Fully Automatic Method for Reliable Spinal Cord Compartment Segmentation in Multiple Sclerosis.

BACKGROUND AND PURPOSE: Fully automatic quantification methods of spinal cord compartments are needed to study pathologic changes of the spinal cord GM and WM in MS in vivo. We propose a novel method for automatic spinal cord compartment segmentation (SCORE) in patients with MS. MATERIALS AND METHODS: The cervical spinal cords of 24 patients with MS and 24 sex- and age-matched healthy controls were scanned on a 3T MR imaging system, including an averaged magnetization inversion recovery acquisition sequence. Three experienced raters manually segmented the spinal cord GM and WM, anterior and posterior horns, gray commissure, and MS lesions. Subsequently, manual segmentations were used to train neural segmentation networks of spinal cord compartments with multidimensional gated recurrent units in a 3-fold cross-validation fashion. Total intracranial volumes were quantified using FreeSurfer. RESULTS: The intra- and intersession reproducibility of SCORE was high in all spinal cord compartments (eg, mean relative SD of GM and WM: ≤ 3.50% and ≤1.47%, respectively) and was better than manual segmentations (all P < .001). The accuracy of SCORE compared with manual segmentations was excellent, both in healthy controls and in patients with MS (Dice similarity coefficients of GM and WM: ≥ 0.84 and ≥0.92, respectively). Patients with MS had lower total WM areas (P < .05), and total anterior horn areas (P < .01 respectively), as measured with SCORE. CONCLUSIONS: We demonstrate a novel, reliable quantification method for spinal cord tissue segmentation in healthy controls and patients with MS and other neurologic disorders affecting the spinal cord. Patients with MS have reduced areas in specific spinal cord tissue compartments, which may be used as MS biomarkers.

Pretreatment Normal WM Magnetization Transfer Ratio Predicts Risk of Radiation Necrosis in Patients with Medulloblastoma.

BACKGROUND AND PURPOSE: Radiation necrosis, for which abnormal WM enhancement is a hallmark, is an uncommon complication of craniospinal irradiation in children with medulloblastoma. The magnetization transfer ratio measures macromolecular content, dominated by myelin in the WM. We investigated whether the pretreatment supratentorial (nonsurgical) WM magnetization transfer ratio could predict patients at risk for radiation necrosis after radiation therapy for medulloblastoma. MATERIALS AND METHODS: Ninety-five eligible patients with medulloblastoma (41% female; mean age, 11.0 [SD, 5.4] years) had baseline balanced steady-state free precession MR imaging before proton or photon radiation therapy. Associations among baseline supratentorial magnetization transfer ratio, radiation necrosis (spontaneously resolving/improving parenchymal enhancement within the radiation field)3, age, and the presence of visible brain metastases were explored by logistic regression and parametric/nonparametric techniques as appropriate. RESULTS: Twenty-three of 95 (24.2%) children (44% female; mean age, 10.7 [SD, 6.7] years) developed radiation necrosis after radiation therapy (19 infratentorial, 1 supratentorial, 3 both). The mean pretreatment supratentorial WM magnetization transfer ratio was significantly lower in these children (43.18 versus 43.50, P = .03). There was no association between the supratentorial WM magnetization transfer ratio and age, sex, risk/treatment stratum, or the presence of visible brain metastases. CONCLUSIONS: A lower baseline supratentorial WM magnetization transfer ratio may indicate underlying structural WM susceptibility to radiation necrosis and may identify children at risk for developing radiation necrosis after craniospinal irradiation for medulloblastoma.

Automatic Spinal Cord Gray Matter Quantification: A Novel Approach.

