(Radiographics. 2002;22:e3-e3.)
© RSNA, 2002
Multiparametric Color-encoded Brain MR Imaging in Talairach Space1
Kenneth L. Weiss, MD,
Qian Dong, MD,
William J. Weadock, MD,
Robert C. Welsh, PhD and
Gaurang V. Shah, MD
1 From the Department of Radiology, University of Michigan, Ann Arbor, Mich. Presented as a scientific exhibit at the 2000 RSNA scientific assembly. Received March 14, 2001; revision requested July 3; revision received November 21; accepted December 3. Supported by grant VIF-FY 98.004 from the University of Michigan. Address correspondence to: K.L.W., Departments of Radiology, Bioengineering, and Psychiatry, University of Cincinnati, P.O. Box 670762, 234 Goodman St, Cincinnati, OH45267-0762. (e-mail: weisskl{at}ucmail.uc.edu)
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Abstract
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Clinical magnetic resonance (MR) imaging of the brain is typically performed in the standard three orthogonal planes of the magnet, with little regard to head positioning. Multiple sequences with different imaging parameters are performed, and gray-scale images are obtained and displayed separately. The authors have implemented, and currently advocate, the routine acquisition of coregistered transverse images after roll, yaw, and pitch correction to Talairach space. Talairach, anterior commissure (AC)—posterior commissure (PC) referenced, stereotactic space has been widely embraced by the neuroscience community. This standardization should lead to more reproducible and readily interpretable MR examinations. A method is described to obtain direct AC-PC referenced (Talairach space) MR images. Sample protocols are provided. Coregistered T1-weighted, T1-weighted with contrast material administration, T2-weighted, fluid-attenuated inversion recovery (FLAIR), MR angiography, fractional anisotropy, and functional MR imaging sequences are presented, depicting a wide range of imaging parameters applied to normal brain anatomy in Talairach space. Illustrative examples of pathology are also provided. Color encoding is discussed and exploited to display and integrate multiparameter MR imaging contrast and white-matter-tract direction (anisotropy). The color composites may reduce the number of images needed for review by a factor of three or four and facilitate interpretation.
© RSNA, 2002
Index Terms: Brain, diffusion, Brain, function, Brain, infarction, 10.781 • Brain, ischemia, 10.781 • Brain, MR, 10.12141, 10.12143, 10.12144 10.12146 • Magnetic resonance (MR), diffusion study, 10.12144 • Magnetic resonance (MR), functional imaging, 10.12143, 10.12144 10.12146 • Magnetic resonance (MR), tissue characterization, 10.12146
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Introduction
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Talairach Space
While the anterior commissure (AC)–posterior commissure (PC) reference line has been widely embraced by the neuroscience community, especially in a research setting, routine clinical magnetic resonance (MR) imaging of the brain is still typically performed in the standard three orthogonal planes of the magnet, with little attention paid to patient positioning. This makes intra- and interpatient comparisons and interpretations difficult. Advances in MR hardware and software have made patient-optimized oblique imaging in a standard reference frame more viable and more readily implemented than when originally reported by Talairach and Tournoux (1).
Talairach and coworkers defined their intercommissural basal brain line as passing through the superior edge of the AC and the inferior edge of the PC (Fig 1).
The stereotactic atlas of Talairach and Tournoux, based on the brain of a 60-year-old right-handed French woman, has become the de facto standard reference almost universally used in functional brain imaging (1,2). Researchers often transform their structural and functional data into Talairach space, which serves as a common coordinate reference system (3).
Color Encoding
Color-encoded display has the potential to facilitate integration of MR images obtained with different parameters. Numerous techniques have been proposed. The most popular and simplest to implement is the assignment of individual red (R), green (G), and blue (B) channels to separately encode three coregistered images with different contrast into a single composite pseudocolor image. For example, T1-weighted, proton-density, or fluid-attenuated inversion-recovery (FLAIR) and T2-weighted images might be combined. Alternatively, calculated tissue parameter maps such as T1 relaxation, T2 relaxation, proton density, magnetization transfer ratio, or apparent diffusion coefficient (ADC) might be integrated into a color composite (4–6).
Numerous color schemes have been proposed to display diffusion anisotropy and in particular encode the orientation of white matter tracts. Nakada and Matsuzawa (7) described a simple technique for color encoding fiber direction applicable to routine diffusion-weighted imaging. (For more information on diffusion-weighted imaging, see Appendix.) They assigned the complement of an R, G, or B color channel to diffusion-weighted images obtained with one of three orthogonal Stejskal-Tanner (ST) gradients. As a result of the trichromatic property of color, isotropic tissue will display no hue. Orientation of the subject in relationship to the ST gradients will, however, influence the appearance of anisotropic tissue.
