Optimizing isotropic three-dimensional fast spin-echo methods for imaging the knee.

PURPOSE: To optimize acquisition parameters for three dimensional fast spin-echo (3D FSE) imaging of the knee. MATERIALS AND METHODS: The knees of eight healthy volunteers were imaged in a 3 Tesla MRI scanner using an eight-channel knee coil. A total of 146 intermediate weighted isotropic resolution 3D FSE (3D-FSE-Cube)images with varied acquisition parameter settings were acquired with an additional reference scan performed for subjective image quality assessment. Images were graded for overall quality, parallel imaging artifact severity and blurring. Cartilage, muscle, and fluid signal-to-noise ratios and fluid-cartilage contrast-to-noise ratios were quantified by acquiring scans without radio frequency excitation and custom-reconstructing the k-space data.Mixed effects regression modeling was used to determine statistically significant effects of different parameters on image quality. RESULTS: Changes in receiver bandwidth, repetition time and echo train length significantly affected all measurements of image quality (P < 0.05). Reducing band width improved all metrics of image quality with the exception of blurring. Reader agreement was slight to fair for subjective metrics, but overall trends in quality ratings were apparent. CONCLUSION: We used a systematic approach to optimize 3D-FSE-Cube parameters for knee imaging. Image quality was overall improved using a receiver bandwidth of 631.25 kHz, and blurring increased with lower band width and longer echo trains.

Three-dimensional brain MRI for DBS patients within ultra-low radiofrequency power limits.

BACKGROUND: For patients with deep brain stimulators (DBS), local absorbed radiofrequency (RF) power is unknown and is much higher than what the system estimates. We developed a comprehensive, high-quality brain magnetic resonance imaging (MRI) protocol for DBS patients utilizing three-dimensional (3D) magnetic resonance sequences at very low RF power. METHODS: Six patients with DBS were imaged (10 sessions) using a transmit/receive head coil at 1.5 Tesla with modified 3D sequences within ultra-low specific absorption rate (SAR) limits (0.1 W/kg) using T2 , fast fluid-attenuated inversion recovery (FLAIR) and T1 -weighted image contrast. Tissue signal and tissue contrast from the low-SAR images were subjectively and objectively compared with routine clinical images of six age-matched controls. RESULTS: Low-SAR images of DBS patients demonstrated tissue contrast comparable to high-SAR images and were of diagnostic quality except for slightly reduced signal. CONCLUSIONS: Although preliminary, we demonstrated diagnostic quality brain MRI with optimized, volumetric sequences in DBS patients within very conservative RF safety guidelines offering a greater safety margin.

High resolution navigated three-dimensional T1-weighted hepatobiliary MRI using gadoxetic acid optimized for 1.5 Tesla.

PURPOSE: To determine optimal delay times and flip angles for T1-weighted hepatobiliary imaging at 1.5 Tesla (T) with gadoxetic acid and to demonstrate the feasibility of using a high-resolution navigated optimized T1-weighted pulse sequence to evaluate biliary disease. MATERIALS AND METHODS: Eight healthy volunteers were scanned at 1.5T using a T1-weighted three-dimensional (3D)-SPGR pulse sequence following the administration of 0.05 mmol/kg of gadoxetic acid. Navigator-gating enabled acquisition of high spatial resolution (1.2 × 1.4 × 1.8 mm(3) , interpolated to 0.7 × 0.7 × 0.9 mm(3) ) images in approximately 5 min of free-breathing. Multiple breath-held acquisitions were performed at flip angles between 15° and 45° to optimize T1 weighting. To evaluate the performance of this optimized sequence in the setting of biliary disease, the image quality and biliary excretion of 51 consecutive clinical scans performed to assess primary sclerosing cholangitis (PSC) were evaluated. RESULTS: Optimal hepatobiliary imaging occurs at 15-25 min, using a 40° flip angle. The image quality and visualization of biliary excretion in the PSC scans were excellent, despite the decreased liver function in some patients. Visualization of reduced excretion often provided diagnostic information that was unavailable by conventional magnetic resonance cholangiopancreatography (MRCP). CONCLUSION: High-resolution navigated 3D-SPGR hepatobiliary imaging using gadoxetic acid and optimized scan parameters is technically feasible and can be clinically useful, even in patients with decreased hepatobiliary function.

