- Email: [email protected]
Hippocampal and cerebellar volumetry in serially acquired MRI volume scans
Magnetic Resonance Imaging 18 (2000) 1027–1033
Technical note
Hippocampal and cerebellar volumetry in serially acquired MRI volume scans L. Lemieux*, Rebecca S.N. Liu, John S. Duncan Epilepsy Research Group, University Department of Clinical Neurology, Institute of Neurology, University College London, 33 Queen Square, London WC1N 3BG, United Kingdom National Society for Epilepsy, Chalfont St Peter, Buckinghamshire SL9 0RJ, United Kingdom Received 19 July 2000; accepted 2 September 2000
Abstract In this work, we describe methodologies for serial volumetric measurements of hippocampi and cerebella. Serial scans were co-registered and intensity matched prior to the volumetric measurements. Manual drawing was used to define the boundaries of the hippocampi. For the cerebellar volumetric measurements, the brain was automatically segmented from the co-registered scans; manual drawing was used to define the boundary between the cerebellum and the cerebrum and brainstem. The operator was blinded to the nature of the subject (patient or normal control) and the chronological order of the scans. The coefficient of reliability of hippocampal volume measurements in a group of 20 controls was 0.078 cm3 (3.1% of the mean baseline volume); for the cerebellum, the value was 3.8 cm3 (3.0% of the mean baseline volume). We conclude that the methods presented are valid and that the software provides a useful integrated tool for the quantitative analysis of structural changes in serially acquired volume MRI data in prospective, blinded studies. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Volumetry; Hippocampus; Cerebellum; Longitudinal
1. Introduction Longitudinal imaging studies offer a unique opportunity to study the morphological evolution of the brain. In particular, the analysis of co-registered serial MRI data has already led to new insights in the progression of neurological disorders [1–3] and the effects of treatment [4,5]. The usefulness of co-registration relies on the fact that the sensitivity to change can be improved by correcting for differences in scan position, orientation and intensity prior to the analysis. This work is concerned with methodological issues related to the following question: what morphological changes are associated with chronic epilepsy? One of the most debated issues in the field of epilepsy research is the causal relationship between hippocampal sclerosis and epilepsy [6 –10]. It is therefore of great interest to determine the time-course of hippocampal volume changes in relation to the disease onset and subsequent evolution in a longitudinal, rather than cross-sectional, fashion. Currently, hip* Corresponding author. Tel.: ⫹44-1494-601361; fax: ⫹44-1494875666. E-mail address: [email protected] (L. Lemieux).
pocampal volume measurements are largely performed by manual delineation in individual slices from volumetric MRI data. Automation of this process is made difficult by the small volume of the structure and the lack of clearly visible boundaries with certain neighbouring structures, e.g. amygdala, para-hippocampal gyrus. The sensitivity of the manual technique to change is therefore limited by its subjectivity. In serial imaging, where we are mainly interested in differences between repeated measurements, the variability of repeated measurements will be larger than that of single-scan repeated measurements. Co-registration of the serial scans should lead to an improvement in the variability of repeated measurements by providing a consistent presentation of the scans for individual subjects. The quantification of cerebellar atrophy in a prospective study of patients with epilepsy is of interest since recurrent seizures and chronic use of certain anti-epileptic drugs, particularly phenytoin, have been associated with cerebellar atrophy [11]. Although a large proportion of the cerebellum’s boundary, i.e. cortex/cerebrospinal fluid (CSF), is clearly and easily defined, cerebellar volumetry requires the operator to make subjective decisions to define the boundary with the brainstem [12].
0730-725X/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 0 ) 0 0 2 1 0 - 1
1028
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
The aim of this study was to determine the reproducibility of hippocampal and cerebellar volume measurements in serial MRI data after co-registration and intensity matching. Jack et al. have published a study on the reproducibility of hippocampal volume measurements following co-registration [13]. However, some of the methodological details (e.g., whether the operator can visualise the two datasets side-by-side, and if so, whether the traces drawn in the first scan were visualised for drawing in the second scan) were not clearly presented. Furthermore, the observers were not blinded to the nature of the subject when assessing the reproducibility of their technique as only controls were used. In this work, the reproducibility of the volumetric measurements in normal subjects was assessed in a group of subjects containing a blinded mixture of normal subjects and patients with chronic localisation-related epilepsy. The operator was blinded to the chronological order of the scans during the measurements. As the method is geared towards maximum objectivity and reproducibility, the results in the small patient group are used to assess the ability of the technique to detect known changes.
2. Computer methods The software MRreg was used to perform all image manipulation and analyses described in this work, except for the two following operations: signal non-uniformity correction, which was performed prior to co-registration, using the publicly available software package N3 [14]; and automatic brain segmentation (prior to co-registration and for cerebellar volumetry), which is performed using our software Exbrain [15]. In the following section, we briefly describe the basic functionality of the MRreg software. The application of the methods to our experimental data is described in the following sections. 2.1. MRreg software functionality for serial volumetry The MRreg software runs on UNIX computers (Sun Microsystems, Palo Alto, California, USA) and can be operated in two modes: command line or window-based. All operations can be performed in the window-based mode, while certain operations cannot be performed in the command line mode, i.e. any that require visualisation of the images. The input of the software is two volume datasets, typically a baseline and a repeat scan. The basic operations that can be performed are: side-by-side display of two datasets; co-registration of two datasets; intensity matching and reconstruction of the co-registered repeat scan; subtraction of the baseline from the matched repeat scan; volumetric, signal change and signal-to-noise ratio (SNR) measurements [16].
Fig. 1. Flowchart of image processing for serial volumetry. “NUC” means Non-uniformity correction.