To obtain rotationally invariant diffusion images, the full diffusion tensor must be determined, requiring at least seven measurements (8). (For more information of diffusion tensor MR imaging, see Appendix.) High-resolution directionally encoded color maps can then be generated. Pajevic and Pierpaoli have implemented a robust heuristic approach to direction-encoded color mapping of white matter fiber tracts (9) (Fig 2).
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Figure 2a. Sequential coronal directionally encoded color (DEC) maps from most anterior (a) to most posterior (d). The color wheels (upper-right-hand corners) map the directions of fibers. The cingulum appears green as it courses along the anteroposterior axis, and can be seen in all images (a, green arrow). The AC (a, red arrow) appears red as it courses along the right-left axis. The corticospinal tracts (b, blue arrow) and the sensory pathways (c, blue arrow) appear blue as they course along the superoinferior axis.
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Figure 2b. Sequential coronal directionally encoded color (DEC) maps from most anterior (a) to most posterior (d). The color wheels (upper-right-hand corners) map the directions of fibers. The cingulum appears green as it courses along the anteroposterior axis, and can be seen in all images (a, green arrow). The AC (a, red arrow) appears red as it courses along the right-left axis. The corticospinal tracts (b, blue arrow) and the sensory pathways (c, blue arrow) appear blue as they course along the superoinferior axis.
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Technique
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A rapid three-step technique (Fig 3) sequentially corrects for patient roll (y rotation, or rotation about the anteroposterior axis), yaw (z rotation, or rotation about the superoinferior axis), and pitch (x rotation, or rotation about the left-right axis). A 2-second coronal T1-weighted gradient-echo scout image is obtained, from which a 2-second transverse oblique T1-weighted gradient-echo sequence is prescribed, correcting for patient roll. Subsequently, a midline 2-second sagittal double-oblique single-shot fast-spin-echo sequence is prescribed from this transverse oblique image, correcting for yaw. Finally, coregistered transverse triple-oblique (roll, yaw, and pitch corrected) sequences are prescribed from the adjusted midline sagittal image parallel to the AC-PC line, as described by Talairach (1) (Fig 3).
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Figure 3a. Illustration of the technique for prescribing a transverse MR imaging sequence in Talairach space. (a) Technique for correcting roll. (b) Technique for correcting yaw. (c) Localization of the AC and PC on sagittal T2-weighted image. (d) Landmarks for identifying the AC. (e) Landmarks for identifying the PC. (f) Pitch standardization to the Talairach reference line.
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Figure 3b. Illustration of the technique for prescribing a transverse MR imaging sequence in Talairach space. (a) Technique for correcting roll. (b) Technique for correcting yaw. (c) Localization of the AC and PC on sagittal T2-weighted image. (d) Landmarks for identifying the AC. (e) Landmarks for identifying the PC. (f) Pitch standardization to the Talairach reference line.
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Figure 3c. Illustration of the technique for prescribing a transverse MR imaging sequence in Talairach space. (a) Technique for correcting roll. (b) Technique for correcting yaw. (c) Localization of the AC and PC on sagittal T2-weighted image. (d) Landmarks for identifying the AC. (e) Landmarks for identifying the PC. (f) Pitch standardization to the Talairach reference line.
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Figure 3d. Illustration of the technique for prescribing a transverse MR imaging sequence in Talairach space. (a) Technique for correcting roll. (b) Technique for correcting yaw. (c) Localization of the AC and PC on sagittal T2-weighted image. (d) Landmarks for identifying the AC. (e) Landmarks for identifying the PC. (f) Pitch standardization to the Talairach reference line.
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Figure 3e. Illustration of the technique for prescribing a transverse MR imaging sequence in Talairach space. (a) Technique for correcting roll. (b) Technique for correcting yaw. (c) Localization of the AC and PC on sagittal T2-weighted image. (d) Landmarks for identifying the AC. (e) Landmarks for identifying the PC. (f) Pitch standardization to the Talairach reference line.
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Figure 3f. Illustration of the technique for prescribing a transverse MR imaging sequence in Talairach space. (a) Technique for correcting roll. (b) Technique for correcting yaw. (c) Localization of the AC and PC on sagittal T2-weighted image. (d) Landmarks for identifying the AC. (e) Landmarks for identifying the PC. (f) Pitch standardization to the Talairach reference line.
