Fast MRI Research Articles

Variable radio frequency proton–electron double-resonance imaging: Application to pH mapping of aqueous samples

Proton–electron double-resonance imaging (PEDRI) offers rapid image data collection and high resolution for spatial distribution of paramagnetic probes. Recently we developed the concept of variable field (VF) PEDRI which enables extracting a functional map from a limited number of images acquired at pre-selected EPR excitation fields using specific paramagnetic probes (Khramtsov et al., J. Magn. Reson. 202 (2010) 267–273). In this work, we propose and evaluate a new modality of PEDRI-based functional imaging with enhanced temporal resolution which we term variable radio frequency (VRF) PEDRI. The approach allows for functional mapping (e.g., pH mapping) using specifically designed paramagnetic probes with high quality spatial resolution and short acquisition times. This approach uses a stationary magnetic field but different EPR RFs. The ratio of Overhauser enhancements measured at each pixel at two different excitation frequencies corresponding to the resonances of protonated and deprotonated forms of a pH-sensitive nitroxide is converted to a pH map using a corresponding calibration curve. Elimination of field cycling decreased the acquisition time by exclusion periods of ramping and stabilization of the magnetic field. Improved magnetic field homogeneity and stability allowed for the fast MRI acquisition modalities such as fast spin echo. In total, about 30-fold decrease in EPR irradiation time was achieved for VRF PEDRI (2.4 s) compared with VF PEDRI (70 s). This is particularly important for in vivo applications enabling one to overcome the limiting stability of paramagnetic probes and sample overheating by reducing RF power deposition.

Relating function to structure: fMRI in (transgenic) mice and the Waxholm Space Reference System

Relating function to structure: fMRI in (transgenic) mice and the Waxholm Space Reference System

Myocardial Delayed Enhancement in Pulmonary Hypertension: Pulmonary Hemodynamics, Right Ventricular Function, and Remodeling

The purpose of this study was to assess predictors of MRI-identified septal delayed enhancement mass at the right ventricular (RV) insertion sites in relation to RV remodeling, altered regional mechanics, and pulmonary hemodynamics in patients with suspected pulmonary hypertension (PH). Thirty-eight patients with suspected PH were evaluated with right heart catheterization and cardiac MRI. Ten age- and sex-matched healthy volunteers acted as controls for MRI comparison. Septal delayed enhancement mass was quantified at the RV insertions. Systolic septal eccentricity index, global RV function, and remodeling indexes were quantified with cine images. Peak systolic circumferential and longitudinal strain at the sites corresponding to delayed enhancement were measured with conventional tagging and fast strain-encoded MRI acquisition, respectively. PH was diagnosed in 32 patients. Delayed enhancement was found in 31 of 32 patients with PH and in one of six patients in whom PH was suspected but proved absent (p = 0.001). No delayed enhancement was found in controls. Delayed enhancement mass correlated with pulmonary hemodynamics, reduced RV function, increased RV remodeling indexes, and reduced eccentricity index. Multiple linear regression analysis showed RV mass index was an independent predictor of total delayed enhancement mass (p = 0.017). Regional analysis showed delayed enhancement mass was associated with reduced longitudinal strain at the basal anterior septal insertion (r = 0.6, p < 0.01). Regression analysis showed that basal longitudinal strain remained an independent predictor of delayed enhancement mass at the basal anterior septal insertion (p = 0.02). In PH, total delayed enhancement burden at the RV septal insertions is predicted by RV remodeling in response to increased afterload. Local fibrosis mass at the anterior septal insertion is associated with reduced regional longitudinal contractility at the base.

A Daily Cone Beam CT Pre and Post Treatment Study for Prostate Cancer RT: Relation between Treatment Time and Dose Uncertainty

