- Email: [email protected]
New scans from old: digital reformatting knee MRI
Radiography (2001) 7, 221–226 doi:10.1053/radi.2001.0335, available online at http://www.idealibrary.com on
New scans from old: digital reformatting knee MRI M. Glisson, MAppSci, Radiographer and Senior Lecturer* A. Forbes, PhD, Senior Lecturer† K. Morris, ME FIEAust, Chief Researcher‡ S. Stuckey, M Med, FRANZCR, Director MRI Unit§ and F. Cicuttini, MB BS, PhD, FRACP, Rheumatologist and Assoc. Professor† *Department of Radiography and Medical Imaging, Monash University; †Department of Epidemiology and Preventive Medicine, Monash University Medical School, Alfred Hospital, Prahran, Victoria, Australia; ‡Victorian Epilepsy Centre, St Vincent’s Hospital, Fitzroy, Victoria, Australia; §Department of Magnetic Resonance Imaging, Alfred Hospital, Prahran, Victoria, Australia (Received 14 September 2000; revised 30 March 2001; second revision 29 June 2001; accepted 16 July 2001)
Key words: knee cartilage; MRI; reformatted images.
Purpose: To determine the accuracy of cartilage volume and bone areas measured from a 3D knee MRI sequence reformatted in different planes. Methods: MRI of 16 adult subjects (9 females, 7 males, age range 45–68 years) were acquired in the sagittal plane using a 3D T1-weighted fat suppressed spoiled gradient echo sequence. Medial and lateral tibial cartilage volumes were determined by processing images acquired in the sagittal plane and from the same image data reformatted in the coronal plane. Tibial plateau areas were determined by processing images acquired in the sagittal plane and reformatted in the axial plane. Results: Cartilage volumes calculated from the original sagittal acquisition and data reformatted into the coronal plane were similar. The average over- or under-estimation of the lateral and medial cartilage volume from the reformatted coronal scans compared to the sagittal sequences was 4.6% and 9.8% respectively. Similar medial and lateral tibial plateau areas were obtained when the sagittal data was reformatted in the axial plane. The average over- and under-estimate of lateral and medial tibial plateau areas from the reformatted axial scans compared to the originally acquired sagittal sequences was 6.5% and 6.8% respectively. Conclusion: Knee data acquired via MRI in one plane can be reformatted into different planes, providing comparable cartilage volumes and bone areas. As different planes through the knee may provide better visualization of different joint structures, this method may be useful clinically and as a research tool, while avoiding the cost associated with the prolonged scanning times associated with acquiring multiple planes.
Introduction There has been increasing interest in the use of magnetic resonance imaging (MRI) in the measurement of knee cartilage volume as a possible outcome measure in arthritis [1–3]. MRI has been shown to be a valid measure of cartilage volume Correspondence should be addressed to: Assoc Prof. Flavia Cicuttini, Department Epidemiology and Preventive Medicine, Alfred Hospital, Prahran, Victoria, 3181, Australia. Tel: 61-3-9903 0555; Fax: 61-39903 0556; E-mail: [email protected]
1078–8174/01/040221+06 $35.00/0
© 2001 The College of Radiographers
when MRI cartilage volume is compared to anatomical dissection and to be reproducible with coefficient of variations of approximately 2% [2]. This technique has been used to explore factors that influence knee cartilage in healthy adults [2] and children [3]. MRI has a further potential advantage in that the sectional and digital nature of MRI image acquisition affords, the researcher and clinician alike, access to image processing and manipulations not available with planar images acquired by plain © 2001 The College of Radiographers
222
radiography which are imaged and stored by analog film media [4–9]. It is possible to perform one acquisition of data from the MRI but reformat the data so that different views of the subject’s knee can be examined. This may be useful, as different views may be needed to examine different components of the knee joint such as bone or cartilage. Reformatting the data from a single acquisition has the potential advantage of avoiding prolonged scanning time and reducing the overall cost of MRI. In this study, we examined the accuracy of cartilage volume and bone area measured from a MRI sequence reformatted in different planes.