BACKGROUND AND PURPOSE: Currently, accurate and reproducible spinal cord GM segmentation remains challenging and a noninvasive broadly accepted reference standard for spinal cord GM measurements is still a matter of ongoing discussion. Our aim was to assess the reproducibility and accuracy of cervical spinal cord GM and WM cross-sectional area measurements using averaged magnetization inversion recovery acquisitions images and a fully-automatic postprocessing segmentation algorithm. MATERIALS AND METHODS: The cervical spinal cord of 24 healthy subjects (14 women; mean age, 40 ± 11 years) was scanned in a test-retest fashion on a 3T MR imaging system. Twelve axial averaged magnetization inversion recovery acquisitions slices were acquired over a 48-mm cord segment. GM and WM were both manually segmented by 2 experienced readers and compared with an automatic variational segmentation algorithm with a shape prior modified for 3D data with a slice similarity prior. Precision and accuracy of the automatic method were evaluated using coefficients of variation and Dice similarity coefficients. RESULTS: The mean GM area was 17.20 ± 2.28 mm2 and the mean WM area was 72.71 ± 7.55 mm2 using the automatic method. Reproducibility was high for both methods, while being better for the automatic approach (all mean automatic coefficients of variation, ≤4.77%; all differences, P < .001). The accuracy of the automatic method compared with the manual reference standard was excellent (mean Dice similarity coefficients: 0.86 ± 0.04 for GM and 0.90 ± 0.03 for WM). The automatic approach demonstrated similar coefficients of variation between intra- and intersession reproducibility as well as among all acquired spinal cord slices. CONCLUSIONS: Our novel approach including the averaged magnetization inversion recovery acquisitions sequence and a fully-automated postprocessing segmentation algorithm demonstrated an accurate and reproducible spinal cord GM and WM segmentation. This pipeline is promising for both the exploration of longitudinal structural GM changes and application in clinical settings in disorders affecting the spinal cord.

Feasibility of Flat Panel Detector CT in Perfusion Assessment of Brain Arteriovenous Malformations: Initial Clinical Experience.

The different results from flat panel detector CT in various pathologies have provoked some discussion. Our aim was to assess the role of flat panel detector CT in brain arteriovenous malformations, which has not yet been assessed. Five patients with brain arteriovenous malformations were studied with flat panel detector CT, DSC-MR imaging, and vessel-encoded pseudocontinuous arterial spin-labeling. In glomerular brain arteriovenous malformations, perfusion was highest next to the brain arteriovenous malformation with decreasing values with increasing distance from the lesion. An inverse tendency was observed in the proliferative brain arteriovenous malformation. Flat panel detector CT, originally thought to measure blood volume, correlated more closely with arterial spin-labeling-CBF and DSC-CBF than with DSC-CBV. We conclude that flat panel detector CT perfusion depends on the time point chosen for data collection, which is triggered too early in these patients (ie, when contrast agent appears in the superior sagittal sinus after rapid shunting through the brain arteriovenous malformation). This finding, in combination with high data variability, makes flat panel detector CT inappropriate for perfusion assessment in brain arteriovenous malformations.

Optimization of Scan Time in MRI for Total Hip Prostheses: SEMAC Tailoring for Prosthetic Implants Containing Different Types of Metals.

INTRODUCTION: Magnetic resonance imaging (MRI) of soft tissues after total hip arthroplasty is of clinical interest for the diagnosis of various pathologies that are usually invisible with other imaging modalities. As a result, considerable effort has been put into the development of metal artifact reduction MRI strategies, such as slice encoding for metal artifact correction (SEMAC). Generally, the degree of metal artifact reduction with SEMAC directly relates to the overall time spent for acquisition, but there is no specific consensus about the most efficient sequence setup depending on the implant material. The aim of this article is to suggest material-tailored SEMAC protocol settings. MATERIALS AND METHODS: Five of the most common total hip prostheses (1. Revision prosthesis (S-Rom), 2. Titanium alloy, 3. Müller type (CoNiCRMo alloy), 4. Old Charnley prosthesis (Exeter/Stryker), 5. MS-30 stem (stainless-steel)) were scanned on a 1.5 T MRI clinical scanner with a SEMAC sequence with a range of artifact-resolving slice encoding steps (SES: 2-23) along the slice direction (yielding a total variable scan time ranging from 1 to 10 min). The reduction of the artifact volume in comparison with maximal artifact suppression was evaluated both quantitatively and qualitatively in order to establish a recommended number of steps for each case. RESULTS: The number of SES that reduced the artifact volume below approximately 300 mm(3) ranged from 3 to 13, depending on the material. Our results showed that although 3 SES steps can be sufficient for artifact reduction for titanium prostheses, at least 11 SES should be used for prostheses made of materials such as certain alloys of stainless steel. CONCLUSION: Tailoring SES to the implant material and to the desired degree of metal artifact reduction represents a simple tool for workflow optimization of SEMAC imaging near total hip arthroplasty in a clinical setting. KEY POINTS: Five of the most common total hip prostheses have been investigated in vitro. Tailored SEMAC protocols - in terms of scan duration - have been determined. Tailoring was similar for T1-weighted and inversion recovery SEMAC MRI. The suggested prosthesis-related SEMAC adaptation shortens clinical scan times.