In practice co-registration, intensity matching and reconstruction of the matched scans are performed using a series of MRreg command-line mode calls combined with calls to Exbrain and N3 for automatic brain segmentation and nonuniformity correction (see next section and Fig. 1). Coregistration can be performed based on either six parameter (three rotation and three translation) or nine (three rotation, three translation, three scaling) rigid-body transformations. MRreg has standard image display facilities, such as display magnification (up to ⫻8 linear) and intensity windowing. Two methods for defining regions of interest (ROI) in individual slices are available in MRreg’s volume measurement tool: manual tracing and region growing. In the region-growing tool, a threshold level is selected and a seed is placed in the slice by mouse clicking. Automatic 2D region growing is then used to connect the seed point to all neighbouring voxels, in the original, unmagnified, image, that have an intensity value equal to or above the threshold value. “Spillage” of the connected region across anatomical boundaries (e.g., from the cerebellum to the brainstem) can be corrected by tracing borders manually. Multiple seeds can be used simultaneously. The seeds are automatically carried to the next slice. The volume of the ROI is calculated by multiplying the voxel volume (mm3) by: for the manual method, the number of screen pixels (at the current degree of magnification) contained within the trace, i.e. excluding the trace itself, divided by the square of the
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
magnification factor; for the region-growing method, the number of connected voxels in the original image data is counted, i.e. independent of the magnification1. The volumes in each individual slice are summed to give the total structure volume. A full record of the volumetric measurement parameters and resulting values is kept in files for output, future reference or re-application to the same or a different image: display window levels, volume in each slice, manual tracings, seed positions, region growing threshold levels, manually drawn borders. The measurements can be done in the baseline, matched repeat or difference scan, either in the original slices or perpendicularly reconstructed (e.g., sagittal or axial from original coronal data). 2.2. Processing of serial MRI The sequence of automatic processing steps leading to the volumetric measurements is described in the flowchart of Fig. 1. After an initial automatic brain segmentation of the baseline and repeat scan using a 2D version of Exbrain [16], non-uniformity correction (NUC) is performed, using N3. A final automatic brain segmentation of the baseline scan is then performed using the 3D version of Exbrain, resulting in an accurate delineation of the cerebrum, cerebellum and brainstem [15]. In the segmented scans, all voxels outside the brain are set to zero intensity. The repeat scan is then co-registered and intensity matched to the segmented baseline scan. The matched repeat scan is then resampled using sinc-based interpolation, with a kernel radius of 5 voxels. A final automatic segmentation of the brain in the matched repeat scan is then performed. In order to take full advantage of the matching of the mean intensities, the foreground and grey matter (GM) threshold values calculated by Exbrain during the segmentation in the baseline scan are used directly (i.e. not recalculated) for the segmentation of the brain in the matched repeat scan [17]. The reproducibility of the total brain volume calculated using Exbrain in matched repeated scans in normal controls was of the order of 1%. Hippocampal volumetry is performed on the baseline and matched repeat scan. Cerebellar volumetry is performed on the matched, segmented images. 3. Experiments 3.1. Subjects, image data and blinding procedure Twenty normal subjects (9 males; mean age at baseline: 31.7 y) were scanned twice with a mean inter-scan interval 1
The connection operation must be performed in the original data in order to avoid the possible effect of interpolation artifacts in the displayed images. On the other hand, the calculation of the area in the manual method must reflect the actual drawing, and therefore the calculation is performed with magnification.
1029
(ISI) of 9.1 months using a 1.5T GE Signa Horizon Echospeed (GE Medical Systems, Milwaukee, WI, USA) MRI scanner. The sequence used was a fast, inversion recoveryprepared, spoiled gradient-recalled (IRSPGR) T1-weighted volume sequence (TI/TR/TE (effective): 450/17.4/4.2 msec, flip angle: 20°, matrix size: 256 ⫻ 192, 24 ⫻ 18 cm FOV, 124, 1.5 mm thick coronal slices). Five patients (2 males; mean age at baseline: 29.4 y) with chronic partial epilepsy were scanned twice using the same sequence, at a mean interval of 22.9 months. Informed consent was obtained from all subjects after the nature of the procedure had been fully explained. The scans were processed as described in the previous section. A nine-parameter rigid body transformation was used for co-registration. The identity of the resulting baseline and matched repeat scans was then hidden and the images displayed side-by-side to the operator (RSNL) in a random order. Therefore, the operator did not know whether the images were from a normal subject or patient, and whether the scan in the left-hand side window was the baseline or matched repeat scan for the current subject. 3.2. Volume measurements 3.2.1. Hippocampal volumetry The matched data sets for a given subject were displayed side-by-side one slice at a time, as shown in Fig. 2. An in-plane magnification factor of ⫻4 was used and the same intensity windowing settings were used in both windows. The hippocampi were delineated using a mouse-driven cursor in accordance with a previously described protocol [18] where the entire antero-posterior extent is measured. The posterior boundary was defined as the oblique coronal section in which the forniceal crura could be seen in its full profile. Delineation continued anteriorly in each contiguous slice reaching the head of the hippocampus which was distinguished from the overlying amygdala by the presence of the alveus or uncal recess. The right hippocampus (RH) was measured first, followed by the left hippocampus (LH). The traces for the RH in the left window (LW) were then reloaded for display purposes in the LW and the RH was then outlined by the operator in the right window (RW) (Fig. 3); a slave cursor was displayed in the LW. Finally, all traces were erased, the traces for the LH in the LW were displayed and the LH was delineated in the RW. The entire procedure for one subject took approximately 75 minutes to perform. 3.2.2. Cerebellar volumetry The two segmented datasets for a given subject were displayed, as described above. An in-plane magnification factor of 2 was used and the same intensity windowing settings were used in both windows. In the region-growing
1030
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
Fig. 2. Display of the matched scans for hippocampal volumetry for a patient (23 y, M, ISI: 12.9 months). In this case, the scan on the left is the baseline scan.
mode, the threshold level is set to unity2, and one or multiple seeds are deposited in the cerebellum. If the slice contains only a section of the body of the cerebellum, then this operation is usually sufficient to obtain a good ROI. If the slice contains cerebellar tissue connected to brainstem or cerebrum tissue, then manual editing is required (see Fig. 4). Volume measurements were performed in every second slice in the coronal plane with simultaneous display of the corresponding sagittal image for better anatomic orientation. Slice measurements progressed sequentially in a rostro-caudal fashion. The rostral border of the ROI was defined as the first section on which cerebellar grey matter became visible and distinguishable from the cerebellar peduncles. An arbitrary border plane was defined as the most rostral slice that cut through the superior and inferior vermis. We therefore used the following definition of the boundaries [19] for total cerebellar volume: hemispheric grey matter, white matter and part of the cerebellar peduncles caudal to the arbitrary plane, cerebellar tonsils, vermis and corpus medullare were included. The 4th ventricle, venous sinuses and the part of the cerebellar peduncles rostral to the arbitrary plane were excluded from the ROI. Thus only grey matter was included rostral to this 2
Since all non-brain voxels have a value of zero.
arbitrary plane since most of the white matter on these sections consisted of the middle cerebellar peduncle. Measurements continued until the cerebellar hemispheres were no longer visible. 3.2.3. Repeatability measures The difference between repeated measurements, d, is defined as: d ⫽ V repeat ⫺ V base Assuming that the mean of d is not significantly different from zero (i.e., small relative to the standard error), we can use the coefficient of repeatability (CR), which is defined as 1.96 times the standard deviation of the differences, [20].3 Another measure, the coefficient of variation (CV), is sometimes used and is defined as the standard deviation of the repeated measurements (pair-wise) expressed as a percentage of the mean measurement [see 13]. However, the CV is most useful when the variability of the measurements is expected to be related to the mean, which is not the case for our type of measurement [21]. In this work, the value of CV
3 This expresses the fact that we expect 95% of differences to be less than 1.96.
Fig. 3. Illustration of hippocampal volumetry procedure in a control subject (33 y, M, ISI: 5.5 months). The baseline scan is on the right. The previously drawn right hippocampal ROI in the left-hand scan is displayed (white trace) while the volume measurement is performed in the right-hand scan (green trace).