Methods Patients and MRI sequence The MRI sequences of 16 adult subjects participating in a longitudinal study (9 females, 7 males, and age range 45–68 years) were examined. The patients knees were imaged in the sagittal plane on a 1.5-T whole body magnetic resonance unit (Sigma Advantage GE Medical Systems, Milwaukee, WIS, U.S.A.) with use of a commercial receive only extremity coil as previously described [2]. The following image sequence was used: a T1-weighted fat saturated 3D gradient recall acquisition in the steady state; flip angle 15; repetition time 25.9 ms; echo time 3.1 ms; field of view 16 cm; bandwidth 15.6 kHz, partition width 1.5 mm, one acquisition.
Glisson et al.
from the original sagittal plane, where all 60 slices were measured. The above process was repeated with the images transformed into the axial plane to create a third set of images. Measurement of cartilage volumes and tibial plateau areas Areas of articular cartilages were determined individually by manually drawing contours around the target cartilage boundaries on a slice by slice basis. Contours were placed using image processing routines in the Osiris Software package (University of Geneva) and a graphic tablet or mouse on a relatively low cost personal computer platform (IBM compatible). This system was used to yield measurement of individual slice cartilage area. The product of the summation of individual cartilage areas, and the image acquisition or reformation slice thickness then calculated total cartilage volumes. The areas of the medial and lateral tibial plateau were directly measured by manually drawing contours on the reformatted axial data. From the sagittal data, each tibial plateau area was calculated by measuring the length of the plateau in each section, multiplying by the inter-section width (1.5 mm) and summing the resultant areas. The coronal measurements were performed by two independent observers, blinded to the results obtained from the sagital planes. Data analysis
Reformatting the original sagittal data set into coronal and axial planes To transform the images to the coronal plane, the Analyse Software package developed by the Mayo Clinic was employed, the images were re-sized to an isotropic volume of 256256144 slices with a voxel size of 0.624 mm. This volume was then transformed to the coronal plane and padded to 256 pixels to retain consistent XY dimensions. The measurement of all 256 interpolated slices that resulted does not improve accuracy over the original acquisition set of 60 slices, as the interpolated slices do not contain any ‘new’ data. Using every third slice resulted in 86 slices of thickness 1.281 mm and a set size of 25625686 based on a voxel size of 0.427 mm0.427 mm 1.281 mm. The accuracy of the coronal measurements was tested by comparing to measurements
The accuracy of the reformatted MRI cartilage volume and tibial plateau area estimate techniques were assessed for systematic error using the median difference between the measurements of the reformatted data and sagittal methods. Absolute errors (i.e. irrespective of direction) are reported as the median of the absolute values of the differences between the reformatted data and sagittal measurements. Absolute percentage error was obtained by dividing the absolute error by the ‘sagittal’ measured volume or area. The coefficient of variation (CV) and intra-rater reproducibility of MR imaging quantification of cartilage volume and tibial plateau area were computed using a one-way random effects ANOVA model [10] to compute the appropriate variance components. In order to avoid reliance on questionable parametric distributional assumptions, 95% confidence intervals for
Reformatting MRI knee scans
223
Figure 1. (a) Single T1-weighted fat saturation sagittal image of a study subject’s knee using SPGR imaging; (b) A similar image of the same subject’s knee from the coronal image reformation; (c) A similar image of the same subject’s knee from the axial image reformation.
reproducibility coefficients were computed using the bootstrap method with 1000 re-samples of the original data and applying the bias-corrected percentile method [11]. Statistical analyses were performed using the STATA package (Stata Corporation, Texas, U.S.A.); random effects models used the SAS MIXED procedure (SAS Institute, Carey, North Carolina, U.S.A.).
coefficients, indicating high reproducibility for lateral and medial tibiofemoral cartilages (Table 1). Similar medial and lateral tibial plateau areas were obtained from the reformatted axial view and the original sagittal data acquisition. The average over- and under-estimate was 6.5% for the lateral tibial plateau and 6.8% for the medial tibial plateau. The intraobserver variability was low with the coefficients of variation of 4.8% and 4.3% for lateral and medial tibial plateau areas respectively.
