BACKGROUND:MR-guided radiotherapy enables real-time imaging and adaptive treatment but may introduce magnetic field effects that alter dose deposition. Accurate dose calculation in such settings requires detailed Monte Carlo (MC) modeling. PURPOSE:To develop and validate a detailed MC model of the 0.5 T bi-planar Linac-MR with an integrated, custom-designed multileaf collimator (MLC) module in TOPAS. METHODS:A MC Model of the 0.5 T bi-planar Linac-MR with a 6 MV FFF beam, commercialized as the Aurora-RT (MagnetTx Oncology Solutions, Canada), is developed in TOPAS. A custom 3D magnetic field vector map, tracking with gantry angle, is incorporated into this TOPAS model. An electron source is used for X-ray generation, and all components of the linac head from the target downward are modeled in detail. The MLCs are modeled from stereolithography (STL) design files and controlled via an empirically driven mechanism developed in this work. Water tank measured percent depth dose (PDD) curves and profiles (3 × $\times$ 3  cm 2 ${\rm cm}^2$ to 25 × $\times$ 25  cm 2 ${\rm cm}^2$ ) are compared to MC simulated data to optimize the electron source energy and radial distribution. Additionally, output factors are simulated and compared to measurements. MLC transmission at 10 cm depth in solid water is simulated and measured using GAFChromic EBT3 film. MLC positioning accuracy is evaluated by comparing off-axis MLC-defined field profiles measured at 10 cm depth in water. The MC model's dose calculation accuracy is further evaluated by comparing measured and simulated surface doses using EBT3 film, and PDDs in slab phantoms using parallel plate chambers. Surface dose is measured by placing films at the surface and 5 cm depth in a solid water phantom. PDDs are measured and simulated in the following slab phantom configurations: polystyrene and polystyrene-bone-lung-polystyrene. RESULTS:A 5.5 MeV electron source energy and a 1.3 mm radial distribution (FWHM) provides the best match between measurement and MC. Simulated PDDs pass 1% | $\vert$ 1 mm gamma criteria at 100% compared to measurements for all fields investigated. Simulated profiles at various depths for all fields (3 × $\times$ 3  cm 2 ${\rm cm}^2$ to 25 × $\times$ 25  cm 2 ${\rm cm}^2$ ) score > $>$ 96.7% in 2% | $\vert$ 2 mm compared to measurement. Evaluated output factors are in good agreement with measurement (within 1%) for all fields except 3 × $\times$ 3  cm 2 ${\rm cm}^2$ (within 1.5%). The 2% | $\vert$ 2 mm gamma pass-rates for MLC defined off-axis fields are > $>$ 98%. The maximum mean Distance-To-Agreement (DTA) in the penumbra region (1% criteria) and mean dose difference in central region for inline and crossline profiles, are 1 mm and 1%, respectively. MC simulated MLC transmission at central axis (0.28% - 0.29%) is in good agreement with measurement (0.28% - 0.44%). Film surface dose relative to Dmax is 74.4% in measurement and 73.6% in simulation. Lastly heterogeneous phantom PDDs passed 1% | $\vert$ 1 mm gamma criteria at > 98 $>98$ % compared to measurement. CONCLUSIONS:The developed TOPAS MC model of the 0.5 T Linac-MR demonstrates high accuracy for dose verification in magnetic fields. The MLC module, including its coordinate positioning mechanism, is fully validated for open aperture applications. This MC model provides a reliable framework for dose simulations in a magnetic field.