Results: All patients consented to the Phase II study had respiratory motion evaluated for 10-20 seconds on lateral and anterior fluoroscopy using soft-tissue or a biliary stent as a surrogate for the pancreas tumor. Five hundred ninety-one at-treatment frames were assessed for 6 treatment beams over 73 fractions (mean 24 fractions/patient) acquired at 2 frames/second. Beams were evaluated if it was possible to (1) visualize the surrogate and (2) acquire at minimum 4 image frames. In 12% of fractions, degraded image quality, due to abdominal gas, prevented clear visualization of surrogates on $1 beam. The mean (range) surrogate motion seen at planning was 1.2 cm (0.9, 1.8), and 0.7 cm (0.5, 0.8) in the CC and AP directions. RL motion of 0.3cm was evaluable in only one patient. The mean (range) at-treatment motion was 0.4 cm (0.2, 0.8), 0.2 cm (0.1, 0.3) and 0.2 cm (0.1, 0.4) in the CC , AP and RL directions, respectively, based on MV cine exit images. Intra-patient inter-fraction motion varied minimally with SDs \2 mm in all patients for all directions. This strategy continues to be used clinically. Conclusions: In this study, fluoroscopic soft-tissue and stent motion seen at treatment planning over-estimated at-treatment respiratory motion in all patients seen on MV exit beams. This preliminary work corresponds well with reported observations from fiducial marker studies. However further studies are required comparing surrogate motion to volumetric pancreas motion from 4DCT or fast MRI and respiratory correlated cone beam CT in larger patient cohorts. In the absence of volumetric motion imaging, fluoroscopy and cine-MV exit imaging, is an inexpensive, non-invasive method that may suffice to estimate/confirm respiratory motion, and may be suitable for quality control in multi-centre trials.

Fast multislice pH‐weighted chemical exchange saturation transfer (CEST) MRI with Unevenly segmented RF irradiation

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging technique for measuring microenvironment properties via dilute CEST labile groups. Conventionally, CEST MRI is implemented with a long radiofrequency irradiation module, followed by fast image acquisition to obtain the steady state CEST contrast. Nevertheless, the sensitivity, scan time, and spatial coverage of the conventional CEST MRI method may not be optimal. Our study proposed a segmented radiofrequency labeling scheme that includes a long primary radiofrequency irradiation module to generate the steady state CEST contrast and repetitive short secondary radiofrequency irradiation module immediately after the image acquisition so as to maintain the steady state CEST contrast for multislice acquisition and signal averaging. The proposed CEST MRI method was validated experimentally with a tissue-like pH phantom and optimized for the maximal contrast-to-noise ratio. In addition, the proposed sequence was evaluated for imaging ischemic acidosis via pH-weighted endogenous amide proton transfer MRI, which showed similar contrast as conventional amide proton transfer MRI. In sum, a fast multislice relaxation self-compensated CEST MRI sequence was developed, with significantly improved sensitivity and suitable for in vivo applications.

Diffuse liver diseases in neonatal and pediatric liver transplant candidates: a pictorial essay

A wide spectrum of common and uncommon diffuse liver diseases affecting neonatal and pediatric liver transplant candidates is presented and analyzed using 16 and 64 multi-detector row helical CT (MDCT) and 1.5 T MRI fast imaging. Correlation of imaging findings and explanted liver or histology is illustrated in representative cases. Associated uncommon congenital anomalies are shown. In conclusion, in pediatric liver transplant candidates, 16-MDCT and 1.5 T fast MRI are useful for diagnosis and staging of liver disease, as well as for the evaluation of associated congenital anomalies.

Abstract #4: Global vs. Regional Spinal Cord Atrophy in Multiple Sclerosis

Objective To assess whole and regional spinal cord atrophy in patients with multiple sclerosis (MS) using 3T MRI. Background Spinal cord damage is common in MS and provides a unique and clinically relevant means to monitor disease progression and therapeutic effects. Whereas regional spinal cord atrophy has been studied extensively in MS, whole spinal cord atrophy has not been previously assessed. Design/Methods Whole spinal cord 3 mm thick T2-weighted axial fast spin-echo MRI images were obtained at 3T on 34 patients with MS [26 women and 8 men; 26 relapsing–remitting, 2 clinically isolated syndrome, 4 secondary progressive, 2 primary progressive; Expanded Disability Status Scale (EDSS) (mean ± SD) 1.95 ± 1.72, range 0–6.5] and 15 age-matched normal controls [11 women and 4 men]. The global and regional (cervical, C2–3, and thoracic) cord volumes were determined using a semi-automated tool and were normalized by the segment length and intracranial volume. Spinal cord lesions were also determined using a semi-automated edge finding tool. Results Whole cord, C2–3, cervical, and thoracic volumes were lower in progressive vs. relapsing patients or normal controls (all p Conclusions The established method of estimating spinal cord atrophy, C2–3 cross-sectional volume, provides a useful surrogate of overall global or regional spinal cord atrophy. Spinal cord atrophy is most common in progressive forms of MS and is related to neurologic disability. Support This study was supported by grants from the National Institutes of Health ( 1R01NS055083-01 ) and the National Multiple Sclerosis Society ( RG3705A1 ; RG3798A2 ).

