Maximising BOLD sensitivity through automated EPI protocol optimisation.

Gradient echo echo-planar imaging (GE EPI) is used for most fMRI studies but can suffer substantially from image distortions and BOLD sensitivity (BS) loss due to susceptibility-induced magnetic field inhomogeneities. While there are various post-processing methods for correcting image distortions, signal dropouts cannot be recovered and therefore need to be addressed at the data acquisition stage. Common approaches for reducing susceptibility-related BS loss in selected brain areas are: z-shimming, inverting the phase encoding (PE) gradient polarity, optimizing the slice tilt and increasing spatial resolution. The optimization of these parameters can be based on atlases derived from multiple echo-planar imaging (EPI) acquisitions. However, this requires resource and time, which imposes a practical limitation on the range over which parameters can be optimised meaning that the chosen settings may still be sub-optimal. To address this issue, we have developed an automated method that can be used to optimize across a large parameter space. It is based on numerical signal simulations of the BS loss predicted by physical models informed by a large database of magnetic field (B0) maps acquired on a broad cohort of participants. The advantage of our simulation-based approach compared to previous methods is that it saves time and expensive measurements and allows for optimizing EPI protocols by incorporating a broad range of factors, including different resolutions, echo times or slice orientations. To verify the numerical optimisation, results are compared to those from an earlier study and to experimental BS measurements carried out in six healthy volunteers.

Neurocognitive Development of the Resolution of Selective Visuo-Spatial Attention: Functional MRI Evidence From Object Tracking.

Our ability to select relevant information from the environment is limited by the resolution of attention - i.e., the minimum size of the region that can be selected. Neural mechanisms that underlie this limit and its development are not yet understood. Functional magnetic resonance imaging (fMRI) was performed during an object tracking task in 7- and 11-year-old children, and in young adults. Object tracking activated canonical fronto-parietal attention systems and motion-sensitive area MT in children as young as 7 years. Object tracking performance improved with age, together with stronger recruitment of parietal attention areas and a shift from low-level to higher-level visual areas. Increasing the required resolution of spatial attention - which was implemented by varying the distance between target and distractors in the object tracking task - led to activation increases in fronto-insular cortex, medial frontal cortex including anterior cingulate cortex (ACC) and supplementary motor area, superior colliculi, and thalamus. This core circuitry for attentional precision was recruited by all age groups, but ACC showed an age-related activation reduction. Our results suggest that age-related improvements in selective visual attention and in the resolution of attention are characterized by an increased use of more functionally specialized brain regions during the course of development.

Comparison of two quantitative proton density mapping methods in multiple sclerosis.

OBJECTIVE: Proton density (PD) mapping requires correction for the receive profile (RP), which is frequently performed via bias-field correction. An alternative RP-mapping method utilizes a comparison of uncorrected PD-maps and a value ρ(T1) directly derived from T1-maps via the Fatouros equation. This may be problematic in multiple sclerosis (MS), if respective parameters are only valid for healthy brain tissue. We aimed to investigate whether the alternative method yields correct PD values in MS patients. MATERIALS/METHODS: PD mapping was performed on 27 patients with relapsing-remitting MS and 27 healthy controls, utilizing both methods, yielding reference PD values (PDref, bias-field method) and PDalt (alternative method). RESULTS: PDalt-values closely matched PDref, both for patients and controls. In contrast, ρ(T1) differed by up to 3 % from PDref, and the voxel-wise correlation between PDref and ρ(T1) was reduced in a patient subgroup with a higher degree of disability. Still, discrepancies between ρ(T1) and PDref were almost identical across different tissue types, thus translating into a scaling factor, which cancelled out during normalization to 100 % in CSF, yielding a good agreement between PDalt and PDref. CONCLUSION: RP correction utilizing the auxiliary parameter ρ(T1) derived via the Fatouros equation provides accurate PD results in MS patients, in spite of discrepancies between ρ(T1) and actual PD values.

The Relationship between Gray Matter Quantitative MRI and Disability in Secondary Progressive Multiple Sclerosis.