Results Figure 1 shows a single image of a typical subject in the sagittal, coronal and axial planes. Joint cartilage can be best visualized in the sagittal and coronal views while the size of the tibial plateau is best seen in the axial view. In 16 subjects, similar lateral and medial tibial cartilage volumes were obtained from sagittal and coronal MR sequences (Figs 2 and 3). The results were more accurate for the lateral tibial cartilage where there was only 4.6% difference in the volume obtained using coronal compared to the sagittal method. There was a difference of 9.8% between the coronal and sagittal methods in determining medial tibial cartilage volume. There was little evidence of systematic error of the coronal method as measured by mean differences between the methods (Table 1). The median absolute percentage difference between the sagittal and coronal methods for the lateral and medial tibial cartilage volumes were 5.9% and 10.6% respectively (Table 1). The coefficients of variation, as a measure of intraobserver variability, were low, ranging from 3.8% to 6.9%, as were the reproducibility
Discussion We have shown that knee measures determined from data acquired from MRI in the sagittal plane and transformed to either the coronal or axial plane is reproducible and is comparable to knee measures of volume or area determined from the original sagittal data which has previously been validated [1, 2]. MRI data acquired in a volumetric or 3D data set can be reformatted in virtually any plane. The clinical potential of this is probably currently underutilized. Multiplanar reformatted images using a three-dimensional Fourier transformed MRI data set have been shown to be useful for assessing torn ulnar collateral ligaments, intraarticular loose bodies, and capitellar fragments in male patients suffering from throwing injuries [6]. In a cadaver model, MR imaging and reformatting of data along the long axis of each tendon enabled accurate determination of different degrees of flexor tendon tears in the hand [7]. It has also been shown that three-dimensional reconstruction of the magnetic
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Glisson et al.
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Figure 2. Graph of coronal vs sagittal measurements of lateral cartilage volume in 16 subjects. The circles represent the sagittal measurement and the ‘+’s represent duplicate coronal measurements.
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Figure 3. Graph of coronal vs sagittal measurements of medial cartilage volume in 16 subjects. The circles represent the sagittal measurement and the ‘+’s represent duplicate coronal measurements.
resonance data in patients with chronic partial epilepsy reveals additional, useful clinical information in up to 75% of patients [8]. It has also been
suggested that reformatted MRI data of the proximal femoral anatomy may improve on current methods for prosthetic hip design [9]. However, no
Reformatting MRI knee scans
225
Table 1. Descriptive statistics and comparison of coronal vs sagittal methods of MR imaging of cartilage volume (ml) for 16 subjects
Mean (SD) of sagittal Difference* Range 95% CI for median % Difference* Range 95% CI for median Absolute difference** Range 95% CI for median Absolute % difference** Range 95% CI for median Intra-rater CV% Intra-rater reproducibility 95% CI
Lateral
Medial
2.43 (0.65) 0.08 0.17 to 0.41 (0.02, 0.15) 4.6% 8.4% to 10.3% (0.7%, 7.1%) 0.12 0.05 to 0.41 (0.07, 0.16) 5.9% 1.8% to 10.3% (3.1%, 7.8%) 3.83% 0.982 (0.962, 0.990)
2.05 (0.44) 0.18 0.27 to 0.41 (0.10, 0.27) 9.8% 15.1% to 26.8% (4.29, 12.58) 0.20 0.10 to 0.41 (0.16, 0.27) 10.6% 4.7% to 26.8% (8.2%, 13.0%) 6.91% 0.917 (0.815, 0.954)
*Median difference and median percentage difference of coronal vs sagittal methods. **Median absolute differences and median absolute percentage differences of coronal vs sagittal methods.