BackgroundThe world's first clinical 0.5 T inline rotating biplanar Linac-MR system is commissioned for clinical use. For reference dosimetry, unique features to device, including an SAD = 120 cm, bore clearance of 60 cm x 110 cm, as well as 0.5 T inline magnetic field, provide some challenges to applying a standard dosimetry protocol (i.e., TG-51).PurposeIn this work, we propose a simple and practical clinical reference dosimetry protocol for the 0.5T biplanar Linac-MR and validated its results.MethodsOur dosimetry protocol for this system is as follows: tissue phantom ratios at 20 and 10 cm are first measured and converted into %dd10x beam quality specifier using equations provided and Kalach and Rogers. The converted %dd10x is used to determine the ion chamber correction factor, using the equations in the TG-51 addendum for the Exradin A12 farmer chamber used, which is cross-calibrated with one calibrated at a standards laboratory. For a 0.5 T parallel field, magnetic field effect on chamber response is assumed to have no effect and is not explicitly corrected for. Once the ion chamber correction factor for a non-standard SAD (kQ,msr) is determined, TG-51 is performed to obtain dose at a depth of 10 cm at SAD = 120 cm. The dosimetry protocol is repeated with the magnetic field ramped down. To validate our dosimetry protocol, Monte Carlo (EGSnrc) simulations are performed to confirm the determined kQ,msr values. MC Simulations and magnetic Field On versus Field Off measurements are performed to confirm that the magnetic field has no effect. To validate our overall dosimetry protocol, external dose audits, based on optical simulated luminescent dosimeters, thermal luminescent dosimeters, and alanine dosimeters are performed on the 0.5 T Linac-MR system.MethodsOur dosimetry protocol for this system is as follows: tissue phantom ratios at 20 and 10 cm are first measured and converted into %dd10x beam quality specifier using equations provided and Kalach and Rogers. The converted %dd10x is used to determine the ion chamber correction factor, using the equations in the TG-51 addendum for the Exradin A12 farmer chamber used, which is cross-calibrated with one calibrated at a standards laboratory. For a 0.5 T parallel field, magnetic field effect on chamber response is assumed to have no effect and is not explicitly corrected for. Once the ion chamber correction factor for a non-standard SAD (kQ,msr) is determined, TG-51 is performed to obtain dose at a depth of 10 cm at SAD = 120 cm. The dosimetry protocol is repeated with the magnetic field ramped down. To validate our dosimetry protocol, Monte Carlo (EGSnrc) simulations are performed to confirm the determined kQ,msr values. MC Simulations and magnetic Field On versus Field Off measurements are performed to confirm that the magnetic field has no effect. To validate our overall dosimetry protocol, external dose audits, based on optical simulated luminescent dosimeters, thermal luminescent dosimeters, and alanine dosimeters are performed on the 0.5 T Linac-MR system.ResultsOur EGSnrc results confirm our protocol-determined kQ,msr values, as well as our assumptions about magnetic field effects (kB = 1) within statistical uncertainty for the A-12 chamber. Our external dosimetry procedures also validated our overall dosimetry protocol for the 0.5 T biplanar Linac-MR hybrid. Ramping down the magnetic field has resulted in a dosimetric difference of 0.1%, well within experimental uncertainty.ConclusionWith the 0. 5 T parallel magnetic field having minimal effect on the ion chamber response, a TPR20,10 approach to determine beam quality provides an accurate method to perform clinical dosimetry for the 0.5 T biplanar Linac-MR.

Background: Stereotactic body radiotherapy (SBRT) is an evolving treatment for the local management of pancreatic cancer (PC). The main purpose of this study is to report our initial experience in terms of local control (LC) and toxicity for PC patients treated with SBRT. Methods: We conducted a retrospective review of patients treated with SBRT using abdominal compression (AC) or an end-expiratory breath-holding (EEBH) technique. The median prescribed dose was 35 Gy, delivered in five fractions. Toxicities were recorded using Common Terminology Criteria for Adverse Events (CTCAE) v5.0, and survival was estimated using the Kaplan–Meier method. Results: From 2017 to 2023, 17 PC patients were offered SBRT. Their median age was 69 years. The median follow-up from the date of diagnosis was 22.37 months. The overall survival (OS) was 94% at 1 year and 60.9% at 2 years. The progression-free survival (PFS) was 63.1% at 6 months and 56.1% at 9 months. The median OS was 26.3 months, and the median PFS was 20.6 months. The 6-month and 1-year LC rates were 71% and 50.8%, respectively. Conclusion: We are successful in implementing the SBRT program at our centre. SBRT appears to be a promising treatment option for achieving LC with limited acute toxicities.