PURPOSE: In secondary progressive Multiple Sclerosis (SPMS), global neurodegeneration as a driver of disability gains importance in comparison to focal inflammatory processes. However, clinical MRI does not visualize changes of tissue composition outside MS lesions. This quantitative MRI (qMRI) study investigated cortical and deep gray matter (GM) proton density (PD) values and T1 relaxation times to explore their potential to assess neuronal damage and its relationship to clinical disability in SPMS. MATERIALS AND METHODS: 11 SPMS patients underwent quantitative T1 and PD mapping. Parameter values across the cerebral cortex and deep GM structures were compared with 11 healthy controls, and correlation with disability was investigated for regions exhibiting significant group differences. RESULTS: PD was increased in the whole GM, cerebral cortex, thalamus, putamen and pallidum. PD correlated with disability in the whole GM, cerebral cortex, putamen and pallidum. T1 relaxation time was prolonged and correlated with disability in the whole GM and cerebral cortex. CONCLUSION: Our study suggests that the qMRI parameters GM PD (which likely indicates replacement of neural tissue with water) and cortical T1 (which reflects cortical damage including and beyond increased water content) are promising qMRI candidates for the assessment of disease status, and are related to disability in SPMS.

Multimodal quantitative MRI assessment of cortical damage in relapsing-remitting multiple sclerosis.

PURPOSE: To investigate magnetization transfer ratio (MTR), T1 relaxation time, and proton density (PD) as indicators of gray matter damage in relapsing-remitting multiple sclerosis (RRMS), reflecting different aspects of microstructural damage and as imaging correlates of clinical disability. We aimed to determine which of these parameters may optimally quantify cortical damage, and serve as an imaging surrogate of clinical disability. In this study, cortical values of MTR, a surrogate for demyelination in MS, of PD, reflecting replacement of neural tissue by water, and of T1 , indicating a complex array of microstructural changes, were assessed in a group of RRMS patients in comparison to healthy controls (HC). MATERIALS AND METHODS: 22 RRMS patients with varying disease duration (4.0 ± 6.54 years) and 10 HC received quantitative 3T magnetic resonance imaging (MRI) with MTR, T1 , and PD mapping. We tested for differences in cortical measurements between patients and HC. Additionally, correlation with disability as quantified by the Expanded Disability Status Scale was investigated. RESULTS: Cortical parameter values were significantly altered in the RRMS group, with increased values of T1 (P = 0.008) and PD (P = 0.028) and reduced values of MTR (P = 0.043). Only cortical T1 was correlated with clinical disability measurements (P = 0.001, r = 0.65). Receiver operating characteristic analysis demonstrated the best discriminatory power for T1 (area under the curve 0.79, PD: 0.75, MTR 0.73). CONCLUSION: Out of the parameters studied, cortical T1 is best suited to detect cortical damage as an imaging surrogate of clinical disability in RRMS. J. Magn. Reson. Imaging 2016;44:1600-1607.

Quantitative T2, T2*, and T2' MR imaging in patients with ischemic leukoaraiosis might detect microstructural changes and cortical hypoxia.

INTRODUCTION: Quantitative MRI with T2, T2*, and T2' mapping has been shown to non-invasively depict microstructural changes (T2) and oxygenation status (T2* and T2') that are invisible on conventional MRI. Therefore, we aimed to assess whether T2 and T2' quantification detects cerebral (micro-)structural damage and chronic hypoxia in lesions and in normal appearing white matter (WM) and gray matter (GM) of patients with ischemic leukoaraiosis (IL). Measurements were complemented by the assessment of the cerebral blood flow (CBF) and the degree of GM and WM atrophy. METHODS: Eighteen patients with IL and 18 age-matched healthy controls were included. High-resolution, motion-corrected T2, T2*, and T2' mapping, CBF mapping (pulsed arterial spin labeling, PASL), and segmentation of GM and WM were used to depict specific changes in both groups. All parameters were compared between patients and healthy controls, using t testing. Values of p < 0.05 were accepted as statistically significant. RESULTS: Patients showed significantly increased T2 in lesions (p < 0.01) and in unaffected WM (p = 0.045) as well as significantly increased T2* in lesions (p = 0.003). A significant decrease of T2' was detected in patients in unaffected WM (p = 0.027), while no T2' changes were observed in GM (p = 0.13). Both unaffected WM and GM were significantly decreased in volume in the patient-group (p < 0.01). No differences of PASL-based CBF could be shown. CONCLUSION: Non-invasive quantitative MRI with T2, T2*, and T2' mapping might be used to detect subtle structural and metabolic changes in IL. Assessing the grade of microstructural damage and hypoxia might be helpful to monitor disease progression and to perform risk assessment.

An improved method for retrospective motion correction in quantitative T2* mapping.