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
1031
Fig. 4. Illustration of the cerebellar volumetry procedure in a control subject (30 y, M, ISI: 7.8 months). The baseline scan is on the left. The scans were segmented using Exbrain. The region growing threshold level was set to unity. A seed has been deposited in the cerebellum (red cross) and the boundary of the connected component is shown in green; separation of the cerebellum and cerebrum has been done by manual drawing (red lines).
is calculated to facilitate the comparison of our results with previously published results based on that measure. 4. Results Regarding the blinding of the images, the operator noted that in one case it was visually obvious that the scans were from a patient (the repeat scan showed signs of a brain operation) and therefore this data was removed from the study, leaving 4 patients. In all other cases, the operator was unable to determine the nature of the subject (control or patient) or the chronological order of the scans. 4.1. Hippocampal volumetry In the patients, the mean hippocampal volumes in the baseline scans were 2.500 cm3 and 2.542 cm3 for the RH and LH, respectively, and the mean volume change, d, was ⫺0.080 cm3 and ⫺0.116 cm3. The mean hippocampal baseline volumes in the control group were 2.497 cm3 and 2.501 cm3 for the RH and LH, respectively. The mean value of d was 0.003 cm3, ⫺0.006 cm3 and ⫺0.003 cm3 for the RH, LH and combined hippocampi (CH), respectively. The value of was 0.032 cm3, 0.039 cm3 and 0.058 cm3 for the RH, LH and CH, respectively, corresponding to CR values of 0.064 cm3, 0.078 cm3 and 1.16 cm3. Regarding the CV, the median value was 0.38%, 0.57% and 0.36% (range: 0.01%–1.30%) for the RH, LH and CH, respectively. 4.2. Cerebellar volumetry For the patients, the mean cerebellar baseline volume was 112.0 cm3 and the mean volume change was ⫹0.48 cm3.
The mean baseline volume in the controls was 127.7 cm3 and the mean value of d was ⫺1.21 cm3. The value of was 1.90 cm3 corresponding to a CR value of 3.8 cm3. Regarding the CV, the median value was 0.66% (range: 0.01%– 1.54%).
5. Discussion Our approach to serial measurements is based on the use of co-registration and side-by-side display of the matched images. Furthermore the display of the boundaries drawn in one of the scans while measuring in the other is designed to help the user in making the subjective drawing decisions more consistently. On the other hand such an approach may introduce bias, i.e. reduce sensitivity to genuine differences through imitation. The validation of our approach therefore required that the operator be blinded to the clinical status of the subject and the chronological order of the scans. This was achieved by randomly mixing normal controls and patients of a similar age range and randomly re-ordering the scans chronologically for each subject. In the small group of patients chosen, there was a suspicion that some hippocampal and cerebellar damage may have taken place in the inter-scan interval. While performing the measurements, we found that the operator was unable to guess whether a given subject was a normal control or a patient (except in one case, as noted previously). On the other hand, the measurements revealed larger hippocampal volume differences in the patients than in the controls; in particular one patient had significant volume losses in both hippocampi (0.277 cm3 in the RH and 0.426 cm3 for the LH). Therefore, this indicates that the operator was performing the task correctly. Furthermore, we have recently found significant hippocampal and cerebellar volume increases in a subject following a dra-
1032
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
matic change in alcohol consumption habit using the methodology presented here [22]. This indicates that reproducibility has not been maximized at the cost of loss of sensitivity. The usefulness of CR as a measure of repeatability in our study relies on the assertion that the mean difference in the repeated measures in the control group was small. The choice of an inter-scan interval of the order of 7 months can be justified as a compromise between the need to minimise any biological effect, i.e. due to natural volume loss, and the need to test the ability of the scan matching process to compensate for variability in the scanner’s performance over a long period; In particular, we note that fluctuations in the performance of the scanner’s gradients can lead to variations of the order of 1% in the voxel dimensions [23]. Although the registration process should compensate for this, by including linear scaling, it is likely that the registration error depends on the amount of scaling correction. Taking the largest value of CR, our results indicate that individual hippocampal volume changes greater than 0.078 cm3 can be detected and measured reliably in individual subjects using our method; for the sum of the RH and LH volumes, the threshold of detectability is 0.116 cm3; these are equivalent to 3.1% and 2.3% of the mean baseline individual and combined volumes, respectively. For the cerebellar volume, changes greater than 3.8 cm3 can be detected reliably, which is equivalent to 3.0% of the mean baseline normal volume. The mean volume change in the cerebellum was larger than the standard measurement error4, indicating a slight bias (0.9% of the mean cerebellar volume). This may be due to either of the following effects: firstly, a difference in the visual appearance of the baseline and matched images, due to the interpolation of the latter following registration; secondly, a bias in the automated segmentation algorithm resulting in a larger volume of the brain; thirdly, normal biological variation. The first effect is subtle due to the use of a sinc interpolation kernel with a radius of 5 (see Fig. 4). The second effect is unlikely, as our previous results on the segmentation of the whole brain in repeated scans have shown [15]. Therefore, we conclude that the measured volume changes in our control group result from a combination of all the above effects. We also note that the mean CV change in the patient group is less than CR. Regarding the CV as a measure of repeatability, we find that our results are comparable to those obtained by Jack et al. for the hippocampi. The slightly higher median CV for CH may be explained by a higher degree of scanner variability (signal to noise, geometric distortions, etc) due to the much longer inter-scan interval in our study. We also note that the CV, as a measure or repeatability, appears superficially to give much more favourable results than the CR. We must emphasise that we have not attempted to esti-
4
公( 2 /n) ⫽ 0.42 cm3.
mate the accuracy and reproducibility of volume changes that may be measured in patients using our method. Regarding the accuracy, this is problematic due to the difficulty of obtaining an independent measure. The reproducibility of measurements of changes can be assessed using the methodology described here and will be the subject of a further study once sufficient patient data as been gathered. Finally, our results validate our approach which is based on mixing repeated scans of patients and normal controls and randomizing the scan order, combined with side-byside display: to our knowledge this has not been done previously and should lead to improved sensitivity while minimizing subjectivity.