Background Magnetic resonance (MR) imaging devices have been integrated with medical linear accelerators (linac) in radiation therapy. Both perpendicular linac-MR (LMR-B perpendicular to) and parallel (LMR-B parallel to) systems exist, where due to the MR's magnetic field dose can be perturbed in the patient. Dose perturbations from the electron return effect (ERE) and electron streaming effects (ESEs) are present in LMR-B perpendicular to systems, where a dose collimating effect has been observed in LMR-B parallel to systems . Purpose To report on an asymmetric dose perturbation which is present at the interface between two different materials during treatment in parallel linac-MR (LMR-B parallel to) systems. To the best of our knowledge, these asymmetric dose effects, "Lateral Scattered Electron Return Effect" (LS-ERE) have not been previously reported. Methods BEAMnrc and EGSnrc Monte Carlo (MC) radiation transport codes were used with the EEMF macro to emulate a 6 FFF beam from the 0.5-T Alberta linac-MR (LMR). Simulations were performed at 0.5 and 1.5 T in several different phantom material-interface combinations and field sizes including from modulated MLC-like fields. MC simulations quantified LS-ERE in patient CT datasets for the head, breast, and lung. LS-ERE cancellation techniques were investigated. LS-ERE asymmetries were quantified by subtracting an antiparallel dose from the parallel dose, dividing by two and normalizing to the global 0-T maximum dose. GafChromic film measurements were made in the 0.5-T Alberta LMR-B parallel to system using solid water at the water-air interface to validate MC simulations. ERE was simulated for an emulated LMR-B perpendicular to system and compared to LMR-B parallel to dose perturbations. Results LS-ERE is mostly independent of field size for fields >1 x 1 cm(2). For 5 x 5-cm(2) fields at 0.5T/1.5T, LS-ERE asymmetries are <=+/- 6.9%/6.9% at bone-air and <=+/- 9.0%/7.0% at tissue-air for nonair doses, and <=+/- 4.1%/5.5% at tissue-lung interfaces. LS-ERE increases as the density gradient increases, where the magnitude and extent of LS-ERE are reduced as field strength increases. For a single 5 x 5-cm(2) field at 0.5T/1.5T, the LS-ERE asymmetry is <=+/- 10.2%/8.5% at the tissue-air sinus interface for head, <=+/- 4.2%/5.3% at the spine-lung interface for the lung, and <=+/- 5.7%/4.9% at the skin-air interface for a breast tangent plan at 0.5T/1.5T. POP fields mostly remove LS-ERE asymmetries, with magnetic field reversal during treatment being the most effective method. Skin dose was investigated and compared to 0-T treatments for 0.5T/1.5T LMR-B parallel to single field breast and head treatments. Including all dosimetric magnetic field perturbations, a 21%/24% and 22%/22% increase in skin dose to head and breast, respectively, was observed, of which LS-ERE is responsible for approximately 30% of the total. Measured LS-ERE asymmetries and dose enhancements at the water-air interface using GafChromic film were in excellent agreement with MC simulations. ERE in 1.5-T LMR-B perpendicular to systems are on average 5.5 times larger than total dose perturbations at 0.5 T in LMR-B parallel to systems. Conclusion LS-ERE is present at the interface between materials and awareness of LS-ERE is crucial for proper TPS evaluation for LMR-B parallel to treatments, especially in areas where large tissue density gradients exist.

Objective.To develop a 2D MR acceleration method utilizing principal component analysis (PCA) in a hybrid fashion for rapid real-time applications.Approach.Retrospective testing was performed on 10 lung, 10 liver and 10 prostate 3T MRI data sets for image quality and target contourability. Sampling of k-space is performed by acquiring central (low-frequency) data in every frame while the high-frequency data is incoherently undersampled such that all of k-space is acquired in a pre-determined number of frames. Firstly, principal components (PCs) representative of intra-frame correlations between central and outer k-space data are used to estimate unsampled data in the frame of interest. Then to add further stability, PCs representative of time-domain fluctuations within a reconstruction window of the most recent frames are fit to outer k-space data (including above estimations) to obtain final estimates in the frame of interest. Accelerated reconstructions between 3x and 8x were tested for image quality and contourability along with the optimal number of PCs for fitting.Main results.It was found that at higher acceleration rates, image quality did not deteriorate significantly. Similarly, it was found that the images were of sufficient quality to contour a target using auto-contouring software at all tested acceleration rates and sites. SSIM values were found to be ⩾0.91 at all accelerations tested. Similarly dice coefficients at the different sites were found to be ⩾0.89 even at 8x accelerations which is on par with or better than intra-observer variation.Significance.This method appears to produce improved image quality and contourability compared to previous PCA methods while also allowing a greater number of PCs to be used in reconstruction. The method can be run using a simple single-channel coil and does not require significant computing power to meet real-time interventional standards (reconstruction times ∼60 ms/frame on Intel i5 CPU).