Characterization of the collagen component of cartilage repair tissue of the talus with quantitative MRI: comparison of T2 relaxation time measurements with a diffusion-weighted double-echo steady-state sequence (dwDESS).

OBJECTIVES: The purpose of this study was to characterize the collagen component of repair tissue (RT) of the talus after autologous matrix-induced chondrogenesis (AMIC) using quantitative T2 and diffusion-weighted imaging. METHODS: Mean T2 values and diffusion coefficients of AMIC-RT and normal cartilage of the talus of 25 patients with posttraumatic osteochondral lesions and AMIC repair were compared in a cross-sectional design using partially spoiled steady-state free precession (pSSFP) for T2 quantification, and diffusion-weighted double-echo steady-state (dwDESS) for diffusion measurement. RT and cartilage were graded with modified Noyes and MOCART scores on morphological sequences. An association between follow-up interval and quantitative MRI measures was assessed using multivariate regression, after stratifying the cohort according to time interval between surgery and MRI. RESULTS: Mean T2 of the AMIC-RT and cartilage were 43.1 ms and 39.1 ms, respectively (p = 0.26). Mean diffusivity of the RT (1.76 µm(2)/ms) was significantly higher compared to normal cartilage (1.46 µm(2)/ms) (p = 0.0092). No correlation was found between morphological and quantitative parameters. RT diffusivity was lowest in the subgroup with follow-up >28 months (p = 0.027). CONCLUSIONS: Compared to T2-mapping, dwDESS demonstrated greater sensitivity in detecting differences in the collagen matrix between AMIC-RT and cartilage. Decreased diffusivity in patients with longer follow-up times may indicate an increased matrix organization of RT. KEY POINTS: • MRI is used to assess morphology of the repair tissue during follow-up. • Quantitative MRI allows an estimation of biochemical properties of the repair tissue. • Differences between repair tissue and cartilage were more significant with dwDESS than T2 mapping.

Attenuation of blood flow pulsatility along the Atlas slope: a physiologic property of the distal vertebral artery?

BACKGROUND AND PURPOSE: Physiologic and pathologic arterial tortuosity may attenuate blood flow pulsatility. The aim of this prospective study was to assess a potential effect of the curved V3 segment (Atlas slope) of the vertebral artery on arterial flow pulsatility. The pulsatility index and resistance index were used to assess blood flow pulsatility. MATERIALS AND METHODS: Twenty-one healthy volunteers (17 men, 4 women; mean age, 32 years) were examined with a 3T MR imaging system. Blood velocities were measured at 2 locations below (I and II) and at 1 location above the V3 segment (III) of the vertebral artery by using a high-resolution 2D-phase-contrast sequence with multidirectional velocity-encoding. RESULTS: Pulsatility and resistance indices decreased along all measurement locations from proximal to distal. The pulsatility index decreased significantly from location II to III and from I to II. However, the decrease was more pronounced along the Atlas slope than in the straight-vessel section below. The decrease of the resistance index was highly significant along the Atlas slope (location II to III). The decrease from location I to II was small and not significant. CONCLUSIONS: The pronounced decrease in pulsatility and resistance indices along the interindividually uniformly bent V3 segment compared with a straight segment of the vertebral artery indicates a physiologic attenuating effect of the Atlas slope on arterial flow pulsatility. A similar effect has been described for the carotid siphon. A physiologic reduction of pulsatility in brain-supplying arteries would be in accordance with several recent publications reporting a correlation of increased arterial flow pulsatility with leukoencephalopathy and lacunar stroke.

Simultaneous T1 and T2 quantification of the myocardium using cardiac balanced-SSFP inversion recovery with interleaved sampling acquisition (CABIRIA).

PURPOSE: To develop a novel sequence for simultaneous quantification of T1 and T2 relaxation times in the myocardium based on the transient phase of the balanced steady-state free precession. METHODS: A new prototype sequence, named "cardiac balanced-SSFP inversion recovery with interleaved sampling acquisition" (CABIRIA) was developed based on a single-shot bSSFP readout following an inversion pulse. With this method, T1 and T2 values can be calculated from the analysis of signal evolution. The scan duration for a single slice in vivo was 8 heartbeats, thus feasible in a breath-hold. The sequence was validated both in vitro by comparing it to conventional inversion recovery and multi-echo spin-echo methods and in 5 healthy volunteers by comparing it to the Modified Look-Locker Inversion Recovery (MOLLI) sequence and to a T2 quantification sequence based on multi-T2 -prepared bSSFP. RESULTS: The method showed good agreement with conventional methods for both T1 and T2 measurements (concordance correlation coefficient ≥ 0.99) in vitro. In healthy volunteers the measured T1 values were 1227 ± 68 ms and T2 values 37.9 ± 2.4 ms, with similar inter- and intrasubject variability with respect to existing methods. CONCLUSION: The proposed CABIRIA method enables simultaneous quantification of myocardial T1 and T2 values with good accuracy and precision.