A new method for motion correction of T2*-weighted data and resulting quantitative T2* maps is presented. For this method, additional data sets with a reduced number of phase encoding steps covering the k-space centre are acquired. Motion correction is based on a 3-step procedure: (1) calculation of improved input data sets with reduced artefact levels from the original data, (2) creation of a target data set free of movement artefacts on the basis of the improved input data sets, and (3) fitting of original data to the target data set, yielding an optimum combination of acquired k-space data which suppresses lines affected by movement. The method was tested on healthy subjects performing pre-trained movement. Motion correction was successful unless the same k-space line was affected by movement in all data sets acquired on a specific subject. The method was applied to patients suffering from subarachnoid haemorrhage (group 1) or tumours (group 2) with accompanying edema in the brain. Motion correction improved the interpretability of T2*-weighted patient data and resulting quantitative T2* maps considerably by allowing a clear delineation between ventricle and edema and a clear localisation of haemorrhage (group 1) or a clear delineation of tumour accompanying edema (group 2) which was not possible in data affected by movement.

Within-lesion differences in quantitative MRI parameters predict contrast enhancement in multiple sclerosis.

PURPOSE: To investigate the relationship between quantitative magnetic resonance imaging (qMRI) and contrast enhancement in multiple sclerosis (MS) lesions. We compared maps of T1 relaxation time, proton density (PD), and magnetization transfer ratio (MTR) between lesions with and without contrast enhancement as quantified by the amount of T1 shortening postcontrast agent (CA). MATERIALS AND METHODS: In 17 patients with relapsing-remitting MS (RRMS), 15 with progressive MS (PMS), and 17 healthy controls, T1, PD, and MTR were measured at 3T and T1-mapping was repeated after CA administration. Manually drawn MS-lesions (3D-FLAIR) were labeled as enhancing if post-CA T1-shortening exceeded mean T1-shortening in normal-appearing white matter (NAWM) by at least 2 standard deviations. Precontrast T1, PD, and MTR were compared in enhancing lesions, nonenhancing lesions, NAWM, and gray matter. RESULTS: Precontrast T1, PD, and MTR differed significantly between enhancing and nonenhancing lesions in RRMS and PMS patients (all P < 0.01). In PMS patients, PD of NAWM, enhancing, and nonenhancing lesions and MTR and T1 of gray matter differed significantly from RRMS and controls. Only MTR of gray matter differed between RRMS and controls. CONCLUSION: Contrast enhancement in MS quantified by relative T1 shortening may be predicted by precontrast abnormalities of T1, PD, and MTR and likely represents blood-brain barrier damage.

T2'- and PASL-based perfusion mapping at 3 Tesla: influence of oxygen-ventilation on cerebral autoregulation.

PURPOSE: To use T2'-mapping together with Pulsed Arterial Spin Labeling (PASL) providing quantitative information of deoxygenation level and cerebral blood flow (CBF) in the cerebral gray matter to obtain simultaneous information about the cerebral oxygen metabolism and the resulting cerebral vasoreactivity under normoxic and hyperoxic conditions. MATERIALS AND METHODS: Twelve young, healthy volunteers underwent MRI under normoxic and hyperoxic conditions performing PASL and high-resolution, motion-corrected T2* and T2-mapping to calculate T2'values. Regions of interest (ROI) were placed in the frontoparietal cortex and thalamus by manual and automatic segmentation. For each ROI, mean normoxic T2'- and CBF values were extracted and compared with the same parameters assessed under hyperoxic ventilation. RESULTS: A hyperoxic-induced decrease of the CBF could be shown in the frontoparietal cortex (P = 0.009). The T2 values of frontoparietal cortex decreased under hyperoxic inhalation compared with normoxia (P = 0.01), whereas T2' remained unchanged. CONCLUSION: Motion-corrected high-resolution T2'-maps can be used together with PASL to evaluate the DeoxyHb content in relation to CBF in the cerebral gray matter. We could show that cortical CBF decreases under hyperoxic inhalation in healthy young subjects, whereas the T2' values remained constant. These data suggest that hyperoxic-induced vasoconstriction may protect the brain against hyperoxemia.

Quantitative proton density mapping: correcting the receiver sensitivity bias via pseudo proton densities.

Most methods for mapping proton densities (PD) in brain tissue are based on measuring all parameters influencing the signal intensity with subsequent elimination of any weighting not related to PD. This requires knowledge of the receiver coil sensitivity profile (RP), the measurement of which can be problematic. Recently, a method for compensating the influence of RP non-uniformities on PD data at a field strength of 3T was proposed, based on bias field correction of spoiled gradient echo image data to remove the low spatial frequency bias imposed by RP variations from uncorrected PD maps. The purpose of the current study was to present and test an independent method, based on the well-known linear relationship between the longitudinal relaxation rate R1 and 1/PD in brain tissue. For healthy subjects, RP maps obtained with this method and the resulting PD maps are very similar to maps based on bias field correction, and quantitative PD values acquired with the new independent method are in very good agreement with literature values. Furthermore, both methods for PD mapping are compared in the presence of several pathologies (multiple sclerosis, stroke, meningioma, recurrent glioblastoma).