previous study has compared the size of similar structures in the original and reformatted data. This is important if data from different sources is to be used interchangeably both clinically and in research. A potential problem with this method is that the images created by reconstruction may only approximate the result of actual scanning. In this study we did not compare the size estimates from reformatted data with those from direct scanning in the same plane. Instead we used the results obtained from direct scanning in the sagittal plane. This may be expected to underestimate the comparability of size estimates between the original and reformatted data given that some of the variability may be due to differences in appearance of the structures in the different planes, not just the effect of reformatting. However, despite this, comparable results were obtained from cartilage volume estimates in the original sagittal plane and the reformatted coronal plane and tibial areas calculated from the sagittal and reformatted axial data. Magnetic resonance image sequences are generally acquired in contiguous parallel slices in one of following scan orientations, sagittal, axial or coronal sections to form an inherently spatially registered, three-dimensional data set. The full potential of this data set is generally not fully utilized at the point of image acquisition. Image processing and spatial measurement methods have been developed
and proven to provide quantitative measurements of knee cartilage volumes derived from individual cartilage areas of MR knee images acquired in the sagittal plane. Due to the geometry of the joint cartilage, certain features can be more clearly seen and measured when different viewing planes are used. For example, the tibial plateau is best visualized in the axial compared to the coronal or sagittal planes. The axial plane also allows clear visualization of the lateral and medial patella cartilage while the component of femoral cartilage that articulates with the tibial cartilage is most clearly seen in the coronal views. These differences will be important to explore when trying to determine the optimum way to measure knee cartilage volume or bone size in patients. Knee data acquired via MRI in one plane can be reformatted into different planes, providing comparable cartilage volumes and bone areas. As different planes through the knee may provide better visualization of different joint structures, this method may be useful clinically and as a research tool, while avoiding the cost and potential discomfort associated with prolonged scanning times. Acknowledgments This work was funded by a NH MRC Project Grant. We would also like to thank Dr Anita Wluka for helpful advice.
226
References 1. Peterfy CG, van Dijke CF, Janzen DL et al. Quantification of articular cartilage in the knee with pulsed saturation transfer subtraction and fat-suppressed MR imaging: optimization and validation. Radiology 1994; 192: 485– 91. 2. Cicuttini F, Forbes A, Morris K, Darling S, Bailey M, Stuckey S. Gender differences in knee cartilage volume as measured by magnetic resonance imaging. Osteoarthritis Cartilage 1999; 7 (3): 265–71. 3. Jones G, Glisson M, Kristen Hynes K, Cicuttini F. Gender and site differences in cartilage development: a possible explanation for variations in knee osteoarthritis in later life. Arthritis & Rheumatism 2000; 43: 2543–9. 4. Smedby O, Rostad H, Klaastad O, Lilleas F, Tillung T, Fosse E. Functional imaging of the thoracic outlet syndrome in an open MR scanner. Eur Radiol 2000; 10 (4): 597–600. 5. Convit A, McHugh P, Wolf OT et al. MRI volume of the amygdala: a reliable method allowing separation from the hippocampal formation. Psychiatry Res 1999; 90 (2): 113–23.
Glisson et al.
6. Sugimoto H, Hyodoh K, Shinozaki T . Throwing injury of the elbow: assessment with gradient threedimensional, fourier transform gradient-echo and short tau inversion recovery images. J Magn Reson Imaging 1998; 8: 487–92. 7. Rubin DA, Kneeland JB, Kitay GS, Naranja RJ Jr. Flexor tendon tears in the hand: use of MR imaging to diagnose degree of injury in a cadaver model. Am J Roentgenol 1996; 166: 615–20. 8. Sisodiya SM, Free SL, Fish DR, Shorvon SD. Increasing the yield from volumetric MRI in patients with epilepsy. Magn Reson Imaging 1995; 13 (8): 1147–52. 9. Traill MR, Kaufman DM, Runge VM et al. Anatomy of the proximal femur as seen with three-dimensional magnetic resonance imaging. J Arthroplasty 1989; 4: 361–7. 10. Milliken GA, Johnson DE. Analysis of Messy Data: Volume 1: Designed Experiments (chapter 18). New York: Van Nostrand Reinhold, 1984. 11. Efron BE, Tibshirani RJ. An Introduction to the Bootstrap, Chapter 14. New York: Chapman and Hall, 1993.