Improved motion correction capabilities for fast spin echo T1 FLAIR propeller using non-cartesian external calibration data driven parallel imaging.

Patient motion is a common challenge in the clinical setting and fast spin echo longitudinal relaxation time fluid attenuating inversion recovery imaging method with motion correction would be highly desirable. The motion correction provided by transverse relaxation time- and diffusion-weighted periodically rotated overlapping parallel lines with enhanced reconstruction methods has seen significant clinical adoption. However, periodically rotated overlapping parallel lines with enhanced reconstruction with fast spin echo longitudinal relaxation time fluid attenuating inversion recovery-weighting has proved challenging since motion correction requires wide blades that are difficult to acquire while also maintaining short echo train lengths that are optimal for longitudinal relaxation time fluid attenuating inversion recovery-weighting. Parallel imaging provides an opportunity to increase the effective blade width for a given echo train lengths. Coil-by-coil data-driven autocalibrated parallel imaging methods provide greater robustness in the event of motion compared to techniques relying on accurate coil sensitivity maps. However, conventional internally calibrated data-driven parallel imaging methods limit the effective acceleration possible for each blade. We present a method to share a single calibration dataset over all imaging blades on a slice by slice basis using the APPEAR non-cartesian parallel imaging method providing an effective blade width increase of 2.45×, enabling robust motion correction. Results comparing the proposed technique to conventional cartesian and periodically rotated overlapping parallel lines with enhanced reconstruction methods demonstrate a significant improvement during subject motion and maintaining high image quality when no motion is present in normal and clinical volunteers.

Optimized double inversion recovery for reduction of T1 weighting in fluid-attenuated inversion recovery.

Fluid-attenuated inversion recovery (FLAIR) is a routinely used technique in clinical practice to detect long T(2) lesions by suppressing the cerebrospinal fluid. Concerns remain, however, that the inversion pulse in FLAIR imparts T(1) weighting that can decrease the detectability and mischaracterize some lesions. Hence, FLAIR is usually acquired in conjunction with a standard T(2) to guard against these concerns. Recently, double inversion recovery (DIR) preparations have highlighted certain types of lesions by suppressing both cerebrospinal fluid and white matter but produce even stronger T(1) contrast than FLAIR. This work shows that the inversion times in a DIR sequence can be optimized to minimize unwanted T(1) weighting, enabling the acquisition of cerebrospinal fluid-suppressed images with pure T(2) weighting. This technique is referred to as T(1) -nulled DIR. The theory to determine the optimized inversion times is discussed and the results are shown by simulations, normal volunteer studies, and multiple sclerosis patient studies. T(1) -nulled DIR provides equivalent or superior contrast between gray and white matters as well as white matter and multiple sclerosis lesion at the same repetition time. Multiple sclerosis lesions appeared sharper on T(1) -nulled DIR compared to FLAIR. T(1) -nulled DIR has the potential to replace the combination of standard T(2) and FLAIR acquisitions in many clinical protocols.

Brain MR imaging at ultra-low radiofrequency power.

PURPOSE: To explore the lower limits for radiofrequency (RF) power-induced specific absorption rate (SAR) achievable at 1.5 T for brain magnetic resonance (MR) imaging without loss of tissue signal or contrast present in high-SAR clinical imaging in order to create a potentially viable MR method at ultra-low RF power to image tissues containing implanted devices. MATERIALS AND METHODS: An institutional review board-approved HIPAA-compliant prospective MR study design was used, with written informed consent from all subjects prior to MR sessions. Seven healthy subjects were imaged prospectively at 1.5 T with ultra-low-SAR optimized three-dimensional (3D) fast spin-echo (FSE) and fluid-attenuated inversion-recovery (FLAIR) T2-weighted sequences and an ultra-low-SAR 3D spoiled gradient-recalled acquisition in the steady state T1-weighted sequence. Corresponding high-SAR two-dimensional (2D) clinical sequences were also performed. In addition to qualitative comparisons, absolute signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) for multicoil, parallel imaging acquisitions were generated by using a Monte Carlo method for quantitative comparison between ultra-low-SAR and high-SAR results. RESULTS: There were minor to moderate differences in the absolute tissue SNR and CNR values and in qualitative appearance of brain images obtained by using ultra-low-SAR and high-SAR techniques. High-SAR 2D T2-weighted imaging produced slightly higher SNR, while ultra-low-SAR 3D technique not only produced higher SNR for T1-weighted and FLAIR images but also higher CNRs for all three sequences for most of the brain tissues. CONCLUSION: The 3D techniques adopted here led to a decrease in the absorbed RF power by two orders of magnitude at 1.5 T, and still the image quality was preserved within clinically acceptable imaging times.