In-vivo assessment of normal T1 values of the right-ventricular myocardium by cardiac MRI.

To test feasibility of myocardial T1 mapping of the right ventricle (RV) at systole when myocardium is more compact and to determine the most appropriate imaging plane. 20 healthy volunteers (11 men; 33 ± 8 years) were imaged on a 1.5T scanner (MAGNETOM Avanto, Siemens AG, Erlangen, Germany). A modified look-locker inversion-recovery sequence was acquired at mid-ventricular short axis (SAX), as horizontal long-axis view and as transversal view at systole (mean trigger time 363 ± 37 ms). Myocardial T1 time of the left-ventricular and RV myocardium was measured within a region of interest (ROI) on generated T1-maps. The most appropriate imaging plane for the RV was determined by the ability to draw a ROI including the largest amount of myocardium without including adjacent tissue or blood. At systole, when myocardium is thicker, measurements of the RV myocardium were feasible in 18/20 subjects. Average size of the ROI was 0.42 ± 0.28 cm(2). In 10/18 subjects, short axis was the most appropriate imaging plane to obtain measurements (p = 0.034). Average T1 time of the RV myocardium was 1,016 ± 61 ms, and average T1 of the left-ventricular (LV) was 956 ± 25 ms (p < 0.001). T1 mapping of the RV myocardium is feasible during systole in the majority of healthy subjects but with a small ROI only. SAX plane was the optimal imaging plane in the majority of subjects. Native myocardial T1 time of the RV is significantly longer compared to the LV, which might be explained by the naturally higher collagen content of the RV.

Delayed gadolinium-enhanced MRI of cartilage of the ankle joint: results after autologous matrix-induced chondrogenesis (AMIC)-aided reconstruction of osteochondral lesions of the talus.

AIM: To assess cartilage quality using delayed gadolinium-enhanced magnetic resonance imaging after repair of osteochondral lesions of the talus using autologous matrix-induced chondrogenesis (AMIC). MATERIALS AND METHODS: A three-dimensional (3D) spoiled gradient-echo (SGE) sequence at 3 T was used to obtain quantitative T1 relaxation times before and after Gd-DTPA2 (Magnevist, 0.2 mM/kg bod weight) administration to assess 23 cases of AMIC-aided repair of osteochondral lesions of the talus. Delta relaxation rates (ΔR1) for reference cartilage (RC) and repair tissue (RT), and the relative delta relaxation rate (rΔR1) were calculated. The morphological appearance of the cartilage RT was graded on sagittal dual-echo steady-state (DESS) views according to the "magnetic resonance observation of cartilage repair tissue" (MOCART) protocol. The study was approved by the institutional review board and written consent from each patient was obtained. RESULTS: The AMIC cases had a mean T1 relaxation time of 1.194 s (SD 0.207 s) in RC and 1.470 s (SD 0.384 s) in RT before contrast medium administration. The contrast-enhanced T1 relaxation time decreased to 0.480 s (SD 0.114 s) in RC and 0.411 s (SD 0.096 s) in RT. There was a significant difference (p > 0.05) between the ΔR1 in RC (1.372 × 10(-3)/s, range 0.526-3.201 × 10(-3)/s, SD 0.666 × 10(-3)/s) and RT (1.856 × 10(-3)/s, range 0.93-3.336 × 10(-3)/s, SD 0.609 × 10(-3)/s). The mean rΔR1 was 1.49, SD 0.45). The mean MOCART score at follow-up was 62.6 points (range 30-95, SD 15.3). CONCLUSION: The results of the present study suggest that repair cartilage resulting from AMIC-aided repair of osteochondral lesions of the talus has a significantly lower glycosaminoglycan (GAG) content than normal hyaline cartilage, but can be regarded as having hyaline-like properties.