New scans from old: digital reformatting knee MRI M. Glisson, MAppSci, Radiographer and Senior Lecturer* A. Forbes, PhD, Senior Lecturer† K. Morris, ME FIEAust, Chief Researcher‡ S. Stuckey, M Med, FRANZCR, Director MRI Unit§ and F. Cicuttini, MB BS, PhD, FRACP, Rheumatologist and Assoc. Professor† *Department of Radiography and Medical Imaging, Monash University; †Department of Epidemiology and Preventive Medicine, Monash University Medical School, Alfred Hospital, Prahran, Victoria, Australia; ‡Victorian Epilepsy Centre, St Vincent’s Hospital, Fitzroy, Victoria, Australia; §Department of Magnetic Resonance Imaging, Alfred Hospital, Prahran, Victoria, Australia (Received 14 September 2000; revised 30 March 2001; second revision 29 June 2001; accepted 16 July 2001)
Key words: knee cartilage; MRI; reformatted images.
Purpose: To determine the accuracy of cartilage volume and bone areas measured from a 3D knee MRI sequence reformatted in different planes. Methods: MRI of 16 adult subjects (9 females, 7 males, age range 45–68 years) were acquired in the sagittal plane using a 3D T1-weighted fat suppressed spoiled gradient echo sequence. Medial and lateral tibial cartilage volumes were determined by processing images acquired in the sagittal plane and from the same image data reformatted in the coronal plane. Tibial plateau areas were determined by processing images acquired in the sagittal plane and reformatted in the axial plane. Results: Cartilage volumes calculated from the original sagittal acquisition and data reformatted into the coronal plane were similar. The average over- or under-estimation of the lateral and medial cartilage volume from the reformatted coronal scans compared to the sagittal sequences was 4.6% and 9.8% respectively. Similar medial and lateral tibial plateau areas were obtained when the sagittal data was reformatted in the axial plane. The average over- and under-estimate of lateral and medial tibial plateau areas from the reformatted axial scans compared to the originally acquired sagittal sequences was 6.5% and 6.8% respectively. Conclusion: Knee data acquired via MRI in one plane can be reformatted into different planes, providing comparable cartilage volumes and bone areas. As different planes through the knee may provide better visualization of different joint structures, this method may be useful clinically and as a research tool, while avoiding the cost associated with the prolonged scanning times associated with acquiring multiple planes.
Introduction There has been increasing interest in the use of magnetic resonance imaging (MRI) in the measurement of knee cartilage volume as a possible outcome measure in arthritis [1–3]. MRI has been shown to be a valid measure of cartilage volume Correspondence should be addressed to: Assoc Prof. Flavia Cicuttini, Department Epidemiology and Preventive Medicine, Alfred Hospital, Prahran, Victoria, 3181, Australia. Tel: 61-3-9903 0555; Fax: 61-39903 0556; E-mail: [email protected]
1078–8174/01/040221+06 $35.00/0
© 2001 The College of Radiographers
when MRI cartilage volume is compared to anatomical dissection and to be reproducible with coefficient of variations of approximately 2% [2]. This technique has been used to explore factors that influence knee cartilage in healthy adults [2] and children [3]. MRI has a further potential advantage in that the sectional and digital nature of MRI image acquisition affords, the researcher and clinician alike, access to image processing and manipulations not available with planar images acquired by plain © 2001 The College of Radiographers
222
radiography which are imaged and stored by analog film media [4–9]. It is possible to perform one acquisition of data from the MRI but reformat the data so that different views of the subject’s knee can be examined. This may be useful, as different views may be needed to examine different components of the knee joint such as bone or cartilage. Reformatting the data from a single acquisition has the potential advantage of avoiding prolonged scanning time and reducing the overall cost of MRI. In this study, we examined the accuracy of cartilage volume and bone area measured from a MRI sequence reformatted in different planes.