Correction of systematic errors in quantitative proton density mapping.

Interest in techniques yielding quantitative information about brain tissue proton densities is increasing. In general, all parameters influencing the signal amplitude are mapped in several acquisitions and then eliminated from the image data to obtain pure proton density weighting. Particularly, the measurement of the receiver coil sensitivity profile is problematic. Several methods published so far are based on the reciprocity theorem, assuming that receive and transmit sensitivities are identical. Goals of this study were (1) to determine quantitative proton density maps using an optimized variable flip angle method for T(1) mapping at 3 T, (2) to investigate if systematic errors can arise from insufficient spoiling of transverse magnetization, and (3) to compare two methods for mapping the receiver coil sensitivity, based on either the reciprocity theorem or bias field correction. Results show that insufficient spoiling yields systematic errors in absolute proton density of about 3-4 pu. A correction algorithm is proposed. It is shown that receiver coil sensitivity mapping based on the reciprocity theorem yields erroneous proton density values, whereas reliable data are obtained with bias field correction. Absolute proton density values in different brain areas, evaluated on six healthy subjects, are in excellent agreement with recent literature results.

Quantitative T*2-mapping based on multi-slice multiple gradient echo flash imaging: retrospective correction for subject motion effects.

Numerous clinical and research applications for quantitative mapping of the effective transverse relaxation time T*(2) have been described. Subject motion can severely deteriorate the quality and accuracy of results. A correction method for T*(2) maps acquired with multi-slice multiple gradient echo FLASH imaging is presented, based on acquisition repetition with reduced spatial resolution (and consequently reduced acquisition time) and weighted averaging of both data sets, choosing weighting factors individually for each k-space line to reduce the influence of motion. In detail, the procedure is based on the fact that motion artifacts reduce the correlation between acquired and exponentially fitted data. A target data set is constructed in image space, choosing the data yielding best correlation from the two acquired data sets. The k-space representation of the target is subsequently approximated as linear combination of original raw data, yielding the required weighting factors. As this method only requires a single acquisition repetition with reduced spatial resolution, it can be employed on any clinical system offering a suitable sequence with export of modulus and phase images. Experimental results show that the method works well for sparse motion, but fails for strong motion affecting the same k-space lines in both acquisitions.

[Automated delivery of codes for charge in radiotherapy]. / Automatisierte Leistungsübermittlung in der Strahlenheilkunde.

BACKGROUND AND PURPOSE: For the medical billing of Radiotherapy every fraction has to be encoded, including date and time of all administered treatments. With fractions averaging 30 per patient and about 2,500 new patients every year the number of Radiotherapy codes reaches an amount of 70,000 and more. Therefore, an automated proceeding for transferring and processing therapy codes has been developed at the Department of Radiotherapy Freiburg, Germany. This is a joint project of the Department of Radiotherapy, the Administration Department, and the Central IT Department of the University Hospital of Freiburg. MATERIAL AND METHODS: The project consists of several modules whose collaboration makes the projected automated transfer of treatment codes possible. The first step is to extract the data from the department's Clinical Information System (MOSAIQ). These data are transmitted to the Central IT Department via an HL7 interface, where a check for corresponding hospitalization data is performed. In the further processing of the data, a matching table plays an important role allowing the transformation of a treatment code into a valid medical billing code. In a last step, the data are transferred to the medical billing system. RESULTS AND CONCLUSION: After assembling and implementing the particular modules successfully, a first beta test was launched. In order to test the modules separately as well as the interaction of the components, extensive tests were performed during March 2006. Soon it became clear that the tested procedure worked efficiently and accurately. In April 2006, a pilot project with a few qualities of treatment (e.g., computed tomography, simulation) was put into practice. Since October 2006, nearly all Radiation Therapy codes (approximately 75,000) are being transferred to the comprehensive Hospital Information System (HIS) automatically in a daily routine.

Dual-echo EPI for non-equilibrium fMRI - implications of different echo combinations and masking procedures.