6. Conclusions We have demonstrated a methodology for serial MRI volumetric measurements aimed at detecting small volume changes in the hippocampus and cerebellum. We have tested the repeatability of the measurements in a blinded study designed to simulate a prospective study containing normal controls and subjects with epilepsy as realistically as possible. Based on the results in our control group, volume changes greater than 0.078 cm3 for the hippocampus and 3.8 cm3 for the cerebellum can be detected reliably.
Acknowledgments The authors would like to thank Dr. K. Krakow, of the Epilepsy Research Group, Institute of Neurology, London, UK, and Dr. G. Hagemann, of Heinrich-Heine University, Du¨sseldorf, Germany, for their advice on the cerebellar volume measurements. This study is partly funded by the Wellcome Trust. The generous support of the National Society for Epilepsy (UK) is also acknowledged.
References [1] Hajnal JV, Saeed N, Oatridge A, Williams EJ, Young IR, Bydder GM. Detection of subtle brain changes using subvoxel registration and subtraction of serial MR images. J Comput Assist Tomogr 1995; 19:677–91. [2] Fox NC, Freeborough PA, Rossor MN. Visualisation and quantification of rates of atrophy in Alzheimer’s disease. Lancet 1996;348: 94 –7. [3] Wieshmann UC, Woermann FG, Lemieux L, Free SL, Bartlett PA, Smith SJM, Duncan JS, Stevens JM, Shorvon SD. The development of hippocampal sclerosis: a serial MRI study over a 58 month period in a patient after status. Epilepsia 1997;38:1238 – 41. [4] Jager HR, Williams EJ, Savage DG, Rule SAJ, Hajnal JV, Sikora K, Goldman JM, Bydder GM. Assessment of brain shape changes with registered MR before and after bone marrow transplantation for chronic myeloid leukemia. Am J Neuroradiol 1996;17:1275– 82. [5] Ellamushi H, Moran NF, Kitchen ND, Stevens JM, Kendall BE, Lemieux L. Generalised cerebral atrophy following temporal lobec-
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
[6]
[7]
[8]
[9]
[10] [11] [12]
[13]
[14]
tomy for intractable epilepsy associated with mesial temporal sclerosis. J Magn Reson Imaging 2000;18:269 –74. Cendes F, Andermann F, Gloor P, Lopes-Cendes I, Andermann E, Melanson D, Jones-Gotman M, Robitaille Y, Evans A, Peters T. Atrophy of mesial structures in patients with temporal lobe epilepsy: cause or consequence of repeated seizures? Ann Neurol 1993;34: 795– 801. Mathern GW, Babb TL, Leite JP, Pretorius JK, Yeoman KM, Kuhlman PA. The pathogenic and progressive features of chronic human hippocampal epilepsy. Epilepsy Res 1996;26:151– 61. Van Paesschen W, Connelly A, King MD, Jackson GD, Duncan JS. The spectrum of hippocampal sclerosis: a quantitative magnetic resonance imaging study. Ann Neurol 1997;41:41–51. Theodore WH, Bhatia S, Hatta J, Fazilat S, DeCarli C, Bookheimer SY, Gaillard WD. Hippocampal atrophy, epilepsy duration, and febrile seizures in patients with partial seizures. Neurology 1999;52:132– 6. Jefferys JGR. Hippocampal sclerosis and temporal lobe epilepsy: cause or consequence? Brain 1999;122:1007– 8. McLain LW, Martin JT, Allen JH. Cerebellar degeneration due to chronic phenytoin therapy. Ann Neurol 1980;7:18 –23. Hagemann G, Krakow K, Free SL, Kendall BE, Everitt AD, Lemieux L, Stevens JM, Duncan JS. Cerebellar atrophy in a prospectively studied population with epilepsy: a volumetric MRI study. Neuroimage 1999;9:S567. Jack CR, Petersen RC, Xu Y, O’Brien PC, Smith GE, Ivnik RJ, Tangalos EG, Kokmen E. Rate of medial temporal lobe atrophy in typical aging and Alzheimer’s disease. Neurology 1998;51:993–9. Sled JG, Zijdenbos AP, Evans AC. A nonparametric method for automatic correction of intensity nonuniformity in MRI data. IEEE Trans Med Imag 1998;17:87–97.
1033
[15] Lemieux L, Hagemann G, Krakow K, Woermann FG. Fast, accurate and reproducible automatic segmentation of the brain in T1-weighted volume magnetic resonance image data. Magn Reson Med 1999;42: 127–35. [16] Lemieux L, Wieshmann UC, Moran NF, Fish DR, Shorvon SD. The detection and significance of subtle changes in mixed-signal brain lesions by serial MRI scan matching and spatial normalization. Med Im Anal 1998;2:227– 42. [17] Lemieux L. Automatic segmentation of grey and white matter and cerebrospinal fluid in serial T1-weighted volume MRI data. Proceedings of Medical Image Analysis and Understanding ’99, Oxford, England. [18] Cook JM, Fish DR, Shorvon SD, Straughan K, Stevens JM. Hippocampal volumetric and morphometric studies in frontal and temporal lobe epilepsy. Brain 1992;115:1001–15. [19] Raz N, Dupuis JH, Briggs SD, McGavran C, Acker JD. Differential effects of age and sex on the cerebellar hemispheres and the vermis: a prospective MRI study. Am J Neuroradiol 1998;19:65–71. [20] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;327: 307–10. [21] Moran NF, Lemieux L, Maudgil DD, Kitchen ND, Fish DR, Shorvond SD. Analysis of temporal lobe resections in MR images. Epilepsia 1999;40:1077– 84. [22] Liu RSN, Lemieux L, Shorvon SD, Sisodiya SM, Sander JWAS, Bell G, Duncan JS. Increase in brain size with abstinence from alcohol. Lancet 2000;355:1969 –70. [23] Lemieux L, Barker GJ. Measurement of small inter-scan fluctuations in voxel dimensions in magnetic resonance images using registration. Med Phys 1998;25:1049 –54.