Methods Patients and MRI sequence The MRI sequences of 16 adult subjects participating in a longitudinal study (9 females, 7 males, and age range 45–68 years) were examined. The patients knees were imaged in the sagittal plane on a 1.5-T whole body magnetic resonance unit (Sigma Advantage GE Medical Systems, Milwaukee, WIS, U.S.A.) with use of a commercial receive only extremity coil as previously described [2]. The following image sequence was used: a T1-weighted fat saturated 3D gradient recall acquisition in the steady state; flip angle 15; repetition time 25.9 ms; echo time 3.1 ms; field of view 16 cm; bandwidth 15.6 kHz, partition width 1.5 mm, one acquisition.
Glisson et al.
from the original sagittal plane, where all 60 slices were measured. The above process was repeated with the images transformed into the axial plane to create a third set of images. Measurement of cartilage volumes and tibial plateau areas Areas of articular cartilages were determined individually by manually drawing contours around the target cartilage boundaries on a slice by slice basis. Contours were placed using image processing routines in the Osiris Software package (University of Geneva) and a graphic tablet or mouse on a relatively low cost personal computer platform (IBM compatible). This system was used to yield measurement of individual slice cartilage area. The product of the summation of individual cartilage areas, and the image acquisition or reformation slice thickness then calculated total cartilage volumes. The areas of the medial and lateral tibial plateau were directly measured by manually drawing contours on the reformatted axial data. From the sagittal data, each tibial plateau area was calculated by measuring the length of the plateau in each section, multiplying by the inter-section width (1.5 mm) and summing the resultant areas. The coronal measurements were performed by two independent observers, blinded to the results obtained from the sagital planes. Data analysis
Reformatting the original sagittal data set into coronal and axial planes To transform the images to the coronal plane, the Analyse Software package developed by the Mayo Clinic was employed, the images were re-sized to an isotropic volume of 256256144 slices with a voxel size of 0.624 mm. This volume was then transformed to the coronal plane and padded to 256 pixels to retain consistent XY dimensions. The measurement of all 256 interpolated slices that resulted does not improve accuracy over the original acquisition set of 60 slices, as the interpolated slices do not contain any ‘new’ data. Using every third slice resulted in 86 slices of thickness 1.281 mm and a set size of 25625686 based on a voxel size of 0.427 mm0.427 mm 1.281 mm. The accuracy of the coronal measurements was tested by comparing to measurements
The accuracy of the reformatted MRI cartilage volume and tibial plateau area estimate techniques were assessed for systematic error using the median difference between the measurements of the reformatted data and sagittal methods. Absolute errors (i.e. irrespective of direction) are reported as the median of the absolute values of the differences between the reformatted data and sagittal measurements. Absolute percentage error was obtained by dividing the absolute error by the ‘sagittal’ measured volume or area. The coefficient of variation (CV) and intra-rater reproducibility of MR imaging quantification of cartilage volume and tibial plateau area were computed using a one-way random effects ANOVA model [10] to compute the appropriate variance components. In order to avoid reliance on questionable parametric distributional assumptions, 95% confidence intervals for
Reformatting MRI knee scans
223
Figure 1. (a) Single T1-weighted fat saturation sagittal image of a study subject’s knee using SPGR imaging; (b) A similar image of the same subject’s knee from the coronal image reformation; (c) A similar image of the same subject’s knee from the axial image reformation.
reproducibility coefficients were computed using the bootstrap method with 1000 re-samples of the original data and applying the bias-corrected percentile method [11]. Statistical analyses were performed using the STATA package (Stata Corporation, Texas, U.S.A.); random effects models used the SAS MIXED procedure (SAS Institute, Carey, North Carolina, U.S.A.).
coefficients, indicating high reproducibility for lateral and medial tibiofemoral cartilages (Table 1). Similar medial and lateral tibial plateau areas were obtained from the reformatted axial view and the original sagittal data acquisition. The average over- and under-estimate was 6.5% for the lateral tibial plateau and 6.8% for the medial tibial plateau. The intraobserver variability was low with the coefficients of variation of 4.8% and 4.3% for lateral and medial tibial plateau areas respectively.