High-resolution Fourier-encoded sub-millisecond echo time musculoskeletal imaging at 3 Tesla and 7 Tesla.

PURPOSE: The feasibility of imaging musculoskeletal fibrous tissue components, such as menisci, ligaments, and tendons, with a conventional spoiled gradient echo technique is explored in vivo at 3 T and 7 T. METHODS: To this end, the echo time (TE1 ) of a conventional Fourier-encoded multicontrast three-dimensional SGPR sequence is minimized by using nonselective excitation pulses, highly asymmetric readouts, and a variable TE1 along the phase and slice encoding direction. In addition, a fully sampled second echo image (with TE2 >> TE1 ) can be used to highlight components with short transverse relaxation times in a difference image with positive contrast. RESULTS: Fourier-encoded spoiled gradient echo sequences are able to provide sub-millisecond TE1 of about 800 µs for typical in-plane resolutions of about 0.5 x 0.5 mm(2) . As a result, high-resolution positive contrast images of fibrous tissues can be generated within clinically feasible scan-time of about 2-7 minutes. CONCLUSION: After optimization, Fourier-encoded spoiled gradient echo provides a highly robust and flexible imaging technique for high-resolution positive contrast imaging of fibrous tissue that can readily be used in the clinical routine.

Fast high-resolution brain imaging with balanced SSFP: Interpretation of quantitative magnetization transfer towards simple MTR.

Magnetization transfer (MT) reflects the exchange of magnetization between protons bound to macromolecules, such as lipids and proteins, and protons in free liquid, and thus might be an early marker for subtle and undetermined pathologic changes in tissue. Detailed analysis of the entire MT phenomenon, however, commonly requires extensive data acquisition and scanning time, and hence is only of limited clinical interest. Therefore, in practice, magnetization transfer effects are commonly confined into a simple ratio measure, the so-called magnetization transfer ratio (MTR), calculated from a MT-weighted and a non-MT-weighted image. However, subtle physiologic and pathologic changes in tissue, invaluable for specific diagnostic imaging, may be lost since MTR-values depend not only on quantitative magnetization transfer (qMT) parameters but also on sequence parameters and relaxation properties. In order to evaluate and assess the diagnostic specificity of MTR versus qMT, high-resolution whole brain MT data was collected from twelve healthy volunteers using balanced steady-state free precession (bSSFP). In contrast to common MT imaging based on spoiled gradient echo (SPGR) sequences, whole brain qMT imaging can be performed with MT-sensitized bSSFP within a clinically feasible acquisition time. Hence, MT-sensitized bSSFP provides access to both MTR and qMT parameters within a clinical setting. The reliability and possible diagnostic value of MTR are analyzed for twelve white matter (WM) and eleven gray matter (GM) structures of the normal appearing brain. Strong correlations were found within and between longitudinal and transverse relaxation times (T1, T2) and MT parameters (ratio between macromolecular and water protons, F, and magnetization exchange rate, kf), whereas weaker correlations were observed between MTR-values and relaxation times or MT parameters. Structures with highly similar MTR-values, such as the crus cerebri and the anterior commissure in the WM, or the pallidum and the amygdala in the GM, however, were also found that showed significant differences in most quantitative parameters. This observation was confirmed from simulations revealing that the overall effect on MTR from an increase (decrease) in relaxation times may be counterbalanced with a decrease (increase) in MT parameters. These findings corroborate the expectation that qMT is superior to MTR imaging, especially for the evaluation and assessment of pathologic or physiological changes in healthy and pathologic brain tissue.

Fast diffusion-weighted steady state free precession imaging of in vivo knee cartilage.

Quantification of molecular diffusion with steady state free precession (SSFP) is complicated by the fact that diffusion effects accumulate over several repetition times (TR) leading to complex signal dependencies on transverse and longitudinal magnetization paths. This issue is commonly addressed by setting TR > T(2), yielding strong attenuation of all higher modes, except of the shortest ones. As a result, signal attenuation from diffusion becomes T(2) independent but signal-to-noise ratio (SNR) and sequence efficiency are remarkably poor. In this work, we present a new approach for fast in vivo steady state free precession diffusion-weighted imaging of cartilage with TR << T(2) offering a considerable increase in signal-to-noise ratio and sequence efficiency. At a first glance, prominent coupling between magnetization paths seems to complicate quantification issues in this limit, however, it is observed that diffusion effects become rather T(2) (ΔD ≈ 1/10 ΔT(2)) but not T(1) independent (ΔD ≈ 1/2 ΔT(1)) for low flip angles α ≈ 10 - 15°. As a result, fast high-resolution (0.35 × 0.35 - 0.50 × 0.50 mm(2) in-plane resolution) quantitative diffusion-weighted imaging of human articular cartilage is demonstrated at 3.0 T in a clinical setup using estimated T(1) and T(2) or a combination of measured T(1) and estimated T(2) values.