Interleaved variable density sampling with a constrained parallel imaging reconstruction for dynamic contrast-enhanced MR angiography.

For MR applications such as contrast-enhanced MR angiography, it is desirable to achieve simultaneously high spatial and temporal resolution. The current clinical standard uses view-sharing methods combined with parallel imaging; however, this approach still provides limited spatial and temporal resolution. To improve on the clinical standard, we present an interleaved variable density (IVD) sampling method that pseudorandomly undersamples each individual frame of a 3D Cartesian ky-kz plane combined with parallel imaging acceleration. From this dataset, time-resolved images are reconstructed with a method that combines parallel imaging with a multiplicative constraint. Total acceleration factors on the order of 20 are achieved for contrast-enhanced MR angiography of the lower extremities, and improvements in temporal fidelity of the depiction of the contrast bolus passage are demonstrated relative to the clinical standard.

T(2)-weighted 3D fast spin echo imaging with water-fat separation in a single acquisition.

PURPOSE: To develop a robust 3D fast spin echo (FSE) T(2)-weighted imaging method with uniform water and fat separation in a single acquisition, amenable to high-quality multiplanar reformations. MATERIALS AND METHODS: The Iterative Decomposition of water and fat with Echo Asymmetry and Least squares estimation (IDEAL) method was integrated with modulated refocusing flip angle 3D-FSE. Echoes required for IDEAL processing were acquired by shifting the readout gradient with respect to the Carr-Purcell-Meiboom-Gill echo. To reduce the scan time, an alternative data acquisition using two gradient echoes per repetition was implemented. Using the latter approach, a total of four gradient echoes were acquired in two repetitions and used in the modified IDEAL reconstruction. RESULTS: 3D-FSE T(2)-weighted images with uniform water-fat separation were successfully acquired in various anatomies including breast, abdomen, knee, and ankle in clinically feasible scan times, ranging from 5:30-8:30 minutes. Using water-only and fat-only images, in-phase and out-of-phase images were reconstructed. CONCLUSION: 3D-FSE-IDEAL provides volumetric T(2)-weighted images with uniform water and fat separation in a single acquisition. High-resolution images with multiple contrasts can be reformatted to any orientation from a single acquisition. This could potentially replace 2D-FSE acquisitions with and without fat suppression and in multiple planes, thus improving overall imaging efficiency.

Evaluation of uterine anomalies: 3D FRFSE cube versus standard 2D FRFSE.

OBJECTIVE: The purpose of this study was to compare a novel MRI sequence-3D fast-recovery fast spin-echo (FRFSE) cube-with a standard 2D FRFSE sequence for the investigation of uterine anomalies. CONCLUSION: Compared with 2D FRFSE, 3D FRFSE cube provides superior image quality and improved 3D reconstructions in a shorter acquisition time and enables excellent visualization of uterine anatomy in any orientation, regardless of the original scanning plane.

Increased volume of coverage for abdominal contrast-enhanced MR angiography with two-dimensional autocalibrating parallel imaging: initial experience at 3.0 Tesla.