Technical note
Hippocampal and cerebellar volumetry in serially acquired MRI volume scans L. Lemieux*, Rebecca S.N. Liu, John S. Duncan Epilepsy Research Group, University Department of Clinical Neurology, Institute of Neurology, University College London, 33 Queen Square, London WC1N 3BG, United Kingdom National Society for Epilepsy, Chalfont St Peter, Buckinghamshire SL9 0RJ, United Kingdom Received 19 July 2000; accepted 2 September 2000
Abstract In this work, we describe methodologies for serial volumetric measurements of hippocampi and cerebella. Serial scans were co-registered and intensity matched prior to the volumetric measurements. Manual drawing was used to define the boundaries of the hippocampi. For the cerebellar volumetric measurements, the brain was automatically segmented from the co-registered scans; manual drawing was used to define the boundary between the cerebellum and the cerebrum and brainstem. The operator was blinded to the nature of the subject (patient or normal control) and the chronological order of the scans. The coefficient of reliability of hippocampal volume measurements in a group of 20 controls was 0.078 cm3 (3.1% of the mean baseline volume); for the cerebellum, the value was 3.8 cm3 (3.0% of the mean baseline volume). We conclude that the methods presented are valid and that the software provides a useful integrated tool for the quantitative analysis of structural changes in serially acquired volume MRI data in prospective, blinded studies. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Volumetry; Hippocampus; Cerebellum; Longitudinal
1. Introduction Longitudinal imaging studies offer a unique opportunity to study the morphological evolution of the brain. In particular, the analysis of co-registered serial MRI data has already led to new insights in the progression of neurological disorders [1–3] and the effects of treatment [4,5]. The usefulness of co-registration relies on the fact that the sensitivity to change can be improved by correcting for differences in scan position, orientation and intensity prior to the analysis. This work is concerned with methodological issues related to the following question: what morphological changes are associated with chronic epilepsy? One of the most debated issues in the field of epilepsy research is the causal relationship between hippocampal sclerosis and epilepsy [6 –10]. It is therefore of great interest to determine the time-course of hippocampal volume changes in relation to the disease onset and subsequent evolution in a longitudinal, rather than cross-sectional, fashion. Currently, hip* Corresponding author. Tel.: ⫹44-1494-601361; fax: ⫹44-1494875666. E-mail address: [email protected] (L. Lemieux).
pocampal volume measurements are largely performed by manual delineation in individual slices from volumetric MRI data. Automation of this process is made difficult by the small volume of the structure and the lack of clearly visible boundaries with certain neighbouring structures, e.g. amygdala, para-hippocampal gyrus. The sensitivity of the manual technique to change is therefore limited by its subjectivity. In serial imaging, where we are mainly interested in differences between repeated measurements, the variability of repeated measurements will be larger than that of single-scan repeated measurements. Co-registration of the serial scans should lead to an improvement in the variability of repeated measurements by providing a consistent presentation of the scans for individual subjects. The quantification of cerebellar atrophy in a prospective study of patients with epilepsy is of interest since recurrent seizures and chronic use of certain anti-epileptic drugs, particularly phenytoin, have been associated with cerebellar atrophy [11]. Although a large proportion of the cerebellum’s boundary, i.e. cortex/cerebrospinal fluid (CSF), is clearly and easily defined, cerebellar volumetry requires the operator to make subjective decisions to define the boundary with the brainstem [12].
0730-725X/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 0 ) 0 0 2 1 0 - 1
1028
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
The aim of this study was to determine the reproducibility of hippocampal and cerebellar volume measurements in serial MRI data after co-registration and intensity matching. Jack et al. have published a study on the reproducibility of hippocampal volume measurements following co-registration [13]. However, some of the methodological details (e.g., whether the operator can visualise the two datasets side-by-side, and if so, whether the traces drawn in the first scan were visualised for drawing in the second scan) were not clearly presented. Furthermore, the observers were not blinded to the nature of the subject when assessing the reproducibility of their technique as only controls were used. In this work, the reproducibility of the volumetric measurements in normal subjects was assessed in a group of subjects containing a blinded mixture of normal subjects and patients with chronic localisation-related epilepsy. The operator was blinded to the chronological order of the scans during the measurements. As the method is geared towards maximum objectivity and reproducibility, the results in the small patient group are used to assess the ability of the technique to detect known changes.
2. Computer methods The software MRreg was used to perform all image manipulation and analyses described in this work, except for the two following operations: signal non-uniformity correction, which was performed prior to co-registration, using the publicly available software package N3 [14]; and automatic brain segmentation (prior to co-registration and for cerebellar volumetry), which is performed using our software Exbrain [15]. In the following section, we briefly describe the basic functionality of the MRreg software. The application of the methods to our experimental data is described in the following sections. 2.1. MRreg software functionality for serial volumetry The MRreg software runs on UNIX computers (Sun Microsystems, Palo Alto, California, USA) and can be operated in two modes: command line or window-based. All operations can be performed in the window-based mode, while certain operations cannot be performed in the command line mode, i.e. any that require visualisation of the images. The input of the software is two volume datasets, typically a baseline and a repeat scan. The basic operations that can be performed are: side-by-side display of two datasets; co-registration of two datasets; intensity matching and reconstruction of the co-registered repeat scan; subtraction of the baseline from the matched repeat scan; volumetric, signal change and signal-to-noise ratio (SNR) measurements [16].
Fig. 1. Flowchart of image processing for serial volumetry. “NUC” means Non-uniformity correction.