Introduction: In the last decade, hybrid linear accelerator magnetic resonance imaging (Linac-MR) devices have evolved into FDA-cleared clinical tools, facilitating magnetic resonance guided radiotherapy (MRgRT). The addition of a magnetic field to radiation therapy has previously demonstrated dosimetric and electron effects regardless of magnetic field orientation. Purpose: This study uses Monte Carlo simulations to investigate the importance and efficacy of the magnetic field design in mitigating surface dose enhancement in the Aurora-RT, focusing specifically on contaminant electrons, their origin, and energy spectrum. Methods: The Aurora-RT 0.5 T Biplanar Linac-MR device was modeled using the BEAMnrc package using the updated EM macros, a magnetic field map generated from Opera 3D. Simulation generated phasespace data at the distal side of the first magnetic pole plate (89 cm) and at machine isocenter (120 cm) were analyzed with respect to electron energy spectra and electron creation origins, both with and without the static magnetic field. Results: The presence of the main magnetic field was verified to affect the origin and distribution of contaminant electrons, removing them from the air column up to 60 cm from the target, and focusing them along the CAX within the region below. Analysis of the remaining electron energy fluence reveals the net removal of electrons with energies > 2 MeV and generation of electrons with energies < 2 MeV in the presence of the static magnetic field as compared to no magnetic field. Moreover, in the presence of the magnetic field the integral energy contained in the contaminant electrons increases from 89 cm to isocenter but is still 15% less overall than the integral energy contained in contaminant electrons without the magnetic field. Conclusion: This study provides an analysis of contaminant electrons in the Aurora-RT 0.5 T Linac-MR, emphasizing the role of magnetic field design in successfully minimizing electron contaminants.

Purpose/Objective(s) To assess if stereotactic ablative body radiotherapy (SABR) for oligoprogressive luminal (ER positive, Her-2 negative) advanced breast cancer could delay a change in combination CDK 4/6 inhibitor and an aromatase inhibitor therapy (CDK 4/6 + AI) by ≥ 6 months in > 25% of patients. Herein we report the primary outcome. Materials/Methods AVATAR (ACTRN 12620001212943) enrolled eligible patients with advanced luminal breast cancer who received first or second line systemic treatment in the metastatic setting with a CDK 4/6 + AI for ≥ 6 months. Patients required an ECOG performance status of 0-2 and 1-5 extracranial oligoprogressing lesions amenable to SABR. Patients who had chemotherapy for metastatic disease, leptomeningeal disease, or prior radiotherapy to an oligoprogressing lesion planned for SABR were excluded. At subsequent progression, further SABR was permitted to delay a change in systemic therapy. The primary endpoint was event free survival (EFS) defined as a time to change in systemic therapy after SABR, any progression within 6 months or in > 3 lesions. Secondary endpoints were progression free survival (PFS), overall survival (OS), treatment related toxicity and modified progression free survival (mPFS) defined as progression not amenable to further SABR at any time. Results 32 patients were recruited (Aug 2020 – Nov 2022), with a median follow-up of 15.8 months. The number of patients with 1, 2, 3, or 4 sites of oligoprogression at baseline was 13 (41%), 10(31%), 7 (22), and 2 (6%), respectively. The most common sites of oligoprogression were bone 44 (71%), and nodal 11 (18%). The most common SABR doses were 20 Gy /1 fraction and 24 Gy/2 fractions. The null hypothesis was rejected, with 47% (95% CI: 29-65) of patients remaining event free for ≥ 6 months. The median mPFS was 10.4 months (95% CI: 4.1-not reached) with 46% (95% CI: 27-63) remaining unchanged on systemic therapy for 12 months. Median PFS was 5.2 months (95% CI: 3.1-6.8), with 10/30 (33%) progressions suitable for a second course of SABR for oligoprogression to further delay systemic therapy change. 17 (53%) patients had no treatment related toxicity. 13 (40%) and 2 (13%) patients experienced grade 1 or 2 treatment related toxicities respectively, with no grade 3 or higher toxicities reported. Conclusion This is the first prospective trial investigating SABR as a strategy to maintain CDK 4/6 + AI in patients with oligoprogessive luminal breast cancer. This approach was well tolerated, with a higher than anticipated median time to change in systemic therapy of 10.4 months, and 46% of patients maintained on a CDK 4/6 + AI for 12 months. These findings suggest that patients with oligoprogressive luminal breast cancer should be considered for SABR in lieu of a change in systemic therapy. To assess if stereotactic ablative body radiotherapy (SABR) for oligoprogressive luminal (ER positive, Her-2 negative) advanced breast cancer could delay a change in combination CDK 4/6 inhibitor and an aromatase inhibitor therapy (CDK 4/6 + AI) by ≥ 6 months in > 25% of patients. Herein we report the primary outcome. AVATAR (ACTRN 12620001212943) enrolled eligible patients with advanced luminal breast cancer who received first or second line systemic treatment in the metastatic setting with a CDK 4/6 + AI for ≥ 6 months. Patients required an ECOG performance status of 0-2 and 1-5 extracranial oligoprogressing lesions amenable to SABR. Patients who had chemotherapy for metastatic disease, leptomeningeal disease, or prior radiotherapy to an oligoprogressing lesion planned for SABR were excluded. At subsequent progression, further SABR was permitted to delay a change in systemic therapy. The primary endpoint was event free survival (EFS) defined as a time to change in systemic therapy after SABR, any progression within 6 months or in > 3 lesions. Secondary endpoints were progression free survival (PFS), overall survival (OS), treatment related toxicity and modified progression free survival (mPFS) defined as progression not amenable to further SABR at any time. 32 patients were recruited (Aug 2020 – Nov 2022), with a median follow-up of 15.8 months. The number of patients with 1, 2, 3, or 4 sites of oligoprogression at baseline was 13 (41%), 10(31%), 7 (22), and 2 (6%), respectively. The most common sites of oligoprogression were bone 44 (71%), and nodal 11 (18%). The most common SABR doses were 20 Gy /1 fraction and 24 Gy/2 fractions. The null hypothesis was rejected, with 47% (95% CI: 29-65) of patients remaining event free for ≥ 6 months. The median mPFS was 10.4 months (95% CI: 4.1-not reached) with 46% (95% CI: 27-63) remaining unchanged on systemic therapy for 12 months. Median PFS was 5.2 months (95% CI: 3.1-6.8), with 10/30 (33%) progressions suitable for a second course of SABR for oligoprogression to further delay systemic therapy change. 17 (53%) patients had no treatment related toxicity. 13 (40%) and 2 (13%) patients experienced grade 1 or 2 treatment related toxicities respectively, with no grade 3 or higher toxicities reported. This is the first prospective trial investigating SABR as a strategy to maintain CDK 4/6 + AI in patients with oligoprogessive luminal breast cancer. This approach was well tolerated, with a higher than anticipated median time to change in systemic therapy of 10.4 months, and 46% of patients maintained on a CDK 4/6 + AI for 12 months. These findings suggest that patients with oligoprogressive luminal breast cancer should be considered for SABR in lieu of a change in systemic therapy.

BACKGROUND:The Alberta rotating biplanar linac-MR has a 0.5 T magnetic field parallel to the beamline. When developing a new linac-MR system, interactions of charged particles with the magnetic field necessitate careful consideration of skin dose and tissue interface effects.PURPOSE:To investigate the effect of the magnetic field on skin dose using measurements and Monte Carlo (MC) simulations.METHODS:We develop an MC model of our linac-MR, which we validate by comparison with ion chamber measurements in a water tank. Additionally, MC simulation results are compared with radiochromic film surface dose measurements on solid water. Variations in surface dose as a function of field size are measured using a parallel plate ion chamber in solid water. Using an anthropomorphic computational phantom with a 2 mm-thick skin layer, we investigate dose distributions resulting from three beam arrangements. Magnetic field on and off scenarios are considered for all measurements and simulations.RESULTS:For a 20 × 20 cm2 field size, D 0.2 c c ${D_{0.2cc}}$ (the minimum dose to the hottest contiguous 0.2 cc volume) for the top 2 mm of a simple water phantom is 72% when the magnetic field is on, compared to 34% with magnetic field off (values are normalized to the central axis dose maximum). Parallel plate ion chamber measurements demonstrate that the relative increase in surface dose due to the magnetic field decreases with increasing field size. For the anthropomorphic phantom, D ∼ 0.2 c c ${D_{ \sim 0.2cc}}$ (minimum skin dose in the hottest 1 × 1 × 1 cm3 cube) shows relative increases of 20%-28% when the magnetic field is on compared to when it is off. With magnetic field off, skin D ∼ 0.2 c c ${D_{ \sim 0.2cc}}$ is 71%, 56%, and 21% for medial-lateral tangents, anterior-posterior beams, and a five-field arrangement, respectively. For magnetic field on, the corresponding skin D ∼ 0.2 c c ${D_{ \sim 0.2cc}}$ values are 91%, 67%, and 25%.CONCLUSIONS:Using a validated MC model of our linac-MR, surface doses are calculated in various scenarios. MC-calculated skin dose varies depending on field sizes, obliquity, and the number of beams. In general, the parallel linac-MR arrangement results in skin dose enhancement due to charged particles spiraling along magnetic field lines, which impedes lateral motion away from the central axis. Nonetheless, considering the results presented herein, treatment plans can be designed to minimize skin dose by, for example, avoiding oblique beams and using a larger number of fields.