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Transverse oblique sequences performed clinically include T1-weighted with and without contrast enhancement, fast spin echo, T2-weighted with fat saturation, FLAIR, and echo-planar diffusion-weighted imaging (Table).
Talairach-referenced coronal sequencing can also be directly prescribed perpendicular to the AC-PC line. Alternatively, if a three-dimensional acquisition is used, multiplanar reconstructions can be performed to include such Talairach-referenced coronal images. Click here for illustration of reformatted imaging in the Talairach coronal plane.
Multiparameter transverse oblique images are combined into color-encoded composites for research or illustrative teaching purposes. Typically, we assign red to T1-weighted images, green to FLAIR images, and blue to T2-weighted images obtained with fat saturation to create composite RGB images (Fig 4).
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Figure 4a. Talairach-referenced color-encoded composite RGB images at two transverse levels, the basal ganglion (a) and the Rolandic (central) sulcus (b), and their component T1-weighted, T2-weighted with fat saturation, and FLAIR images. Note that cerebrospinal fluid is blue, reflecting the dominant contribution from the T2-weighted image. Fat appears yellow, reflecting the combination of the T1-weighted (red) and FLAIR (green) images, with the T2-weighted component (blue) suppressed by fat saturation. White and gray matter reflect a combination of all three components. The red channel contributes the most to the RGB encoding of white matter, as white matter appears relatively hyperintense only on T1-weighted images.
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Figure 4b. Talairach-referenced color-encoded composite RGB images at two transverse levels, the basal ganglion (a) and the Rolandic (central) sulcus (b), and their component T1-weighted, T2-weighted with fat saturation, and FLAIR images. Note that cerebrospinal fluid is blue, reflecting the dominant contribution from the T2-weighted image. Fat appears yellow, reflecting the combination of the T1-weighted (red) and FLAIR (green) images, with the T2-weighted component (blue) suppressed by fat saturation. White and gray matter reflect a combination of all three components. The red channel contributes the most to the RGB encoding of white matter, as white matter appears relatively hyperintense only on T1-weighted images.
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Acquiring T1-weighted, T2-weighted, and FLAIR images in the same plane makes tissue characterization easier and permits RGB encoding. A consistent reference plane facilitates inter- and intrasubject comparison. The Talairach plane is the de facto standard for functional imaging and neurostererotaxis. A fourth parameter may be superimposed in white on the composite RGB image. Such parameters may include contrast enhancement (Fig 5), flow (Fig 6), fractional anisotropy, and functional brain activation. Click here for an example of fractional anisotropy and functional brain activation encoding.
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Figure 5. Thresholded contrast maps (T1-weighted postcontrast images [T1+C] minus precontrast T1-weighted images) may be superimposed in gray scale on the composite T1-weighted-FLAIR-T2-weighted RGB image to produce the image in the lower right corner. Click here for more images of this patient with a right frontal glioblastoma multiforme.
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We use a modified Nakada and Matsuzawa (7) method for color encoding clinical 3-ST diffusion-weighted images. The superoinferior ST gradient images are encoded with the color complement of blue (red and green), left-to-right images with the complement of red (green and blue), and anteroposterior images with the complement of green (red and blue). Isotropic tissues, such as gray matter, have no hue, as an equal balance of RGB yields gray-scale images (Fig 7).
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Figure 7. Modified Nakada and Matsuzawa (7) method for color encoding clinical 3-ST diffusion-weighted images. The superioinferior component (S/I) is mapped in red and green, the left-to-right component (L/R) in green and blue, and the anteroposterior component (A/P) in red and blue. Note commissural fibers, such as the corpus callosum (red arrow), which traverse from side to side, are depicted in red, while the corticospinal tracts (blue arrow), which pass perpendicular to the image section, are depicted in blue.
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All MR imaging was performed on a Signa 1.5-T magnet (GE Medical Systems, Milwaukee, Wis) with echo-planar capabilities. Images were transferred to a personal computer with the RADimage 2.65 program (Nevada City, Calif), and color composite images were initially created with PhotoShop 5.0 (Adobe, San Jose, Calif). More recently, a macrocode has been written with Matlab 5.3 (Mathworks, Natick, Mass) to automate color image processing. Color composites, including MR images and fractional anisotropy overlays were created in Matlab 5.3 with SPM 99 software (http://www.fil.ion.ucl.ac.uk/spm/), research diffusion tensor software (GE Medical Systems, Milwaukee, Wis), and algorithms developed at the University of Michigan.