In practice co-registration, intensity matching and reconstruction of the matched scans are performed using a series of MRreg command-line mode calls combined with calls to Exbrain and N3 for automatic brain segmentation and nonuniformity correction (see next section and Fig. 1). Coregistration can be performed based on either six parameter (three rotation and three translation) or nine (three rotation, three translation, three scaling) rigid-body transformations. MRreg has standard image display facilities, such as display magnification (up to ⫻8 linear) and intensity windowing. Two methods for defining regions of interest (ROI) in individual slices are available in MRreg’s volume measurement tool: manual tracing and region growing. In the region-growing tool, a threshold level is selected and a seed is placed in the slice by mouse clicking. Automatic 2D region growing is then used to connect the seed point to all neighbouring voxels, in the original, unmagnified, image, that have an intensity value equal to or above the threshold value. “Spillage” of the connected region across anatomical boundaries (e.g., from the cerebellum to the brainstem) can be corrected by tracing borders manually. Multiple seeds can be used simultaneously. The seeds are automatically carried to the next slice. The volume of the ROI is calculated by multiplying the voxel volume (mm3) by: for the manual method, the number of screen pixels (at the current degree of magnification) contained within the trace, i.e. excluding the trace itself, divided by the square of the
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
magnification factor; for the region-growing method, the number of connected voxels in the original image data is counted, i.e. independent of the magnification1. The volumes in each individual slice are summed to give the total structure volume. A full record of the volumetric measurement parameters and resulting values is kept in files for output, future reference or re-application to the same or a different image: display window levels, volume in each slice, manual tracings, seed positions, region growing threshold levels, manually drawn borders. The measurements can be done in the baseline, matched repeat or difference scan, either in the original slices or perpendicularly reconstructed (e.g., sagittal or axial from original coronal data). 2.2. Processing of serial MRI The sequence of automatic processing steps leading to the volumetric measurements is described in the flowchart of Fig. 1. After an initial automatic brain segmentation of the baseline and repeat scan using a 2D version of Exbrain [16], non-uniformity correction (NUC) is performed, using N3. A final automatic brain segmentation of the baseline scan is then performed using the 3D version of Exbrain, resulting in an accurate delineation of the cerebrum, cerebellum and brainstem [15]. In the segmented scans, all voxels outside the brain are set to zero intensity. The repeat scan is then co-registered and intensity matched to the segmented baseline scan. The matched repeat scan is then resampled using sinc-based interpolation, with a kernel radius of 5 voxels. A final automatic segmentation of the brain in the matched repeat scan is then performed. In order to take full advantage of the matching of the mean intensities, the foreground and grey matter (GM) threshold values calculated by Exbrain during the segmentation in the baseline scan are used directly (i.e. not recalculated) for the segmentation of the brain in the matched repeat scan [17]. The reproducibility of the total brain volume calculated using Exbrain in matched repeated scans in normal controls was of the order of 1%. Hippocampal volumetry is performed on the baseline and matched repeat scan. Cerebellar volumetry is performed on the matched, segmented images. 3. Experiments 3.1. Subjects, image data and blinding procedure Twenty normal subjects (9 males; mean age at baseline: 31.7 y) were scanned twice with a mean inter-scan interval 1
The connection operation must be performed in the original data in order to avoid the possible effect of interpolation artifacts in the displayed images. On the other hand, the calculation of the area in the manual method must reflect the actual drawing, and therefore the calculation is performed with magnification.
1029
(ISI) of 9.1 months using a 1.5T GE Signa Horizon Echospeed (GE Medical Systems, Milwaukee, WI, USA) MRI scanner. The sequence used was a fast, inversion recoveryprepared, spoiled gradient-recalled (IRSPGR) T1-weighted volume sequence (TI/TR/TE (effective): 450/17.4/4.2 msec, flip angle: 20°, matrix size: 256 ⫻ 192, 24 ⫻ 18 cm FOV, 124, 1.5 mm thick coronal slices). Five patients (2 males; mean age at baseline: 29.4 y) with chronic partial epilepsy were scanned twice using the same sequence, at a mean interval of 22.9 months. Informed consent was obtained from all subjects after the nature of the procedure had been fully explained. The scans were processed as described in the previous section. A nine-parameter rigid body transformation was used for co-registration. The identity of the resulting baseline and matched repeat scans was then hidden and the images displayed side-by-side to the operator (RSNL) in a random order. Therefore, the operator did not know whether the images were from a normal subject or patient, and whether the scan in the left-hand side window was the baseline or matched repeat scan for the current subject. 3.2. Volume measurements 3.2.1. Hippocampal volumetry The matched data sets for a given subject were displayed side-by-side one slice at a time, as shown in Fig. 2. An in-plane magnification factor of ⫻4 was used and the same intensity windowing settings were used in both windows. The hippocampi were delineated using a mouse-driven cursor in accordance with a previously described protocol [18] where the entire antero-posterior extent is measured. The posterior boundary was defined as the oblique coronal section in which the forniceal crura could be seen in its full profile. Delineation continued anteriorly in each contiguous slice reaching the head of the hippocampus which was distinguished from the overlying amygdala by the presence of the alveus or uncal recess. The right hippocampus (RH) was measured first, followed by the left hippocampus (LH). The traces for the RH in the left window (LW) were then reloaded for display purposes in the LW and the RH was then outlined by the operator in the right window (RW) (Fig. 3); a slave cursor was displayed in the LW. Finally, all traces were erased, the traces for the LH in the LW were displayed and the LH was delineated in the RW. The entire procedure for one subject took approximately 75 minutes to perform. 3.2.2. Cerebellar volumetry The two segmented datasets for a given subject were displayed, as described above. An in-plane magnification factor of 2 was used and the same intensity windowing settings were used in both windows. In the region-growing
1030
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
Fig. 2. Display of the matched scans for hippocampal volumetry for a patient (23 y, M, ISI: 12.9 months). In this case, the scan on the left is the baseline scan.
mode, the threshold level is set to unity2, and one or multiple seeds are deposited in the cerebellum. If the slice contains only a section of the body of the cerebellum, then this operation is usually sufficient to obtain a good ROI. If the slice contains cerebellar tissue connected to brainstem or cerebrum tissue, then manual editing is required (see Fig. 4). Volume measurements were performed in every second slice in the coronal plane with simultaneous display of the corresponding sagittal image for better anatomic orientation. Slice measurements progressed sequentially in a rostro-caudal fashion. The rostral border of the ROI was defined as the first section on which cerebellar grey matter became visible and distinguishable from the cerebellar peduncles. An arbitrary border plane was defined as the most rostral slice that cut through the superior and inferior vermis. We therefore used the following definition of the boundaries [19] for total cerebellar volume: hemispheric grey matter, white matter and part of the cerebellar peduncles caudal to the arbitrary plane, cerebellar tonsils, vermis and corpus medullare were included. The 4th ventricle, venous sinuses and the part of the cerebellar peduncles rostral to the arbitrary plane were excluded from the ROI. Thus only grey matter was included rostral to this 2
Since all non-brain voxels have a value of zero.
arbitrary plane since most of the white matter on these sections consisted of the middle cerebellar peduncle. Measurements continued until the cerebellar hemispheres were no longer visible. 3.2.3. Repeatability measures The difference between repeated measurements, d, is defined as: d ⫽ V repeat ⫺ V base Assuming that the mean of d is not significantly different from zero (i.e., small relative to the standard error), we can use the coefficient of repeatability (CR), which is defined as 1.96 times the standard deviation of the differences, [20].3 Another measure, the coefficient of variation (CV), is sometimes used and is defined as the standard deviation of the repeated measurements (pair-wise) expressed as a percentage of the mean measurement [see 13]. However, the CV is most useful when the variability of the measurements is expected to be related to the mean, which is not the case for our type of measurement [21]. In this work, the value of CV
3 This expresses the fact that we expect 95% of differences to be less than 1.96.
Fig. 3. Illustration of hippocampal volumetry procedure in a control subject (33 y, M, ISI: 5.5 months). The baseline scan is on the right. The previously drawn right hippocampal ROI in the left-hand scan is displayed (white trace) while the volume measurement is performed in the right-hand scan (green trace).