Intrascanner and interscanner variability of magnetization transfer-sensitized balanced steady-state free precession imaging.

Recently, a new and fast three-dimensional imaging technique for magnetization transfer ratio (MTR) imaging has been proposed based on a balanced steady-state free precession protocol with modified radiofrequency pulses. In this study, optimal balanced steady-state free precession MTR protocol parameters were derived for maximum stability and reproducibility. Variability between scans was assessed within white and gray matter for nine healthy volunteers using two different 1.5 T clinical systems at six different sites. Intrascanner and interscanner MTR measurements were well reproducible (coefficient of variation: c(v) < 0.012 and c(v) < 0.015, respectively) and results indicate a high stability across sites (c(v) < 0.017) for optimal flip angle settings. This study demonstrates that balanced steady-state free precession MTR not only benefits from short acquisition time and high signal-to-noise ratio but also offers excellent reproducibility and low variability, and it is thus proposed for clinical MTR scans at individual sites as well as for multicenter studies.

Finite RF pulse correction on DESPOT2.

Magnetization transfer and finite radiofrequency (RF) pulses affect the steady state of balanced steady state free precession. As quantification of transverse relaxation (T2) with driven equilibrium single pulse observation of T2 is based on two balanced steady state free precession acquisitions, both effects can influence the outcome of this method: a short RF pulse per repetition time (TRF/TR≪1) leads to considerable magnetization transfer effects, whereas prolonged RF pulses (TRF/TR>0.2) minimize magnetization transfer effects, but lead to increased finite pulse effects. A correction for finite pulse effects is thus implemented in the driven equilibrium single pulse observation of T2 theory to compensate for reduced transverse relaxation effects during excitation. It is shown that the correction successfully removes the driven equilibrium single pulse observation of T2 dependency on the RF pulse duration. A reduction of the variation in obtained T2 from over 50% to less than 10% is achieved. We hereby provide a means of acquiring magnetization transfer-free balanced steady state free precession images to yield accurate T2 values using elongated RF pulses.

Characterization of normal appearing brain structures using high-resolution quantitative magnetization transfer steady-state free precession imaging.

Compared to standard spoiled gradient echo (SPGR)-methods, balanced steady-state free precession (bSSFP) provides quantitative magnetization transfer (qMT) images with increased resolution and high signal-to-noise ratio (SNR) in clinically feasible acquisition times. The aim of this study was to acquire 3D high-resolution qMT-data to create standardized qMT-values of many single brain structures that might serve as a baseline for the future characterization of pathologies of the brain. QMT parameters, such as the fractional pool size (F), exchange rate (kf) and relaxation times of the free pool (T1, T2) were assessed in a total of 12 white matter (WM) and 11 grey matter (GM) structures in 12 healthy volunteers with MT-sensitized bSSFP. Our results were compared with qMT-data from previous studies obtained with SPGR-methods using MT-sensitizing preparation pulses with significantly lower resolution. In general, qMT-values were in good accordance with prior studies. As expected, higher F and kf and lower relaxation times were observed in WM as compared to GM structures. However, many significant differences were observed within WM and GM regions and also between different regions of the same structure like in the internal capsule where the posterior limb showed significant higher kf than the anterior limb. Significant differences for all parameters were observed between subjects. In contrast to previous studies, bSSFP allowed assessment of even small brain structures due to its high resolution. The observed differences from previous studies can partly be explained by the reduced partial volume effects. MT-sensitized bSSFP is an ideal candidate for qMT-analysis in the clinical routine as it provides high-resolution 3D qMT-data of even small brain structures in clinically feasible acquisition times. The present qMT-data can serve as a reference for the characterization of cerebral diseases.

Automatic slice positioning (ASP) for passive real-time tracking of interventional devices using projection-reconstruction imaging with echo-dephasing (PRIDE).