Dual-echo EPI is based on the acquisition of two images with different echo times per excitation, thus allowing for the calculation of purely T2(*) weighted data. The technique can be used for the measurement of functional activation whenever the prerequisite of constant equilibrium magnetization cannot be fulfilled due to variable inter-volume delays. The latter is the case when image acquisition is triggered by physiological parameters (e.g. cardiac gating) or by the subject's response. Despite its frequent application, there is currently no standardized way of combining the information obtained from the two acquired echoes. The goal of this study was to quantify the implication of different echo combination methods (quotients of echoes and quantification of T(2)(*)) and calculation modalities, either pre-smoothing data before combination or subjecting unsmoothed combined data to masking (no masking, volume-wise masking, joint masking), on the theoretically predicted signal-to-noise ratio (SNR) of the BOLD response and on activation results of two fMRI experiments using finger tapping and visual stimulation in one group (n=5) and different motor paradigms to activate motor areas in the cortex and the brainstem in another group (n=21). A significant impact of echo combination and masking procedure was found for both SNR and activation results. The recommended choice is a direct calculation of T(2)(*) values, either using joint masking on unsmoothed data, or pre-smoothing images prior to T(2)(*) calculation. This method was most beneficial in areas close to the surface of the brain or adjacent to the ventricles and may be especially relevant to brainstem fMRI.

A fast B1-mapping method for the correction and normalization of magnetization transfer ratio maps at 3 T.

In neuroimaging, there is increasing interest in magnetization transfer (MT) techniques which yield information about bound water protons. One of the main applications is the investigation of the myelin integrity in the central nervous system (CNS). However, several problems may arise, in particular at high magnetic field strengths: B1 inhomogeneities may yield deviations of the MT saturation angle and thus non-uniformities of the measured MT ratio (MTR). This effect can be corrected for but requires in general additional time consuming B1 mapping. Furthermore, increased values of the specific absorption rate (SAR) may require a reduction of the saturation angle for individual subjects, impairing comparability of results. In this work, a B1 mapping method based on magnetization-prepared FLASH with slice selective preparation and excitation pulses and correction for relaxation effects is presented, yielding B1 maps with whole brain coverage, an in-plane resolution of 4 mm, a slice thickness of 3 mm, and a clinically acceptable duration of 46 s. The method is tested both in vitro and in vivo and applied in a subsequent in vivo study to show that MTR values in human brain tissue depend approximately linearly on the preparation angle, with a slope similar to values reported for 1.5 T. Calibration data and B1 maps are applied to B1 inhomogeneity corrections of MTR maps. Subsequently, it is shown that B1-corrected MTR maps acquired at reduced preparation angles due to individual SAR restrictions can be normalized, allowing for a direct comparison with maps acquired at the full angle.

Reduction of susceptibility-induced signal losses in multi-gradient-echo images: application to improved visualization of the subthalamic nucleus.

T2-weighted gradient echo (GE) images yield good contrast of iron-rich structures like the subthalamic nuclei due to microscopic susceptibility induced field gradients, providing landmarks for the exact placement of deep brain stimulation electrodes in Parkinson's disease treatment. An additional advantage is the low radio frequency (RF) exposure of GE sequences. However, T2-weighted images are also sensitive to macroscopic field inhomogeneities, resulting in signal losses, in particular in orbitofrontal and temporal brain areas, limiting anatomical information from these areas. In this work, an image correction method for multi-echo GE data based on evaluation of phase information for field gradient mapping is presented and tested in vivo on a 3 Tesla whole body MR scanner. In a first step, theoretical signal losses are calculated from the gradient maps and a pixelwise image intensity correction is performed. In a second step, intensity corrected images acquired at different echo times TE are combined using optimized weighting factors: in areas not affected by macroscopic field inhomogeneities, data acquired at long TE are weighted more strongly to achieve the contrast required. For large field gradients, data acquired at short TE are favored to avoid signal losses. When compared to the original data sets acquired at different TE and the respective intensity corrected data sets, the resulting combined data sets feature reduced signal losses in areas with major field gradients, while intensity profiles and a contrast-to-noise (CNR) analysis between subthalamic nucleus, red nucleus and the surrounding white matter demonstrate good contrast in deep brain areas.

Rapid single-scan T2*-mapping using exponential excitation pulses and image-based correction for linear background gradients.