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
1031
Fig. 4. Illustration of the cerebellar volumetry procedure in a control subject (30 y, M, ISI: 7.8 months). The baseline scan is on the left. The scans were segmented using Exbrain. The region growing threshold level was set to unity. A seed has been deposited in the cerebellum (red cross) and the boundary of the connected component is shown in green; separation of the cerebellum and cerebrum has been done by manual drawing (red lines).
is calculated to facilitate the comparison of our results with previously published results based on that measure. 4. Results Regarding the blinding of the images, the operator noted that in one case it was visually obvious that the scans were from a patient (the repeat scan showed signs of a brain operation) and therefore this data was removed from the study, leaving 4 patients. In all other cases, the operator was unable to determine the nature of the subject (control or patient) or the chronological order of the scans. 4.1. Hippocampal volumetry In the patients, the mean hippocampal volumes in the baseline scans were 2.500 cm3 and 2.542 cm3 for the RH and LH, respectively, and the mean volume change, d, was ⫺0.080 cm3 and ⫺0.116 cm3. The mean hippocampal baseline volumes in the control group were 2.497 cm3 and 2.501 cm3 for the RH and LH, respectively. The mean value of d was 0.003 cm3, ⫺0.006 cm3 and ⫺0.003 cm3 for the RH, LH and combined hippocampi (CH), respectively. The value of was 0.032 cm3, 0.039 cm3 and 0.058 cm3 for the RH, LH and CH, respectively, corresponding to CR values of 0.064 cm3, 0.078 cm3 and 1.16 cm3. Regarding the CV, the median value was 0.38%, 0.57% and 0.36% (range: 0.01%–1.30%) for the RH, LH and CH, respectively. 4.2. Cerebellar volumetry For the patients, the mean cerebellar baseline volume was 112.0 cm3 and the mean volume change was ⫹0.48 cm3.
The mean baseline volume in the controls was 127.7 cm3 and the mean value of d was ⫺1.21 cm3. The value of was 1.90 cm3 corresponding to a CR value of 3.8 cm3. Regarding the CV, the median value was 0.66% (range: 0.01%– 1.54%).
5. Discussion Our approach to serial measurements is based on the use of co-registration and side-by-side display of the matched images. Furthermore the display of the boundaries drawn in one of the scans while measuring in the other is designed to help the user in making the subjective drawing decisions more consistently. On the other hand such an approach may introduce bias, i.e. reduce sensitivity to genuine differences through imitation. The validation of our approach therefore required that the operator be blinded to the clinical status of the subject and the chronological order of the scans. This was achieved by randomly mixing normal controls and patients of a similar age range and randomly re-ordering the scans chronologically for each subject. In the small group of patients chosen, there was a suspicion that some hippocampal and cerebellar damage may have taken place in the inter-scan interval. While performing the measurements, we found that the operator was unable to guess whether a given subject was a normal control or a patient (except in one case, as noted previously). On the other hand, the measurements revealed larger hippocampal volume differences in the patients than in the controls; in particular one patient had significant volume losses in both hippocampi (0.277 cm3 in the RH and 0.426 cm3 for the LH). Therefore, this indicates that the operator was performing the task correctly. Furthermore, we have recently found significant hippocampal and cerebellar volume increases in a subject following a dra-
1032
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
matic change in alcohol consumption habit using the methodology presented here [22]. This indicates that reproducibility has not been maximized at the cost of loss of sensitivity. The usefulness of CR as a measure of repeatability in our study relies on the assertion that the mean difference in the repeated measures in the control group was small. The choice of an inter-scan interval of the order of 7 months can be justified as a compromise between the need to minimise any biological effect, i.e. due to natural volume loss, and the need to test the ability of the scan matching process to compensate for variability in the scanner’s performance over a long period; In particular, we note that fluctuations in the performance of the scanner’s gradients can lead to variations of the order of 1% in the voxel dimensions [23]. Although the registration process should compensate for this, by including linear scaling, it is likely that the registration error depends on the amount of scaling correction. Taking the largest value of CR, our results indicate that individual hippocampal volume changes greater than 0.078 cm3 can be detected and measured reliably in individual subjects using our method; for the sum of the RH and LH volumes, the threshold of detectability is 0.116 cm3; these are equivalent to 3.1% and 2.3% of the mean baseline individual and combined volumes, respectively. For the cerebellar volume, changes greater than 3.8 cm3 can be detected reliably, which is equivalent to 3.0% of the mean baseline normal volume. The mean volume change in the cerebellum was larger than the standard measurement error4, indicating a slight bias (0.9% of the mean cerebellar volume). This may be due to either of the following effects: firstly, a difference in the visual appearance of the baseline and matched images, due to the interpolation of the latter following registration; secondly, a bias in the automated segmentation algorithm resulting in a larger volume of the brain; thirdly, normal biological variation. The first effect is subtle due to the use of a sinc interpolation kernel with a radius of 5 (see Fig. 4). The second effect is unlikely, as our previous results on the segmentation of the whole brain in repeated scans have shown [15]. Therefore, we conclude that the measured volume changes in our control group result from a combination of all the above effects. We also note that the mean CV change in the patient group is less than CR. Regarding the CV as a measure of repeatability, we find that our results are comparable to those obtained by Jack et al. for the hippocampi. The slightly higher median CV for CH may be explained by a higher degree of scanner variability (signal to noise, geometric distortions, etc) due to the much longer inter-scan interval in our study. We also note that the CV, as a measure or repeatability, appears superficially to give much more favourable results than the CR. We must emphasise that we have not attempted to esti-
4
公( 2 /n) ⫽ 0.42 cm3.
mate the accuracy and reproducibility of volume changes that may be measured in patients using our method. Regarding the accuracy, this is problematic due to the difficulty of obtaining an independent measure. The reproducibility of measurements of changes can be assessed using the methodology described here and will be the subject of a further study once sufficient patient data as been gathered. Finally, our results validate our approach which is based on mixing repeated scans of patients and normal controls and randomizing the scan order, combined with side-byside display: to our knowledge this has not been done previously and should lead to improved sensitivity while minimizing subjectivity.
