T2 and T2* quantification using optimal B1 image reconstruction for multicoil arrays

PURPOSE

To investigate the accuracy of low signal-to-noise ratio (SNR) T(2) and T(2)* measurements using array coils and optimal B(1) image reconstruction (OBR) compared to the standard root sum of squares (RSS) reconstruction.

MATERIALS AND METHODS

Calibrated gels were used for the in vitro study of T(2). T(2) and T(2)* measurements were obtained from a volunteer's knee and liver, respectively. T(2) and T(2)* measurements were performed using multiecho spin echo and multiecho gradient echo sequences, respectively. SNR was deliberately kept low. The same raw data were used for both reconstructions. For the in vivo studies the effect of signal averaging was also investigated.

RESULTS

The optimal reconstructions demonstrated a lower mean background noise level than RSS. In vitro, the T(2) measurements made with OBR images agreed better with a reference high SNR measurement than measurements made from RSS images; the RSS image results overestimated the T(2.) In vivo, increasing the signal averages decreased the difference between the measurements obtained using the OBR and RSS methods, with RSS resulting in longer relaxation times.

CONCLUSION

This work demonstrates improvements to the accuracy of T(2) and T(2)* measurements obtained when OBR is used compared to RSS, particularly in the case of low SNR.

Rapid volumetric MRI using parallel imaging with order-of-magnitude accelerations and a 32-element RF coil array: feasibility and implications

RATIONALE AND OBJECTIVES

Many clinical applications of Magnetic Resonance Imaging are constrained by basic limits on imaging speed. Parallel MRI relaxes these limits by using the sensitivity patterns of arrays of radiofrequency receiver coils to encode spatial information in a manner complementary to traditional encoding with magnetic field gradients. Until now, parallel MRI has been used to achieve modest improvements in imaging speed; order-of-magnitude improvements have been elusive given fundamental losses in signal-to-noise ratio. The goal of this work was to demonstrate that, with appropriate hardware and careful SNR management, rapid volumetric imaging at high accelerations is in fact feasible.

MATERIALS AND METHODS

Contrast-enhanced MRI with an axial 3D spoiled gradient echo imaging sequence was performed in healthy adult subjects using a 32-element RF coil array and a prototype 32-channel MR imaging system. Large imaging volumes were prescribed, in place of traditional limited slabs targeted only to suspect regions.

RESULTS

As much as 16-fold net accelerations of imaging were achieved repeatably using this approach. The use of large 3D volumes allowed comprehensive anatomical coverage at clinically useful spatial and/or temporal resolution. The need for careful, time-consuming, and subject-specific scan prescription was also eliminated.

CONCLUSION

The highly parallel imaging approach presented here allows previously inaccessible volumetric coverage for time-sensitive MRI examinations such as contrast-enhanced MRA, and simultaneously provides a substantially simplified imaging paradigm. The resulting capability for rapid volumetric imaging promises to combine the strengths of MRI with some of the advantages of alternative imaging modalities such as multidetector CT.

Highly parallel volumetric imaging with a 32-element RF coil array

The improvement of MRI speed with parallel acquisition is ultimately an SNR-limited process. To offset acquisition- and reconstruction-related SNR losses, practical parallel imaging at high accelerations should include the use of a many-element array with a high intrinsic signal-to-noise ratio (SNR) and spatial-encoding capability, and an advantageous imaging paradigm. We present a 32-element receive-coil array and a volumetric paradigm that address the SNR challenge at high accelerations by maximally exploiting multidimensional acceleration in conjunction with noise averaging. Geometric details beyond an initial design concept for the array were determined with the guidance of simulations. Imaging with the support of 32-channel data acquisition systems produced in vivo results with up to 16-fold acceleration, including images from rapid abdominal and MRA studies.

Cerebrospinal fluid flow waveforms: MR analysis in chronic adult hydrocephalus

Henry-Feugeas MC, Idy-Peretti I, Baledent O, et al. Cerebrospinal fluid flow waveforms: MR analysis in chronic adult hydrocephalus. Invest Radiol 2001;36:146-154.

RATIONALE AND OBJECTIVES

To analyze changes in cerebrospinal fluid (CSF) hydrodynamics in chronic adult hydrocephalus.

METHODS

Phase-contrast cine-MR acquisitions were used to explore the ventricular system and the upper ventral cervical spaces of 16 patients. The aqueductal jet was explored in 32 control subjects.

RESULTS

The duration of pulsatile caudal CSF flow (ie, CSF systole) was abnormally short in patients with active idiopathic and obstructive hydrocephalus. The duration of CSF cervical systole was normal in patients with stable hydrocephalus. The aqueductal stroke volume could be increased in stable communicating hydrocephalus. Patients who responded to shunting had shortened CSF systoles and hyperpulsatile ventricular patterns. Successful CSF diversion resulted in longer CSF systoles and CSF ventricular patterns that were no longer hyperpulsatile.

CONCLUSIONS

Magnetic resonance analysis of CSF flow can show craniospinal dissociation and limitation of CSF outflow from the ventricles in both obstructive and communicating hydrocephalus; it should help determine the response to shunting in communicating hydrocephalus.

New black blood pulse sequence for studies of the heart

The black blood concept is based on the signal void principle of the sequences in Spin Echo, SE or FSE, which are very useful for studying the mediastinum and heart. In this setting, new sequences are continuously introduced to eliminate the artifacts caused by breathing and heart movements. One such sequence is the Double-IR preparation Black Blood FSE. We report our experience in 97 patients, using this new pulse sequence to evaluate cardiac pathology, and establish comparisons with the conventional Spin Echo sequences. The study comprises mediastinal disease and aorta and heart explorations. We consider this new Double-IR preparation FSE sequence to be an excellent choice for evaluating chest, mediastinal and cardiac images. The sequence offers improved spatial resolution of both the vessels and other chest structures with respect to conventional Spin Echo imaging. With the exception of patients presenting severe heart problems, or in the presence of intense bradycardia, the required 16 cycles in apnea are well tolerated. The purpose of the present study is to present our initial results with this new pulse sequence as applied to cardiac pathology, in comparison with conventional Spin Echo imaging.