A method for fast quantitative T(2)* mapping based on multiple gradient-echo (multi-GE) imaging with correction for static magnetic field inhomogeneities is described, using an exponential excitation pulse. Field gradient maps are obtained from the phase information and modulus data are subsequently corrected, allowing for simple monoexponential T(2)* fitting. Echoes with long echo times suffering from major signal losses due to field inhomogeneities are excluded from the analysis. The acquisition time for a matrix size of 256 x 256, 1 mm in-plane resolution, and 2 mm slice thickness amounts to 15 s per slice. An additional correction for in-plane field gradients further improves accuracy. Phantom experiments show that the method provides accurate T(2)* values for field gradients up to 200 microT/m; for gradients up to 300 microT/m errors do not exceed 15%. In vivo T(2)* values acquired on healthy volunteers at 3T are in excellent agreement with results from the literature.

Exponential excitation pulses for improved water content mapping in the presence of background gradients.

Several water content mapping techniques are based on the acquisition of multiple gradient echoes (GE) with different echo times (TE). However, in the presence of linear magnetic field gradients G(susc) the signal decay is no longer exponential but in the case of a rectangular slice profile weighted by a sinc function, giving rise to erroneous initial amplitudes S(0) in monoexponential fitting. Generally, it can be shown that the signal decay is weighted by the time profile of the excitation pulse. Thus, for an excitation pulse with an exponential time profile, i.e., a Lorentzian slice profile, the signal decay remains exponential and exponential fitting still yields the correct amplitude S(0). Multiecho GE images of a gel phantom and five human volunteers were acquired at 3 T using a sinc-shaped and an exponential excitation pulse. In addition, simulations were performed to investigate the influence of saturation effects due to distortion of the ideal Lorentzian slice profile. A considerable overestimation of S(0) when using a sinc-shaped excitation pulse was observed. Errors were greatly reduced with an exponential excitation pulse. We thus propose the use of excitation pulses with exponential time profile to obtain accurate estimates for S(0) from exponential fitting.

Probing category selectivity for environmental sounds in the human auditory brain.

Earlier studies reported evidence suggesting distinct category-related auditory representations for environmental sounds such as animal vocalizations and tool sounds in superior and middle temporal regions of the temporal lobe. However, the degree of selectivity of these representations remains to be determined. The present study combined functional magnetic resonance imaging (fMRI) adaptation with a silent acquisition protocol to further investigate category-related auditory processing of environmental sounds. To this end, we consecutively presented pairs of sounds taken from the categories 'tool sounds' or 'animal vocalizations' with either the same or different identity/category. We examined the degree of selectivity as evidenced by adaptation effects to both or only one sound category in the course of whole-brain as well as functionally and anatomically constrained region of interest analyses. While most regions predominately in the temporal cortex showed an adaptation to both sound categories, particularly the left superior temporal gyrus (STG) and the left posterior middle temporal gyrus (pMTG) selectively adapted to animal vocalizations and tool sounds, respectively. However, the activation profiles of these regions differed with respect to the general responsiveness to sounds. While tool sounds still produced fMRI signals significantly different from fixation baseline in the STG, this was not the case for animal vocalizations in pMTG. Consistent with the interpretation of STG as an intermediate auditory processing stage, this region might differentiate auditory stimuli into categories based on variations of physical stimulus properties. However, processing in left pMTG seems to be even more restricted to action-related sounds of man-made objects.

Human motor corpus callosum: topography, somatotopy, and link between microstructure and function.

The corpus callosum (CC) is the principal white matter fiber bundle connecting neocortical areas of the two hemispheres. Although an object of extensive research, important details about the anatomical and functional organization of the human CC are still largely unknown. Here we focused on the callosal motor fibers (CMFs) that connect the primary motor cortices (M1) of the two hemispheres. Topography and somatotopy of CMFs were explored by using a combined functional magnetic resonance imaging/diffusion tensor imaging fiber-tracking procedure. CMF microstructure was assessed by fractional anisotropy (FA), and CMF functional connectivity between the hand areas of M1 was measured by interhemispheric inhibition using paired-pulse transcranial magnetic stimulation. CMFs mapped onto the posterior body and isthmus of the CC, with hand CMFs running significantly more anteriorly and ventrally than foot CMFs. FA of the hand CMFs but not FA of the foot CMFs correlated linearly with interhemispheric inhibition between the M1 hand areas. Findings demonstrate that CMFs connecting defined body representations of M1 map onto a circumscribed region in the CC in a somatotopically organized manner. The significant and topographically specific positive correlation between FA and interhemispheric inhibition strongly suggests that microstructure can be directly linked to functional connectivity. This provides a novel way of exploring human brain function that may allow prediction of functional connectivity from variability of microstructure in healthy individuals, and potentially, abnormality of functional connectivity in neurological or psychiatric patients.