A novel and fast approach for passive real-time tracking of interventional devices using paramagnetic markers, termed "projection-reconstruction imaging with echo-dephasing" (PRIDE) is presented. PRIDE is based on the acquisition of echo-dephased projections along all three physical axes. Dephasing is preferably set to 4pi within each projection ensuring that background tissues do not contribute to signal formation and thus appear heavily suppressed. However, within the close vicinity of the paramagnetic marker, local gradient fields compensate for the intrinsic dephasing to form an echo. Successful localization of the paramagnetic marker with PRIDE is demonstrated in vitro and in vivo in the presence of different types of off-resonance (air/tissue interfaces, main magnetic field inhomogeneities, etc). In order to utilize the PRIDE sequence for vascular interventional applications, it was interleaved with balanced steady-state free precession (bSSFP) to provide positional updates to the imaged slice using a dedicated real-time feedback link. Active slice positioning (ASP) with PRIDE is demonstrated in vitro, requiring approximately 20 ms for the positional update to the imaging sequence, comparable to existing active tracking methods.

Quantitative magnetization transfer imaging using balanced SSFP.

It is generally accepted that signal formation in balanced steady-state free precession (bSSFP) is a simple function of relaxation times and flip angle only. This can be confirmed for fluids, but for more complex substances, magnetization transfer (MT) can lead to a considerable loss of steady-state signal. Thus, especially in tissues, the analytical description of bSSFP requires a revision to fully take observed effects into account. In the first part of this work, an extended bSSFP signal equation is derived based on a binary spin-bath model. Based on this new model of bSSFP signal formation, quantitative MT parameters such as the fractional pool size, corresponding magnetization exchange rates, and relaxation times can be explored. In the second part of this work, model parameters are derived in normal appearing human brain. Factors that may influence the quality of the model, such as B(1) field inhomogeneities or off-resonances, are discussed. Overall, good correspondence between parameters derived from two-pool bSSFP and common quantitative MT models is observed. Short repetition times in combination with high signal-to-noise ratios make bSSFP an ideal candidate for the acquisition of high resolution isotropic quantitative MT maps, as for the human brain, within clinically feasible acquisition times.

Optimized spectrally selective steady-state free precession sequences for cartilage imaging at ultra-high fields.

OBJECT: Fat suppressed 3D steady-state free precession (SSFP) sequences are of special interest in cartilage imaging due to their short repetition time in combination with high signal-to-noise ratio. At low-to-high fields (1.5-3.0 T), spectral spatial (spsp) radio frequency (RF) pulses perform superiorly over conventional saturation of the fat signal (FATSAT pulses). However, ultra-high fields (7.0 T and more) may offer alternative fat suppression techniques as a result of the increased chemical shift. MATERIALS AND METHODS: Application of a single, frequency selective, RF pulse is compared to spsp excitation for water (or fat) selective imaging at 7.0 T. RESULTS: For SSFP, application of a single frequency selective RF pulse for selective water or fat excitation performs beneficially over the commonly applied spsp RF pulses. In addition to the overall improved fat suppression, the application of single RF pulses leads to decreased power depositions, still representing one of the major restrictions in the design and application of many pulse sequences at ultra-high fields. CONCLUSION: The ease of applicability and implementation of single frequency selective RF pulses at ultra-high-fields might be of great benefit for a vast number of applications where fat suppression is desirable or fat-water separation is needed for quantification purposes.

Morphing steady-state free precession.

A novel concept for visualization of positive contrast originating from susceptibility-related magnetic field distortions is presented. In unbalanced steady-state free precession (SSFP) the generic, gradient-induced dephasing competes with local gradient fields generated by paramagnetic materials. Thus, within the same image, SSFP may morph its own appearance from unbalanced to balanced SSFP (bSSFP) as a result of local gradient compensation. In combination with low to very low flip angles, unbalanced SSFP signals are heavily suppressed, whereas bSSFP locally produces very high steady-state amplitudes at certain frequency offsets. As a result, bSSFP signals appear hyperintense on an almost completely dark background. In this study, the conceptual issues of local gradient compensation and frequency matching, as well as the feasibility of proper detection of marker materials for interventional MRI from hyperintense pixels locations, are evaluated both in vitro and in vivo. Signal dependencies of morphing SSFP on sequence parameters such as flip angle or repetition time are investigated theoretically and experimentally. In addition to passive tracking of interventional devices, morphing SSFP might also be a promising new concept for the generation of positive contrast from super-paramagnetic iron oxide (SPIO) particles in contrast-enhanced MRI as well as for particle tracking.