Purpose A rapid real-time 2D accelerated method was developed for magnetic resonance imaging (MRI) using principal component analysis (PCA) in the temporal domain. This method employs a moving window of previous dynamic frames to reconstruct the current, real-time frame within this window. This technique could be particularly useful in real-time tracking applications such as in MR-guided radiotherapy, where low latency real-time reconstructions are essential. Methods The method was tested retrospectively on 15 fully-sampled data sets of lung patient data acquired on a 3T Philips Achieva system. High frequency data are incoherently undersampled, while the central low-frequency data are always acquired to characterize the temporal fluctuations through PCA. The undersampling pattern is derived in such a way that all of k-space is acquired within a pre-determined number of frames. The missing data in the current frame are then filled in by fitting the temporal characterizations to the acquired undersampled data, using a pre-determined number of PCs. A subset of six patients was used to test the contour ability of the images. Various accelerations between 3x and 8x were tested along with the optimal number of PCs for fitting. A comparison was also performed with previous work from our group proposed by Dietz et al. as well as with a standard low resolution acquisition. In order to determine how the method would perform at lower signal to noise ratio (SNR), noise levels of 2x, 4x, and 6x were added to the 3T data. Metrics such as normalised mean square error and Dice coefficient were used to measure the reconstruction image quality and contour ability. Results The proposed method demonstrated good temporal robustness as consistent metrics were detected for the duration of the imaging session. It was found that the optimal number of PCs for temporal fitting was dependent on the acceleration rate. For the data tested, five PCs were found to be optimal at the acceleration rates of 3x and 4x. This number decreases to three at accelerations of 5x and 6x and further decreases to two at an acceleration rate of 8x, likely due to greater instability with fewer acquired data points. The use of too many PCs for fitting increased the chances of noisy reconstruction which affected contourability. Conclusions The proposed 2D real-time MR acceleration method demonstrated greater robustness in the metrics over time when compared with previous real-time PCA methods using metrics such as normalised mean squared error, peak SNR and structural similarity up to an acceleration of 8x. Improved temporal robustness of image structure contourability and accurate definition was also demonstrated using several metrics including the Dice coefficient. Reconstruction of raw acquired data can be performed at approximately 50 ms per frame using an Intel core i5 CPU. The method has the advantage of being very flexible in terms of hardware requirements as it can operate successfully on a single coil channel and does not require specialized computing power to implement in real-time.

Magnetic resonance imaging (MRI) provides a means to non-invasively investigate the neurological links with dyslexia, a learning disability that affects one's ability to read. Most previous brain MRI studies of dyslexia and reading skill have used structural or diffusion imaging to reveal regional brain abnormalities. However, volumetric and diffusion MRI lack specificity in their interpretation at the microstructural level. Myelin is a critical neural component for brain function and plasticity, and as such, deficits in myelin may impact reading ability. MRI can estimate myelin using myelin water fraction (MWF) imaging, which is based on evaluation of the proportion of short T2 myelin-associated water from multi-exponential T2 relaxation analysis, but has not yet been applied to the study of reading or dyslexia. In this study, MWF MRI, intelligence, and reading assessments were acquired in 20 participants aged 10-18 years with a wide range of reading ability to investigate the relationship between reading ability and myelination. Group comparisons showed markedly lower MWF by 16-69% in poor readers relative to good readers in the left and right thalamus, as well as the left posterior limb of the internal capsule, left/right anterior limb of the internal capsule, left/right centrum semiovale, and splenium of the corpus callosum. MWF over the entire group also correlated positively with three different reading scores in the bilateral thalamus as well as white matter, including the splenium of the corpus callosum, left posterior limb of the internal capsule, left anterior limb of the internal capsule, and left centrum semiovale. MWF imaging from T2 relaxation suggests that myelination, particularly in the bilateral thalamus, splenium, and left hemisphere white matter, plays a role in reading abilities. Myelin water imaging thus provides a potentially valuable in vivo imaging tool for the study of dyslexia and its remediation.