 208 msec to -208 msec from the first measurement in white matter and plaques, respectively. The 95% confidence intervals for the upper and lower limit for white matter were 35 msec to 151 msec and -35 msec to - 151 msec, respectively. The 95% confidence interval for the upper and lower limit for the plaques was 79 msec to 339 msec and -79 msec to -339 msec, respectively. The mean and SD of the four external standard probes were: 276 msec and 16 msec (SD/mean = 5.8%), 983 msec and 95 msec (9.70/o), 1329 msec and 104 msec (7.8%), and 1913 msec and 179 msec (9.4%), see Fig. 1. These values of the standard probes agree with values obtained by us in a previous study.’ The slope
of the regression line between the numerical value of the differences and the number of pixels included in the ROIs was not significantly different from zero either for the white matter or for the plaques, (p > 0.10). With regard to T2 measurement, the SD of the differences were 9 msec and 13 msec for white matter and plaques, respectively. This means again that 95% of repeated measurements will be within the interval 18 msec to -18 msec and 26 msec to -26 msec from the first measurement in white matter and plaques, respectively. The 95% confidence intervals for the upper and lower limit for white matter were 8 msec to 29 msec and -8 msec to -29 msec, respectively. The 95% confidence interval for the upper and lower limit for the plaques were 10 msec to 43 msec and -10 msec to -43 msec, respectively. The mean and SD of the four external standard probes were: 267 msec and 10 msec (3.7Vo), 853 msec and 24 msec (2.8%), 1014 msec and 59 msec (S.SVo), and 1316 msec and 104 msec (7.90/o), see Fig. 2. The slope of the regression line between the numerical value of the differences and the number of pixels included in the ROIs was not significantly different from zero either for the white matter or for the plaques. Plaques that followed a biexponential T2 relaxation process were not found. This is probably a consequence of the patients selected as having a steady progressive MS form without any acute attack.3
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Diffamncebetwaan pain of measurements
50 40 i
30 * 20 -
+2SD
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Fig. 2. Repeated measures of T, in patients with MS. Difference against mean values. The bars (+ 1 SD) refer to the external standard probes. B: white matter; l : plaque; ------: 2 SD of white matter; -. -. -. -: 2 SD of plaque.
DISCUSSION In previous studies it has been found that both T, and T2in MS plaques show a large variation. I-3 In the present study, we found a SD of the difference (d) between repeated measurements of 106 msec and 13 msec for Tland T,,respectively, corresponding to coefficients of variation of approximately 9%. This confirms our previous conclusion that the large scatter of relaxation times in plaques reflects true biophysical differences in tissue composition, and is not caused by inaccuracies in the measurements.2*3 Figure 1 indicates that the SD of d is proportional to the measured value of T,and that the SD of in vivo measurements are not different from corresponding measurements in phantoms. From the present study and a previous study, l6 it is seen that the SD of T2 measurements in phantoms is about 2.5 to 10 msec for a range of T2values of 52 msec to 267 msec, and that the coefficient of variation for a range of T2values from 52 msec to 1316 msec is about 2.5% to 10%. Again, a SD of in vivo T2 measurements of 9 msec and 13 msec for respectively white matter and plaques does not seem to differ from the SD obtained in phantoms. This tells us that repositioning of the head was satisfactorily performed. The 95% confidence limits of (d + 1.96 SD) for T, and T2 in MS plaques may seem somewhat
large probably also reflecting the small number of patients investigated. However, even the worst case, that is, limits of +339 msec and k43 msec for precision of, respectively, T,and T2 in plaques, does not contradict our interpretation that the large scatter of T,and T2 reflects true biophysical differences. We h&e previously measured the accuracy and the precision in phantoms. For the actual sequences used we have found the accuracy of T,measurements to,be 3-l 5% in the biological range of 500-2000 msec, and the accuracy of T2 was better than 10% for typical values Our precision in phantoms of T2 (So-500 msec). 2*4p16 did not deviate more than 10% from one time to the next (long-term precision) for Tland T2 for relevant biological values. Others have measured accuracy and precision in phantoms but direct comparison is difficult because of different field strength and the use of other methods of calculation. Johnson et al.” calculated Tland T2 from three sets of images; two spin echo images (SE2000,40,SE2000/120)and one inversion recovery image (IR2000,500,40).They found an accuracy of 7% and not much better than 20% for Tland T2 measurements, respectively, and precision of 6% and 8% for T, and T2 measurements, respectively. Breger et al. ‘* used a multiple saturation-recovery sequence with varying inversion time (150, 300,600, and 1200 msec) for T,calculation, and a Carr-Purcell-
TI and
T2of brain of MS patients l H.B.W.