PURPOSE: To assess the feasibility and the quality of abdominal three-dimensional (3D) contrast enhanced MR angiograms acquired at 3.0 Tesla (T) using a new 2D-accelerated autocalibrating parallel reconstruction method for Cartesian sampling (2D-ARC). MATERIALS AND METHODS: With institutional review board approval and written informed consent, a prospective trial in 6 normal healthy volunteers and 23 patients referred for evaluation of suspected renovascular disease was performed. The volunteers underwent abdominal MRA with and without 2D-ARC acceleration. Images were evaluated independently by two blinded vascular radiologists in randomized order. Vessel conspicuity was rated on a five-point scale. Evaluation for significant differences between the scores for each technique was performed using a Wilcoxon signed-rank test. RESULTS: In the series of six volunteers, no statistical significance was found between the image quality scores for 2D-ARC accelerated and nonaccelerated exams. A high proportion of the 23 clinical 2D-ARC exams were graded as diagnostic (vessel conspicuity score >or=2; Reader 1, 96%; Reader 2, 100%) for overall image quality. CONCLUSION: Subjective image quality of 2D-ARC accelerated MRA was equivalent to the conventional MRA method. However, the 2D-ARC accelerated sequence provided a 3.5-fold increase in imaging volume, complete abdominal coverage, and a 30% reduction in voxel volume, all within the same acquisition time.

Knee joint: comprehensive assessment with 3D isotropic resolution fast spin-echo MR imaging--diagnostic performance compared with that of conventional MR imaging at 3.0 T.

PURPOSE: To determine whether a three-dimensional isotropic resolution fast spin-echo sequence (FSE-Cube) has similar diagnostic performance as a routine magnetic resonance (MR) imaging protocol for evaluating the cartilage, ligaments, menisci, and osseous structures of the knee joint in symptomatic patients at 3.0 T. MATERIALS AND METHODS: This prospective, HIPAA-compliant, institutional review board-approved study was performed with a waiver of informed consent. FSE-Cube was added to the routine 3.0-T MR imaging protocol performed in 100 symptomatic patients (54 male patients with a median age of 32 years and 46 female patients with a median age of 33 years) who subsequently underwent arthroscopic knee surgery. All MR imaging studies were independently reviewed twice by two musculoskeletal radiologists. During the first review, the routine MR imaging protocol was used to detect cartilage lesions, ligament tears, meniscal tears, and bone marrow edema lesions. During the second review, FSE-Cube with multiplanar reformations was used to detect these joint abnormalities. With arthroscopic results as the reference standard, the sensitivity and specificity of FSE-Cube and the routine MR imaging protocol in the detection of cartilage lesions, anterior cruciate ligament tears, and meniscal tears were calculated. Permutation tests were used to compare sensitivity and specificity values. RESULTS: FSE-Cube had significantly higher sensitivity (P = .039) but significantly lower specificity (P = .003) than the routine MR imaging protocol for detecting cartilage lesions. There were no significant differences (P = .183-.999) in sensitivity and specificity between FSE-Cube and the routine MR imaging protocol in the detection of anterior cruciate ligament tears, medial meniscal tears, or lateral meniscal tears. FSE-Cube depicted 96.2% of medial collateral ligament tears, 100% of lateral collateral ligament tears, and 85.3% of bone marrow edema lesions identified on images obtained with the routine MR imaging protocol. CONCLUSION: FSE-Cube has similar diagnostic performance as a routine MR imaging protocol for detecting cartilage lesions, cruciate ligament tears, collateral ligament tears, meniscal tears, and bone marrow edema lesions within the knee joint at 3.0 T.

Ankle: isotropic MR imaging with 3D-FSE-cube--initial experience in healthy volunteers.

The purpose of this prospective study was to compare a new isotropic three-dimensional (3D) fast spin-echo (FSE) pulse sequence with parallel imaging and extended echo train acquisition (3D-FSE-Cube) with a conventional two-dimensional (2D) FSE sequence for magnetic resonance (MR) imaging of the ankle. After institutional review board approval and informed consent were obtained and in accordance with HIPAA privacy guidelines, MR imaging was performed in the ankles of 10 healthy volunteers (four men, six women; age range, 25-41 years). Imaging with the 3D-FSE-Cube sequence was performed at 3.0 T by using both one-dimensional- and 2D-accelerated autocalibrated parallel imaging to decrease imaging time. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) with 3D-FSE-Cube were compared with those of the standard 2D FSE sequence. Cartilage, muscle, and fluid SNRs were significantly higher with the 3D-FSE-Cube sequence (P < .01 for all). Fluid-cartilage CNR was similar for both techniques. The two sequences were also compared for overall image quality, blurring, and artifacts. No significant difference for overall image quality and artifacts was demonstrated between the 2D FSE and 3D-FSE-Cube sequences, although the section thickness in 3D-FSE-Cube imaging was much thinner (0.6 mm). However, blurring was significantly greater on the 3D-FSE-Cube images (P < .04). The 3D-FSE-Cube sequence with isotropic resolution is a promising new MR imaging sequence for viewing complex joint anatomy.