Although current clinical software provides only three orthogonal ST gradients, research software permits acquisition of a virtually limitless number of ST gradients, allowing description of the full diffusion tensor. The algorithm described by Pajevic and Pierpaoli (9) may be used to encode research diffusion tensor maps, as illustrated in the introduction. (Fig 1)
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Example Cases
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To view the full data sets of the illustrative cases listed below, click on the case number.
- Case 1. Healthy volunteer: T1-weighted, T2-weighted, FLAIR, and 3-ST RGB composites.
- Case 2. Healthy volunteer: T1-weighted, T2-weighted, FLAIR, and 3-ST RGB composites.
- Case 3. Healthy volunteer: T1-weighted, T2-weighted, FLAIR, and RGB composites.
- Case 4. Right-handed healthy volunteer: T1-weighted, T2-weighted, FLAIR, fractional anisotropy, and functional MR imaging composites.
- Normal anatomy. Anatomy-labeled T1-weighted, T2-weighted, FLAIR, fractional anisotropy, and functional MR imaging composites.
- Case 5. Hematoma.
- Case 6. Pachymeningitis.
- Case 7. Glioblastoma multiforme.
- Case 8. Anaplastic astrocytoma with follow-up.
- Case 9. Acute and chronic ischemia and infarction.
- Case 10. Acute posterior inferior cerebellar artery infarct.
- Case 11. Periventricular leukomalacia.
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Discussion
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Our rapid, sequential reference (localizer) technique provides several potential advantages. The direct correction of roll and yaw facilitates side-to-side (interhemispheric) comparison. Standardization of pitch facilitates inter- and intrasubject comparison. The Talairach reference is an obvious choice, as it has already become the de facto standard for neurostereotaxis and functional imaging studies. Locating critical structures may be simplified. The Rolandic fissure, for example, consistently passes between the vertical (coronal) AC and PC planes (labeled VCA and VCP, respectively, in Fig 8). It originates caudally 0.5 cm in front or behind of the vertical AC and terminates in the midline approximately 1 cm posterior to the vertical PC (1,10).
Selecting the Talairach reference line also typically allows efficient brain coverage in the transverse plane and better approximates routine computed tomographic (CT) angulation. The latter may facilitate the comparison of CT and MR brain examinations.
There are potential disadvantages as well. Oblique prescriptions can add stress to the gradient system and may not be currently compatible with all pulse sequences. Protocol institution requires technologist training and additional, albeit short, set-up and imaging times. Given the requisite pitch correction, comparison to previously obtained, non-Talairach-referenced brain MR studies might be more difficult (11).
As demonstrated, color-encoding schemes may be applied to routine clinical MR studies to facilitate the integration of multiple imaging sequences that vary in tissue contrast. Theoretically, RGB encoding can reduce the number of images to review and mentally integrate by a factor of three. By superimposing an additional imaging parameter in gray scale, such as flow, fractional anisotropy, contrast enhancement, or functional MR imaging activation, this factor may be increased to four. Color may also encode white-matter-tract fiber orientation.
Color composite imaging, however, remains a work in progress. Unlike Talairach space for functional neuroimaging and stereotaxis, no accepted or popularized standard exists for MR color encoding or display. A uniform color space to match human color perception is still an elusive goal. Pseudo–color imaging is highly dependent on the sequence(s) selected for input, the color encoding and display algorithm selected, and window and level settings. To standardize the latter, we are currently working on an automated window and leveling technique that uses segmented tissue histogram analysis (12).
Successful multiparameter-encoded MR display requires precise coregistration of different images. Although computer algorithms exist for coregistering and warping images, the process is far from perfect or "push-button." Reducing inter- and intrasequence patient motion remains paramount. Additional eddy-current correction and warping techniques may improve diffusion tensor mapping and the directionally encoded color display of white matter anisotropy.
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Conclusions
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Direct roll, yaw, and pitch corrected and standardized brain imaging can be readily achieved in clinical practice. This may facilitate intra- and intersubject comparison. The Talairach AC-PC reference is recommended, as it provides efficient brain coverage, closer approximation to CT angulation, and ready integration with functional MR imaging and neurostereotaxis. Color encoding, albeit a work in progress, holds promise for facilitating the integration and interpretation of multiple MR sequences that vary in tissue contrast. The number of images required for review may be reduced by a factor of three or four. White matter tract direction and anisotropy can also be effectively displayed. Given the potential to improve diagnostic accuracy and efficiency, further study of multiparametric color-encoded MR imaging in Talairach space is warranted.
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Appendix
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The text of this Appendix has, for the most part, been taken from reference 15.