Meiboom-Gill multiple spin-echo sequence with four echoes (25, 50, 75, and 100 msec) for T2calculation. They found that the precision in phantoms was 5-14% and 2-10% for T, and T2measurements, respectively. These values do not seem to be very different from what we found in phantoms. In a similar phantom study of Breger et al., l3 the coefficient of variation of T, was found to be about 5%) and 6-9% for T2. However, every measurement used for calculation of the coefficient of variation was in fact a mean of three consecutive measurements, and is not a realistic approach with regard to investigation of patients. In appearance, Kjos et al. ” have also evaluated the reproducibility of relaxation times in a clinical study of CNS. The method they used is not clearly described. Their study is, however, merely an evaluation of relaxation times between patients, and no repeated studies of the individual subject have been performed. There are many sources of errors contributing to inaccuracies and imprecision. Our estimated values of accuracy and precision must necessarily include most of these errors. In conclusion, the present study indicates, that only a change of more than 208 msec for T1 and a change of more than 26 msec for T2may be considered a statistically significant change during a treatment trial or‘ when the time evolution of the plaque is studied. Judging from our own study and others as well, a precision better than 10% for Tland T2would seem unrealistic at present. Furthermore, the large dispersal of T, and T2 we have observed previously in MS plaques’ can only be explained by biophysical differences in tissue composition, and the rapid change of relaxation times observed in acute MS plaques3 is a true reflection of changes in tissue composition. Thus, relaxation times can provide information on MS plaques in vivo, and can be used to monitor the disease during treatment trials. REFERENCES Ormerod, I.E.C.; Bronstein, A.; Rudge, P.; Johnson, G.; MacManus, D.; Halliday, A.M.; Barret, H.; Du Boulay, E.P.G.H.; Kendall, B.E.; Moseley, I.F.; Jones, S.J.; Kriss, A.; Peringer, E. Magnetic resonance imaging in clinically isolated lesions of the brain stem. J. Neurol. Neurosurg. Psychiatry 49:737-743; 1986. Larsson, H.B.W.; Frederiksen, J.; Kjazr, L.; Henriksen, 0.; Olesen, J. In vivo determination of T, and T2 in the brain of patients with severe but stable multiple sclerosis. Magn. Reson. Med. 7~43-55; 1988. Larsson, H.B.W.; Frederiksen, J.; Petersen, J.; Nordenbo, A.; Zeeberg, I.; Henriksen, 0.; Olesen, J. Assessment of demyelination, edema, and gliosisby in vivo determination of T, and T2 in the brain of patients with
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acute attack of multiple sclerosis. Magn. Reson. Med. 11:337-348; 1989. 4. MacDonald, H.L.; Bell, B.A.; Smith, M.A.; Kean, D.M; Tocher, J.L.; Douglas, R.H.B.; Miller, J.D.; Best, J.J.K. Correlation of human NMR T, values measured in vivo and brain water content. Br. J. Radiol. 59:355357; 1986. 5. Barnes, D.; McDonald, W.I.; Johnson, G.; Tofts, P.S.; Landon, D.N. Quantitative nuclear magnetic resonance imaging: characterisation of experimental cerebral oedema. J. Neural. Neurosurg. Psychiatry 50:125-133; 1987. 6. Karlik, S. J.; Strejan, G.; Gilbert, J. J.; Noseworthy, J.H. NMR studies in experimental allergic encephalomyelitis (EAE): Normalization of T, and T2 with parenchymal cellular infiltration. Neurology 36: 1112-l 114; 1986. 