Effects of refocusing flip angle modulation and view ordering in 3D fast spin echo.

Recent advances have reduced scan time in three-dimensional fast spin echo (3D-FSE) imaging, including very long echo trains through refocusing flip angle (FA) modulation and 2D-accelerated parallel imaging. This work describes a method to modulate refocusing FAs that produces sharp point spread functions (PSFs) from very long echo trains while exercising direct control over minimum, center-k-space, and maximum FAs in order to accommodate the presence of flow and motion, SNR requirements, and RF power limits. Additionally, a new method for ordering views to map signal modulation from the echo train into k(y)-k(z) space that enables nonrectangular k-space grids and autocalibrating 2D-accelerated parallel imaging is presented. With long echo trains and fewer echoes required to encode large matrices, large volumes with high in- and through-plane resolution matrices may be acquired with scan times of 3-6 min, as demonstrated for volumetric brain, knee, and kidney imaging.

Three-dimensional fluid attenuated inversion recovery imaging with isotropic resolution and nonselective adiabatic inversion provides improved three-dimensional visualization and cerebrospinal fluid suppression compared to two-dimensional flair at 3 tesla.

OBJECTIVES: In this investigation, we compare two-dimensional (2D) fluid-attenuated inversion recovery (FLAIR) imaging of the brain to an isotropic three-dimensional (3D) FLAIR technique that uses a modulated refocusing flip angle echo train and parallel imaging with 2D acceleration. MATERIALS AND METHODS: Two-dimensional and 3D FLAIR sequences were obtained in 16 patients. All examinations were performed on a 3 Tesla (T) magnetic resonance (MR) system. Flow artifacts within the subarachnoid space and ventricles were scored using a 4-point scale. For 2D and 3D FLAIR, the signal-to-noise ratios and contrast-to-noise ratios were calculated. RESULTS: Compared to 2D FLAIR, the 3D FLAIR images were less degraded by flow artifacts in the subarachnoid space and ventricle (P < 0.03) based on the qualitative imaging scores. Signal-to-noise ratios and contrast-to-noise ratios were higher for 3D FLAIR (P < 0.02) for all variables when compared with 2D FLAIR sequence. CONCLUSIONS: The acquisition time for whole brain isotropic fast spin echo 3D FLAIR can be dramatically reduced by using an extended echo train with flip angle modulation and parallel imaging. The adiabatic, nonselective inversion pulse encompasses the entire volume and provides uniform suppression of the cerebrospinal fluid signal eliminating cerebrospinal fluid pulsation artifacts. Other advantages include reformatting in any desired plane, volume measurements, displays of surface anatomy, and coregistration.

B1-insensitive fast spin echo using adiabatic square wave enabling of the echo train (SWEET) excitation.

Abdominal images at 3T acquired with fast spin echo (FSE) sequences often exhibit signal voids due to RF transmit field inhomogeneities. Theory suggests, however, that the repeated refocusing pulses of FSE are capable of maintaining signal even at reduced RF amplitudes if the magnetization is suitably prepared. Here we propose a modified excitation strategy for FSE that is more robust to transmit field inhomogeneities than conventional FSE. The new excitation approach replaces the standard 90 degrees excitation pulse with a discretely sampled hyperbolic secant pulse that creates a square wave longitudinal magnetization as a function of gradient and off-resonance induced phase shifts between the subsequent echoes of the FSE sequence. This pulse is followed by the conventional train of refocusing pulses except that the first few pulses increase from near zero to the desired refocusing amplitude. Simulations and in vivo results at 3T indicate preserved image quality and much greater robustness of this new sequence to nonuniform RF fields. This robustness comes at the cost of 20% reduction in signal when the RF field is uniform and increased motion sensitivity. This RF field-insensitive sequence may overcome challenges of body imaging at high field and in patients with ascites.