Diffusion-weighted Imaging
Diffusion-weighted (sensitized) MR imaging was first achieved in l985 and has rapidly advanced into the clinical realm (13,14). Diffusion is the random, incoherent motion of molecules driven by thermal energy. Diffusion-weighted imaging typically uses strong ST pulsed gradients. These paired gradients are balanced about a 180° excitation pulse and reduce the signal of diffusing protons relative to more stationary spins. In the presence of ST gradients, random motion leads to uncompensated spin dephasing and, hence, signal attenuation. Stationary protons on the other hand acquire no net phase shift as they experience identical magnetic field perturbations before and after the inversion pulse (15).
Signal attenuation related to random molecular motion involves an exponential with argument (-bD), where D is the diffusion coefficient and b is a gradient factor dependent on pulse sequence. For the ST arrangement, b = g 2 G2 d2 (D - d/3), where g is the gyromagnetic ratio, G is the gradient strength, and d and D are time intervals (15–17) as illustrated in Figure A1.
Diffusion weighting can be used as a contrast mechanism. Additionally, by varying b in a series of diffusion-weighted images, it is possible to calculate a "diffusion coefficient" of water in the direction of the diffusion-sensitizing gradient. This is analogous to creating a T2 relaxation map by varying the echo time or a T1 relaxation map by varying the repetition time. Higher b values denote greater diffusion weighting. Unfortunately, large diffusion gradients induce strong eddy currents and sensitize images to patient motion, blood flow, system radio-frequency instability and gradient instability. These effects degrade images and may substantially influence the experimentally measured value of the diffusion coefficient, which is thus more appropriately termed the apparent diffusion coefficient (ADC) (15).
Diffusion-weighted imaging is the most sensitive imaging technique for evaluating hyperacute stroke. Restricted diffusion has been demonstrated less than 1 hour after the ictus (18). Diffusion-weighted imaging is helpful in distinguishing acute from chronic infarcts, often a vexing clinical problem. In the setting of suspected stroke, trace (orientationally averaged) diffusion images are typically obtained. Trace diffusion images demonstrate contrast associated with the magnitude of the ADC in each voxel, but offer no information concerning the direction of diffusion. Trace images remove confounding white matter anisotropy and improve image quality through signal averaging (15).
Diffusion Tensor MR Imaging
The microstructure in isotropic tissue is randomly ordered rather than regularly ordered, as in anisotropic tissue. In gray matter and other isotropic tissues, the measured ADC is independent of orientation; whereas in white matter and other anisotropic media, the ADC varies with orientation. Since the ADC measures molecular displacements in only one direction, it does not provide enough information to describe the three-dimensional diffusive displacements in anisotropic tissue (15).
To fully describe diffusion in an anisotropic medium, where symmetry has been broken, a tensor approach must be taken. For diffusion in three-dimensional space, the tensor is specified by six unique quantities. In order to measure the tensor with MR imaging, at least seven measurements must be made. A measure with b = 0 gives the overall T2-weighted image, while the measures with b ¹ 0 are taken with diffusion gradients in at least six different directions (8).
The diffusion tensor may be visualized as an ellipsoid with three principal diffusivities (eigenvalues) associated with three mutually perpendicular principal directions (eigenvectors) intrinsic to the tissue. Quantitative characterization of the full diffusion tensor, though technically demanding, is now clinically feasible. Diffusion anisotropy has been shown to vary widely within white matter, reflecting differences in fiber-tract architecture (19). Directionally encoded color maps can help display the wealth of information contained in diffusion tensor MR imaging (7,9,15).
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Acknowledgments
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The authors thank O. Petter Eldevik, MD, PhD, for his help in developing and instituting Talairach-referenced clinical brain protocols, Jane Weiss for background research, Suzanne Meadows Murphy and Kim Gleichert for assistance with manuscript preparation, Devin Walsh for revisions for online submission, and the Department of Psychiatry at the University of Cincinnati for Web hosting.
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Footnotes
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Abbreviations: AC = anterior commissure,
ADC = apparent diffusion coefficient,
FLAIR = fluid-attenuated inversion recovery,
PC = posterior commissure,
RGB = red green blue,
ST = Stejskal-Tanner.
Dr Weadock has a financial interest in RADimage.
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K. L. Weiss, H. Pan, J. Storrs, W. Strub, J. L. Weiss, L. Jia, and O. P. Eldevik
Clinical Brain MR Imaging Prescriptions in Talairach Space: Technologist- and Computer-Driven Methods
AJNR Am. J. Neuroradiol.,
May 1, 2003;
24(5):
922 - 929.
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Abstract
Introduction