Results Figure 1 shows a single image of a typical subject in the sagittal, coronal and axial planes. Joint cartilage can be best visualized in the sagittal and coronal views while the size of the tibial plateau is best seen in the axial view. In 16 subjects, similar lateral and medial tibial cartilage volumes were obtained from sagittal and coronal MR sequences (Figs 2 and 3). The results were more accurate for the lateral tibial cartilage where there was only 4.6% difference in the volume obtained using coronal compared to the sagittal method. There was a difference of 9.8% between the coronal and sagittal methods in determining medial tibial cartilage volume. There was little evidence of systematic error of the coronal method as measured by mean differences between the methods (Table 1). The median absolute percentage difference between the sagittal and coronal methods for the lateral and medial tibial cartilage volumes were 5.9% and 10.6% respectively (Table 1). The coefficients of variation, as a measure of intraobserver variability, were low, ranging from 3.8% to 6.9%, as were the reproducibility
Discussion We have shown that knee measures determined from data acquired from MRI in the sagittal plane and transformed to either the coronal or axial plane is reproducible and is comparable to knee measures of volume or area determined from the original sagittal data which has previously been validated [1, 2]. MRI data acquired in a volumetric or 3D data set can be reformatted in virtually any plane. The clinical potential of this is probably currently underutilized. Multiplanar reformatted images using a three-dimensional Fourier transformed MRI data set have been shown to be useful for assessing torn ulnar collateral ligaments, intraarticular loose bodies, and capitellar fragments in male patients suffering from throwing injuries [6]. In a cadaver model, MR imaging and reformatting of data along the long axis of each tendon enabled accurate determination of different degrees of flexor tendon tears in the hand [7]. It has also been shown that three-dimensional reconstruction of the magnetic
224
Glisson et al.
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Figure 2. Graph of coronal vs sagittal measurements of lateral cartilage volume in 16 subjects. The circles represent the sagittal measurement and the ‘+’s represent duplicate coronal measurements.
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Figure 3. Graph of coronal vs sagittal measurements of medial cartilage volume in 16 subjects. The circles represent the sagittal measurement and the ‘+’s represent duplicate coronal measurements.
resonance data in patients with chronic partial epilepsy reveals additional, useful clinical information in up to 75% of patients [8]. It has also been
suggested that reformatted MRI data of the proximal femoral anatomy may improve on current methods for prosthetic hip design [9]. However, no
Reformatting MRI knee scans
225
Table 1. Descriptive statistics and comparison of coronal vs sagittal methods of MR imaging of cartilage volume (ml) for 16 subjects
Mean (SD) of sagittal Difference* Range 95% CI for median % Difference* Range 95% CI for median Absolute difference** Range 95% CI for median Absolute % difference** Range 95% CI for median Intra-rater CV% Intra-rater reproducibility 95% CI
Lateral
Medial
2.43 (0.65) 0.08 0.17 to 0.41 (0.02, 0.15) 4.6% 8.4% to 10.3% (0.7%, 7.1%) 0.12 0.05 to 0.41 (0.07, 0.16) 5.9% 1.8% to 10.3% (3.1%, 7.8%) 3.83% 0.982 (0.962, 0.990)
2.05 (0.44) 0.18 0.27 to 0.41 (0.10, 0.27) 9.8% 15.1% to 26.8% (4.29, 12.58) 0.20 0.10 to 0.41 (0.16, 0.27) 10.6% 4.7% to 26.8% (8.2%, 13.0%) 6.91% 0.917 (0.815, 0.954)
*Median difference and median percentage difference of coronal vs sagittal methods. **Median absolute differences and median absolute percentage differences of coronal vs sagittal methods.