In vivo T(1rho) mapping in cartilage using 3D magnetization-prepared angle-modulated partitioned k-space spoiled gradient echo snapshots (3D MAPSS).

For T(1rho) quantification, a three-dimensional (3D) acquisition is desired to obtain high-resolution images. Current 3D methods that use steady-state spoiled gradient-echo (SPGR) imaging suffer from high SAR, low signal-to-noise ratio (SNR), and the need for retrospective correction of contaminating T(1) effects. In this study, a novel 3D acquisition scheme-magnetization-prepared angle-modulated partitioned-k-space SPGR snapshots (3D MAPSS)-was developed and used to obtain in vivo T(1rho) maps. Transient signal evolving towards the steady-state were acquired in an interleaved segmented elliptical centric phase encoding order immediately after a T(1rho) magnetization preparation sequence. RF cycling was applied to eliminate the adverse impact of longitudinal relaxation on quantitative accuracy. A variable flip angle train was designed to provide a flat signal response to eliminate the filtering effect in k-space caused by transient signal evolution. Experiments in phantoms agreed well with results from simulation. The T(1rho) values were 42.4 +/- 5.2 ms in overall cartilage of healthy volunteers. The average coefficient-of-variation (CV) of mean T(1rho) values (N = 4) for overall cartilage was 1.6%, with regional CV ranging from 1.7% to 8.7%. The fitting errors using MAPSS were significantly lower (P < 0.05) than those using sequences without RF cycling and variable flip angles.

Isotropic MRI of the knee with 3D fast spin-echo extended echo-train acquisition (XETA): initial experience.

OBJECTIVE: The purpose of our study was to prospectively compare a recently developed method of isotropic 3D fast spin-echo (FSE) with extended echo-train acquisition (XETA) with 2D FSE and 2D fast recovery FSE (FRFSE) for MRI of the knee. SUBJECTS AND METHODS: Institutional review board approval, Health Insurance Portability and Accounting Act (HIPAA) compliance, and informed consent were obtained. We studied 10 healthy volunteers and one volunteer with knee pain using 3D FSE XETA, 2D FSE, and 2D FRFSE. Images were obtained both with and without fat suppression. Cartilage and muscle signal-to-noise ratio (SNR) and cartilage-fluid contrast-to-noise ratio (CNR) were compared using a Student's t test. We also compared reformations of 3D FSE XETA with 2D FSE images directly acquired in the axial plane. RESULTS: Cartilage SNR was higher with 3D FSE XETA (56.8 +/- 9 [SD]) compared with the 2D FSE (45.8 +/- 8, p < 0.01) and 2D FRFSE (32.5 +/- 5.3, p < 0.01). Muscle SNR was significantly higher with 3D FSE XETA (52.1 +/- 4.3) than 2D FSE (45.2 +/- 9, p < 0.01) and 2D FRFSE (23.6 +/- 6.2, p < 0.01). Fluid SNR was significantly higher for 2D FSE (144.9 +/- 33) than 3D FSE XETA (104.7 +/- 18, p < 0.01). Compared with 2D FSE and 2D FRFSE, 3D FSE XETA had lower cartilage-fluid CNR due to higher cartilage SNR (p < 0.01). Three-dimensional FSE XETA acquired volumetric data sets with isotropic resolution. Reformatted images in the axial plane were similar to axial 2D FSE acquisitions but with thinner slices. CONCLUSION: Three-dimensional FSE XETA acquires high-resolution (approximately 0.7 mm) isotropic data with intermediate and T2-weighting that may be reformatted in arbitrary planes. Three-dimensional FSE XETA is a promising technique for MRI of the knee.

Evaluation of real-time single-shot fast spin-echo MRI for visualization of the fetal midline corpus callosum and secondary palate.