MRI of bone tumors: advances in diagnosis and treatment assessment

MRI is a unique imaging technique that allows direct visualization of bone marrow with high spatial resolution and is considered the best tool for local staging of bone tumors. Owing to its high natural contrast resolution, it can depict specific tissue components useful for tumor characterization. Moreover, there is great interest in MRI capability to assess post-chemotherapy changes in order to identify unresponsive tumors early in the course of therapy. To increase the accuracy in the assessment of tumor response to treatment, new and more sophisticated approaches using fast and ultrafast MRI are under evaluation. This article presents an overview of the state-of-the-art of MRI in staging bone lesions, discusses the role that this technique plays in tumor characterization and provides a quick look at future technological advancements.

Minor head injury in children

This review will examine mild closed head injury (CHI) and the current evidence on head computed tomography (CT) imaging risks in children, prediction rules to guide decisions on CT scan use, and issues of concussion after initial evaluation. The current literature offers preliminary evidence on the risks of radiation exposure from CT scans in children. A recent study introduces a validated prediction rule for use in mild CHI, to limit the number of CT scans performed. Concurrent with this progress, fast (or short sequence) MRI represents an emerging technology that may prove to be a viable alternative to CT scan use in certain cases of mild CHI where imaging is desired. The initial emergency department evaluation for mild CHI is the start point for a sequence of follow-up to assure that postconcussive symptoms fully resolve. The literature on sports-related concussion offers some information that may be used for patients with non-sports-related concussion. It is clear that CT scan use should be as safe and limited in scope as possible for children. Common decisions on the use of CT imaging for mild head injury can now be guided by a prediction rule for clinically important traumatic brain injury. Parameters for the follow-up care of patients with mild CHI after emergency department discharge are needed in the future to assure that postconcussive symptoms are adequately screened for full resolution.

A method for measuring the cross sectional area of the anterior portion of the optic nerve in vivo using a fast 3D MRI sequence

To investigate the three-dimensional (3D) fast-recovery fast spin-echo accelerated (FRFSE-XL) sequence as a new application for measuring the intraorbital optic nerve (ION) mean cross-sectional area in vivo and to determine its value within a commonly used high resolution imaging protocol. The entire ION was scanned in nine healthy volunteers (mean age 32 +/- 4 years) using the 3D FRFSE-XL sequence and a commonly used high resolution short-echo fast fluid-attenuated inversion-recovery (sTE fFLAIR) sequence with identical slice locations at 1.5T. The mean cross-sectional area from both sequences was measured on a slice-by-slice basis from 3 mm behind the globe to the orbital apex. The reproducibility of both techniques was assessed from repeated scans (scan-rescan) and repeated image analysis (intraobserver). Measurement of the mean cross-sectional area of the anterior 9 mm segment of the ION was only possible using the 3D FRFSE-XL sequence with a mean area of 11.6 +/- 2.2 mm(2) (scan rescan COV = 3.3 +/- 1.5, intraobserver COV = 2.4 +/- 0.02) whereas the remainder segment of the ION (i.e., 9 mm behind the globe to the orbital apex) could only be measured with the use of the sTE fFLAIR with a mean area of 8.5 +/- 1.7 mm(2) (scan rescan COV = 4.9 +/- 2.5 and intraobserver COV = 3.70 +/- 0.03). The 3D FRFSE-XL allows fast and reproducible measurement of the cross-sectional area of the anterior 9 mm segment of the ION, which is not possible using commonly used imaging sequences due to image degradation from motion, and is of complementary value to the existing imaging protocol for ION atrophy quantification.

Field‐cycling NMR relaxometry with spatial selection

Fast field-cycling MRI offers access to sources of endogenous information not available from conventional fixed-field imagers. One example is the T(1) dispersion curve: a plot of T(1) versus field strength. We present a pulse sequence that combines saturation-recovery/inversion-recovery T(1) determination with field cycling and point-resolved spectroscopy localization, enabling the measurement of dispersion curves from volumes selected from a pilot image. Compared with a nonselective sequence, our method of volume selection does not influence measurement accuracy, even for relatively long echo times and in the presence of radiofrequency field nonuniformity. The measured voxel profile, while not ideal, corresponds with that expected from the image slice profile. On a whole-body fast field-cycling scanner with 59-mT detection, the sensitivity of the experiment is sufficient to reveal distinctive "quadrupole dips" in dispersion curves of protein-rich human tissue in vivo.