7. Bederson, J.B.; Bartkowzki, H.M.; Moon, K.; HalksMiller, M.; Nishimura, M.C.; Brant-Zawadzki, M.; Pitts, L.H. Nuclear magnetic resonance imaging and spectroscopy in experimental brain edema in a rat model. J. Neurosurg. 64:795-802; 1986. 8. O’Brian, J.T.; Noseworthy, J.H.; Gilbert, J.J.; Karlik, S. J. NMR changes in experimental allergic encephalomyelitis: NMR changes precede clinical and pathological events. Magn. Res. Med. 5:109-117; 1987. 9. Kesselring, J.; Miller, D.H.; MacManus, D.G.; Johnson, G.; Milligan, N.M.; Scolding, N.; Composton, D.A.S.; McDonald, W.I. Quantitative magnetic resonance imaging in multiple sclerosis: the effect of high dose intravenous methylprednisolone. J. Neurol. Neurosurg. Psychiatry 52:14-17; 1989. 10. Brainin, M.; Neuhold, A.; Reisner, T.; Maida, E.; Lang, S.; Deecke, L. Changes within the “normal” cerebral white matter of multiple sclerosis patients during acute attacks and during high-dose cortisone therapy assessed by means of quantitative MRI. J. Neurol. Neurosurg. Psychiatry 52:1355-1359; 1989. 11. Kjos, B.O.; Ehman, R.L.; Brant-Zawadzki, M. Reproducibility of TI and T2 relaxation times calculated from routine MR-imaging sequences: Phantom study. AJR 144:1157-l 163; 1985. 12. Breger, R.K.; Wehrli, F.W.; Charles, H.C.; MacFall, J-R.; Haughton, V.M. Reproducibility of relaxation and spin-density parameters in phantoms and the human brain measured by MR imaging at 1.5 T. Magn. Res. Med. 3:649-662; 1986. 13. Breger, R.K.; Rimm, A.A.; Fisher, M.E.; Papke, R.A.; Haughton, V.M. T, and T2 measurements on a 1.5 T commercial MR imager. Radiology 171:273-276; 1989. 14. Kjzr, L.; Thomsen, C.; Henriksen, 0.; Ring, P.; Stubgaard, M.; Pedersen, E.J. Evaluation of relaxation time measurements by magnetic resonance imaging. Acta Radiologica 28:345-351; 1987. 15. Kjzr, L.; Thornsen, C.; Larsson, HBW.; Henriksen. 0.;
Ring, P. Evaluation of biexponential relaxation processes by magnetic resonance imaging. A phantom study. Acta Radiologica 29:473-479; 1988. 16. Thomsen, C.; Jensen, K.E.; Jensen, M.; Olesen, E.R.; Henriksen,
0. MR pulse sequences for selective relax-
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ation time measurements: A phantom study. Magn. Reson. Imaging 8:43-50; 1990. 17. Kjos, B.O.; Ehman, R.L.; Brant-Zawadzki, M.; Kelly, W.M.; Normann, D.; Newton, T.H. Reproducibility of relaxation times and spin density calculated from routine MR imaging sequences: Clinical study of the CNS. AJR 144:1165-1170; 1985. 18. Johnson, G.; Ormerod, I.E.C.; Branes, D.; Tofts, P.S., and McManus, D. Accuracy and precision in the measurement of relaxation times from nuclear magnetic resonance images. Br. .I. Radiol. 60:143-153; 1987. 19. Graumann, R.; Oppelt, A.; Stetter, E. Multiple spinecho imaging with a 2D Fourier method. Mugn. Res. Med. 3:707-721; 1986.
20. Hendrick, R.E.; Nelson, T.R.; Hendee, W.R. Optimizing tissue contrast in magnetic resonance imaging. Magn. Reson. Imaging 2: 193-200; 1984. 21. Kirkegaard, P. A Fortran IV Version of the Sum-of-Exponential Least-Squares Code Exposum. Rise, Denmark: The Danish Atomic Energy Commission; 1970. 22. Sperber, G.; Ericsson, A.; Hemmingsson, A.; Jung, B.; Thuomas, K-A. Improved formulae for signal amplitudes in repeated NMR sequences. Applications in NMR imaging. Magn. Res. Med. 3:685-698; 1986. 23. Bland, J.M.; Altman, D.G. Statistical methods for assessing agreement between two methods of clinical measurement. Lancer 8:307-310; 1986.