OBJECTIVE: The objective of our study was to assess the visibility of the fetal corpus callosum and soft palate on standard single-shot fast spin-echo (SSFSE) imaging versus real-time (RT) SSFSE imaging. SUBJECTS AND METHODS: Part 1 of the study was a prospective analysis using a questionnaire rating the ease of use and utility of RT imaging. Part 2 of the study was a retrospective analysis of 69 fetal MRI studies with RT sagittal midline imaging of the head, face, or both. Standard and RT SSFSE image sets were de-identified, randomized, and shown to three pediatric neuroradiologists who rated on a 5-point scale whether the images were midline and how well they could see and characterize as normal the corpus callosum and secondary palate. The imaging results were correlated with postnatal diagnosis. Statistical methods included the Wilcoxon's signed rank test, McNemar chi-square test, and analysis of variance. RESULTS: Prospectively, the RT SSFSE technique was ranked as excellent in all the categories assessed. Retrospective analysis showed that the midline view obtained with RT SSFSE imaging was helpful in diagnosing the normal and abnormal secondary palate, allowing improved diagnosis of 19 (30.6%) of 62 cases of normal palate and four (57.1%) of seven cases of abnormal palate, when compared with the standard SSFSE technique. RT SSFSE imaging improved the ability to diagnose a normal corpus callosum on the midline view in 13 (27.6%) of 47 fetuses of 20 or more weeks gestational age. CONCLUSION: The RT SSFSE technique can aid in obtaining images in planes that are critical to the evaluation of a moving fetus, particularly when a midline sagittal view of the corpus callosum or palate is required. The use of this technique may lead to improved diagnosis of CNS or orofacial abnormalities in fetuses.

Noninvasive oxygen partial pressure measurement of human body fluids in vivo using magnetic resonance imaging.

RATIONALE AND OBJECTIVES: The oxygen partial pressure (pO2) of human body fluids reflects the oxygenation status of surrounding tissues. All existing fluid pO2 measurements are invasive, requiring either microelectrode/optode placement or fluid removal. The purpose of this study is to develop a noninvasive magnetic resonance imaging method to measure the pO2 of human body fluids. MATERIALS AND METHODS: We developed an imaging paradigm that exploits the paramagnetism of molecular oxygen to create quantitative images of fluid oxygenation. A single-shot fast spin echo pulse sequence was modified to minimize artifacts from motion, fluid flow, and partial volume. Longitudinal relaxation rate (R1 = 1/T1) was measured with a time-efficient nonequilibrium saturation recovery method and correlated with pO2 measured in phantoms. RESULTS: pO2 images of human and fetal cerebrospinal fluid, bladder urine, and vitreous humor are presented and quantitative oxygenation levels are compared with prior literature estimates, where available. Significant pO2 increases are shown in cerebrospinal fluid and vitreous following 100% oxygen inhalation. Potential errors due to temperature, fluid flow, and partial volume are discussed. CONCLUSIONS: Noninvasive measurements of human body fluid pO2 in vivo are presented, which yield reasonable values based on prior literature estimates. This rapid imaging-based measurement of fluid oxygenation may provide insight into normal physiology as well as changes due to disease or during treatment.

Fast spin echo sequences with very long echo trains: design of variable refocusing flip angle schedules and generation of clinical T2 contrast.

Reducing and continuously varying the flip angle of the refocusing RF pulses in a rapid acquisition with relaxation enhancement (RARE; fast/turbo spin echo) sequence is a useful means of addressing high RF power deposition and modulation transfer function (MTF) distortion due to relaxation. This work presents a streamlined technique to generate a sequence of refocusing flip angles on a per-prescription basis that produces relatively high SNR and limits blurring in a wide range of materials encountered in vivo. Since the "effective TE" (traditionally defined as the time at which the center of k-space is sampled) no longer corresponds to the expected amount of spin-echo T2 contrast due to the mixing of stimulated and spin echoes, a "contrast-equivalent" TE is defined and experimentally demonstrated that allows annotation of a more accurate effective TE that matches the contrast produced by 180 degrees refocusing. Furthermore, contrast is shown to be manipulable by the addition of magnetization preparation pulse sequence segments, such as T2-prep, to produce clinically desirable contrast for routine head and body imaging.