Morpho-functional patterns of physiologic oropharyngeal swallowing evaluated with dynamic fast MRI

With the advent of dynamic fast MRI sequences the act of deglutition can be dynamically visualized in cine-mode. Twenty-three healthy volunteers were enrolled in this study to define the morpho-functional patterns of oral and pharyngeal deglutition using new dynamic MRI techniques. All subjects were previously submitted to video endoscopic assessment, to exclude swallowing abnormalities. As contrast material a combination of yogurt mixed with gadolinium-diethylene diamine pentaacetic acid was used. The protocol was divided into three parts: (a) preliminary assessment of the oral cavity, pharynx and laryngeal structures; (b) morphologic assessment of tongue, soft palate, pharynx, epiglottis and larynx-hyoid bone; (c) dynamic assessment of swallowing without administrating any contrast media and, in subsequent phase, by injecting 5 ml of yogurt-based contrast medium in the patient's mouth. The time resolution was 3-4 images/s. The MR protocol revealed to be effective in the evaluation of normal motility patterns of the structures involved in swallowing. Moreover, the evaluation of the bolus progression, slowdown or stagnation, was possible. On the contrary problems were encountered in calculating precisely the bolus progression time, because of the insufficient temporal resolution. However, more energy should be invested to optimize the spatial and temporal resolution of turbo-FLASH sequences, to obtain a better dynamic representation of a complex function such as deglutition.

Multi-contrast high spatial resolution black blood inner volume three-dimensional fast spin echo MR imaging in peripheral vein bypass grafts

The purpose of this study is to primarily evaluate the lumen area and secondarily evaluate wall area measurements of in vivo lower extremity peripheral vein bypass grafts patients using high spatial resolution, limited field of view, cardiac gated, black blood inner volume three-dimensional fast spin echo MRI. Fifteen LE-PVBG patients prospectively underwent ultrasound followed by T1-weighted and T2-weighted magnetic resonance (MR) imaging. Lumen and vessel wall areas were measured by direct planimetry. For graft lumen areas, T1- and T2-weighted measurements were compared with ultrasound. For vessel wall areas, differences between T1- and T2-weighted measurements were evaluated. There was no significant difference between ultrasound and MR lumen measurements, reflecting minimal MR blood suppression artifact. Graft wall area measured from T1-weighted images was significantly larger than that measured from T2-weighted images (P < 0.001). The mean of the ratio of T1- versus T2-weighted vessel wall areas was 1.59 (95% CI: 1.48-1.69). The larger wall area measured on T1-weighted images was due to a significantly larger outer vessel wall boundary. Very high spatial resolution LE-PVBG vessel wall MR imaging can be performed in vivo, enabling accurate measurements of lumen and vessel wall areas and discerning differences in those measures between different tissue contrast weightings. Vessel wall area differences suggest that LE-PVBG vessel wall tissues produce distinct signal characteristics under T1 and T2 MR contrast weightings.

Carotid artery visualization during anterior skull base surgery: a novel protocol for neuronavigation

Detailed knowledge of the vascular anatomy of the anterior skull base is critical to successful surgery in this area. Whereas conventional neuronavigational approaches combine MRI (+/- contrast) for tumor visualization and CT (+/- C) for bony and vascular anatomy, we describe the Canadian and Austrian experiences using a novel protocol integrating MR angiography (MRA) into surgical neuronavigation to provide superior visualization of the carotid arteries. The pre-operative imaging protocol employs a T1-weighted, 3D fast spoiled gradient echo MRI (+/- C) for soft tissue anatomy, a plain CT for bony anatomy, and a 3D time-of-flight MR angiography for carotid anatomy. The series are imported into the Medtronic StealthStation((R)) TREON((R)) Treatment Guidance System; during intra-operative neuronavigation, each series (MRI, CT, MRA) can be viewed individually, or layered and viewed as a composite image. Our protocol has important advantages. First, it provides detailed tissue, tumor, vascular and bony anatomy. Second, a contrast CT is not necessary; this is important, as numerous reports have highlighted the nephrotoxic nature of radiographic contrast material. Third, visualization of the carotid system is superior than can be obtained from CT angiography. We use this unique imaging protocol routinely for our endoscopic transsphenoidal surgeries to provide superior visualization of the carotid arteries during anterior skull base surgery.