6. Conclusions We have demonstrated a methodology for serial MRI volumetric measurements aimed at detecting small volume changes in the hippocampus and cerebellum. We have tested the repeatability of the measurements in a blinded study designed to simulate a prospective study containing normal controls and subjects with epilepsy as realistically as possible. Based on the results in our control group, volume changes greater than 0.078 cm3 for the hippocampus and 3.8 cm3 for the cerebellum can be detected reliably.
Acknowledgments The authors would like to thank Dr. K. Krakow, of the Epilepsy Research Group, Institute of Neurology, London, UK, and Dr. G. Hagemann, of Heinrich-Heine University, Du¨sseldorf, Germany, for their advice on the cerebellar volume measurements. This study is partly funded by the Wellcome Trust. The generous support of the National Society for Epilepsy (UK) is also acknowledged.
References [1] Hajnal JV, Saeed N, Oatridge A, Williams EJ, Young IR, Bydder GM. Detection of subtle brain changes using subvoxel registration and subtraction of serial MR images. J Comput Assist Tomogr 1995; 19:677–91. [2] Fox NC, Freeborough PA, Rossor MN. Visualisation and quantification of rates of atrophy in Alzheimer’s disease. Lancet 1996;348: 94 –7. [3] Wieshmann UC, Woermann FG, Lemieux L, Free SL, Bartlett PA, Smith SJM, Duncan JS, Stevens JM, Shorvon SD. The development of hippocampal sclerosis: a serial MRI study over a 58 month period in a patient after status. Epilepsia 1997;38:1238 – 41. [4] Jager HR, Williams EJ, Savage DG, Rule SAJ, Hajnal JV, Sikora K, Goldman JM, Bydder GM. Assessment of brain shape changes with registered MR before and after bone marrow transplantation for chronic myeloid leukemia. Am J Neuroradiol 1996;17:1275– 82. [5] Ellamushi H, Moran NF, Kitchen ND, Stevens JM, Kendall BE, Lemieux L. Generalised cerebral atrophy following temporal lobec-
L. Lemieux et al. / Magnetic Resonance Imaging 18 (2000) 1027–1033
[6]
[7]
[8]
[9]
[10] [11] [12]
[13]
[14]
tomy for intractable epilepsy associated with mesial temporal sclerosis. J Magn Reson Imaging 2000;18:269 –74. Cendes F, Andermann F, Gloor P, Lopes-Cendes I, Andermann E, Melanson D, Jones-Gotman M, Robitaille Y, Evans A, Peters T. Atrophy of mesial structures in patients with temporal lobe epilepsy: cause or consequence of repeated seizures? Ann Neurol 1993;34: 795– 801. Mathern GW, Babb TL, Leite JP, Pretorius JK, Yeoman KM, Kuhlman PA. The pathogenic and progressive features of chronic human hippocampal epilepsy. Epilepsy Res 1996;26:151– 61. Van Paesschen W, Connelly A, King MD, Jackson GD, Duncan JS. The spectrum of hippocampal sclerosis: a quantitative magnetic resonance imaging study. Ann Neurol 1997;41:41–51. Theodore WH, Bhatia S, Hatta J, Fazilat S, DeCarli C, Bookheimer SY, Gaillard WD. Hippocampal atrophy, epilepsy duration, and febrile seizures in patients with partial seizures. Neurology 1999;52:132– 6. Jefferys JGR. Hippocampal sclerosis and temporal lobe epilepsy: cause or consequence? Brain 1999;122:1007– 8. McLain LW, Martin JT, Allen JH. Cerebellar degeneration due to chronic phenytoin therapy. Ann Neurol 1980;7:18 –23. Hagemann G, Krakow K, Free SL, Kendall BE, Everitt AD, Lemieux L, Stevens JM, Duncan JS. Cerebellar atrophy in a prospectively studied population with epilepsy: a volumetric MRI study. Neuroimage 1999;9:S567. Jack CR, Petersen RC, Xu Y, O’Brien PC, Smith GE, Ivnik RJ, Tangalos EG, Kokmen E. Rate of medial temporal lobe atrophy in typical aging and Alzheimer’s disease. Neurology 1998;51:993–9. Sled JG, Zijdenbos AP, Evans AC. A nonparametric method for automatic correction of intensity nonuniformity in MRI data. IEEE Trans Med Imag 1998;17:87–97.
1033
[15] Lemieux L, Hagemann G, Krakow K, Woermann FG. Fast, accurate and reproducible automatic segmentation of the brain in T1-weighted volume magnetic resonance image data. Magn Reson Med 1999;42: 127–35. [16] Lemieux L, Wieshmann UC, Moran NF, Fish DR, Shorvon SD. The detection and significance of subtle changes in mixed-signal brain lesions by serial MRI scan matching and spatial normalization. Med Im Anal 1998;2:227– 42. [17] Lemieux L. Automatic segmentation of grey and white matter and cerebrospinal fluid in serial T1-weighted volume MRI data. Proceedings of Medical Image Analysis and Understanding ’99, Oxford, England. [18] Cook JM, Fish DR, Shorvon SD, Straughan K, Stevens JM. Hippocampal volumetric and morphometric studies in frontal and temporal lobe epilepsy. Brain 1992;115:1001–15. [19] Raz N, Dupuis JH, Briggs SD, McGavran C, Acker JD. Differential effects of age and sex on the cerebellar hemispheres and the vermis: a prospective MRI study. Am J Neuroradiol 1998;19:65–71. [20] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;327: 307–10. [21] Moran NF, Lemieux L, Maudgil DD, Kitchen ND, Fish DR, Shorvond SD. Analysis of temporal lobe resections in MR images. Epilepsia 1999;40:1077– 84. [22] Liu RSN, Lemieux L, Shorvon SD, Sisodiya SM, Sander JWAS, Bell G, Duncan JS. Increase in brain size with abstinence from alcohol. Lancet 2000;355:1969 –70. [23] Lemieux L, Barker GJ. Measurement of small inter-scan fluctuations in voxel dimensions in magnetic resonance images using registration. Med Phys 1998;25:1049 –54.