previous study has compared the size of similar structures in the original and reformatted data. This is important if data from different sources is to be used interchangeably both clinically and in research. A potential problem with this method is that the images created by reconstruction may only approximate the result of actual scanning. In this study we did not compare the size estimates from reformatted data with those from direct scanning in the same plane. Instead we used the results obtained from direct scanning in the sagittal plane. This may be expected to underestimate the comparability of size estimates between the original and reformatted data given that some of the variability may be due to differences in appearance of the structures in the different planes, not just the effect of reformatting. However, despite this, comparable results were obtained from cartilage volume estimates in the original sagittal plane and the reformatted coronal plane and tibial areas calculated from the sagittal and reformatted axial data. Magnetic resonance image sequences are generally acquired in contiguous parallel slices in one of following scan orientations, sagittal, axial or coronal sections to form an inherently spatially registered, three-dimensional data set. The full potential of this data set is generally not fully utilized at the point of image acquisition. Image processing and spatial measurement methods have been developed
and proven to provide quantitative measurements of knee cartilage volumes derived from individual cartilage areas of MR knee images acquired in the sagittal plane. Due to the geometry of the joint cartilage, certain features can be more clearly seen and measured when different viewing planes are used. For example, the tibial plateau is best visualized in the axial compared to the coronal or sagittal planes. The axial plane also allows clear visualization of the lateral and medial patella cartilage while the component of femoral cartilage that articulates with the tibial cartilage is most clearly seen in the coronal views. These differences will be important to explore when trying to determine the optimum way to measure knee cartilage volume or bone size in patients. Knee data acquired via MRI in one plane can be reformatted into different planes, providing comparable cartilage volumes and bone areas. As different planes through the knee may provide better visualization of different joint structures, this method may be useful clinically and as a research tool, while avoiding the cost and potential discomfort associated with prolonged scanning times. Acknowledgments This work was funded by a NH MRC Project Grant. We would also like to thank Dr Anita Wluka for helpful advice.
226
References 1. Peterfy CG, van Dijke CF, Janzen DL et al. Quantification of articular cartilage in the knee with pulsed saturation transfer subtraction and fat-suppressed MR imaging: optimization and validation. Radiology 1994; 192: 485– 91. 2. Cicuttini F, Forbes A, Morris K, Darling S, Bailey M, Stuckey S. Gender differences in knee cartilage volume as measured by magnetic resonance imaging. Osteoarthritis Cartilage 1999; 7 (3): 265–71. 3. Jones G, Glisson M, Kristen Hynes K, Cicuttini F. Gender and site differences in cartilage development: a possible explanation for variations in knee osteoarthritis in later life. Arthritis & Rheumatism 2000; 43: 2543–9. 4. Smedby O, Rostad H, Klaastad O, Lilleas F, Tillung T, Fosse E. Functional imaging of the thoracic outlet syndrome in an open MR scanner. Eur Radiol 2000; 10 (4): 597–600. 5. Convit A, McHugh P, Wolf OT et al. MRI volume of the amygdala: a reliable method allowing separation from the hippocampal formation. Psychiatry Res 1999; 90 (2): 113–23.
Glisson et al.
6. Sugimoto H, Hyodoh K, Shinozaki T . Throwing injury of the elbow: assessment with gradient threedimensional, fourier transform gradient-echo and short tau inversion recovery images. J Magn Reson Imaging 1998; 8: 487–92. 7. Rubin DA, Kneeland JB, Kitay GS, Naranja RJ Jr. Flexor tendon tears in the hand: use of MR imaging to diagnose degree of injury in a cadaver model. Am J Roentgenol 1996; 166: 615–20. 8. Sisodiya SM, Free SL, Fish DR, Shorvon SD. Increasing the yield from volumetric MRI in patients with epilepsy. Magn Reson Imaging 1995; 13 (8): 1147–52. 9. Traill MR, Kaufman DM, Runge VM et al. Anatomy of the proximal femur as seen with three-dimensional magnetic resonance imaging. J Arthroplasty 1989; 4: 361–7. 10. Milliken GA, Johnson DE. Analysis of Messy Data: Volume 1: Designed Experiments (chapter 18). New York: Van Nostrand Reinhold, 1984. 11. Efron BE, Tibshirani RJ. An Introduction to the Bootstrap, Chapter 14. New York: Chapman and Hall, 1993.