Ferucarbotran-Enhanced Hepatic MRI at 3T Unit: Quantitative and Qualitative Comparison of Fast Breath-hold Imaging Sequences

Purpose : To compare the relative values of various fast breath-hold imaging sequences for superparamagnetic iron-oxide (SPIO)-enhanced hepatic MRI for the assessment of solid focal lesions with a 3T MRI unit. Materials and Methods : 102 consecutive patients with one or more solid malignant hepatic lesions were evaluated by spoiled gradient echo (GRE) sequences with three different echo times (2.4 msec [GRE_2.4], 5.8 msec [GRE_5.8], and 10 msec [GRE_10]) for T2*-weighted imaging in addition to T2-weighted turbo spin echo (TSE) sequence following intravenous SPIO injection. Image qualities of the hepatic contour, vascular landmarks and artifacts were rated by two independent readers using a four-point scale. For quantitative analysis, contrast-to-noise ratio (CNR) was measured in 170 solid focal lesions larger than 1 cm (107 hepatocellular carcinomas, nine cholangiocarcinomas and 54 metastases). Results : GRE_5.8 showed the highest mean points for hepatic contour, vascular anatomy and imaging artifact presence among all of the subjected sequences (p<0.001) and was comparable (p=0.414) with GRE_10 with regard to lesion conspicuity. The mean CNRs were significantly higher (p<0.001) in the following order: GRE_10 (24.4±14.5), GRE_5.8 (14.8±9.4), TSE (9.7±6.3), and GRE_2.4 (7.9 ±6.4). The mean CNRs of CCCs and metastases were higher than those of HCCs for all imaging sequences (p<0.05). Conclusion : Regarding overall performances, GRE using a moderate echo time of 5.8 msec can provide the most reliable data among the various fast breath-hold SPIOenhanced hepatic MRI sequences at 3T unit despite the lower CNR of GRE_5.8 compared to that of GRE_10.

Fast MRI for spatially resolved quantitative information on molecular exchange

Fast MRI for spatially resolved quantitative information on molecular exchange

Performance evaluation of SiPM photodetectors for PET imaging in the presence of magnetic fields

The multi-pixel photon counter (MPPC) or silicon photomultiplier (SiPM), recently introduced as a solid-state photodetector, consists of an array of Geiger-mode photodiodes (microcells). It is a promising device for PET due to its potential for high photon detection efficiency (PDE) and its foreseeable immunity to magnetic fields. It is also easy to use with simple read-outs, has a high gain and a small size. In this work we evaluate the in field performance of three 1×1 mm 2 (with 100, 400 and 1600 microcells, respectively) and one 6×6 mm 2 (arranged as a 2×2 array) Hamamatsu MPPCs for their use in PET imaging. We examine the dependence of the energy resolution and the gain of these devices on the temperature and reverse bias voltage, when coupled to LYSO scintillator crystals under conditions that one would find in a PET system. We find that the 400 and 1600 microcells models and the 2×2 array are suitable for small-size crystals, like those employed in high resolution small animal scanners. We have confirmed the good performance of these devices up to magnetic fields of 7 T as well as their suitability for performing PET acquisitions in the presence of fast switching gradients and high duty radiofrequency MRI sequences.

PO13-TU-21 Fast multimodal MRI assessment of hyperacute stroke thrombolysis in 12-hour time window

PO13-TU-21 Fast multimodal MRI assessment of hyperacute stroke thrombolysis in 12-hour time window

Simulation of coupled-spin systems in the steady-state free-precession acquisition mode for fast magnetic resonance (MR) spectroscopic imaging

The steady-state free-precession (SSFP) acquisition mode may be found useful for fast in vivo proton magnetic resonance spectroscopic imaging in high-field MR systems because of the achievable signal-to-noise ratio and the avoidance of RF pulses with large flip angles. Detection of signals from metabolites with coupled-spin systems under SSFP has not yet been accomplished, but should be possible in high field, albeit with substantial signal truncation. It must be expected that the spin system evolution and the spectra will be affected by the steady-state conditions, which prevent the spin systems from returning to the Boltzmann equilibrium. Computer simulation is needed for the experiment design and spectrum quantification. This work outlines a suitable simulation method (QuaM-EPG), which combines and extends two pre-existing approaches: the density matrix calculation, used in high-resolution NMR, and the extended phase graph method, used to describe cyclic excitation in fast MRI of water protons. The method is illustrated by its application to model molecules and myo-inositol, which is one of the clinically relevant target molecules. It is shown that antiphase and multiple-quantum coherences may represent a considerable portion of the steady-state magnetization in a quantum-mechanical sense and that the spectral patterns are affected thereby.

Spin‐locked balanced steady‐state free‐precession (slSSFP)

A spin-locked balanced steady-state free-precession (slSSFP) pulse sequence is described that combines a balanced gradient-echo acquisition with an off-resonance spin-lock pulse for fast MRI. The transient and steady-state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady-state free-precession (bSSFP) for a range of T(2)/T(1) and flip angles, although the slSSFP steady-state could be maintained with considerably lower radio frequency (RF) power. In both simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal-to-noise ratio (SNR) efficiency to bSSFP, but with significantly lower power.