Needle shear wave driver for magnetic resonance elastography

Magnetic resonance elastography (MRE) is capable of quantitatively depicting the mechanical properties of tissues in vivo. In contrast to mechanical excitation at the surface of the tissue, the method proposed in this study describes shear waves produced by an inserted needle. The results demonstrate that MRE performed with the needle driver provides shear stiffness estimates that correlate well with those obtained using mechanical testing. Comparisons between MRE acquisitions obtained with surface and needle drivers yielded similar results in general. However, the well-defined wave propagation pattern provided by the needle driver in a target region can reduce section orientation-related error in wavelength estimation that occurs with surface drivers in 2D MRE acquisitions. Preliminary testing of the device was performed on animals. This study demonstrates that the needle driver is an effective option that offers advantages over surface drivers for obtaining accurate stiffness estimates in targeted regions that are accessible by the needle.

Autocorrection in MR imaging: adaptive motion correction without navigator echoes

A technique for automatic retrospective correction of motion artifacts on magnetic resonance (MR) images was developed that uses only the raw (complex) data from the MR imager and requires no knowledge of patient motion during the acquisition. The algorithm was tested on coronal images of the rotator cuff in a series of 144 patients, and the improvements in image quality were similar to those achieved with navigator echoes. The results demonstrate that autocorrection can significantly reduce motion artifacts in a technically demanding MR imaging application.

Prospective multiaxial motion correction for fMRI

Corruption of the image time series due to interimage head motion limits the clinical utility of functional MRI. This paper presents a method for real-time prospective correction of rotation and translation in all six degrees of rigid body motion. By incorporating an orbital navigator (ONAV) echo for each of the sagittal, axial, and coronal planes into the fMRI pulse sequence, rotation and translation can be measured and the spatial orientation of the image acquisition sequence that follows can be corrected prospectively in as little as 160 msec. Testing of the method using a computerized motion phantom capable of performing complex multiaxial motion showed subdegree rotational and submillimeter translational accuracy over a range of +/-8 degrees and +/-8 mm of motion. In vivo images demonstrate correction of simultaneous through-plane and in-plane motion and improved detection of fMRI activation in the presence of head motion.

A prospective approach to correct for inter-image head rotation in fMRI

Global head motion occurring between successive image acquisitions during a functional MRI time series can corrupt the signal of physiologic brain activation, potentially invalidating interpretation of the final activation map from that particular fMRI time series. By approximating the head as a rigid body, multiaxial global head motion can be decomposed into orthogonal linear and rotational components. This paper describes a method using orbital navigator echoes to provide prospective correction for both through-plane and in-plane inter-image head rotation in functional MRI. The dynamic detection and correction of rotation can be performed in <100 ms. Phantom experiments demonstrate accurate correction of rotational motion over a range of +/-0.36 degrees to +/-12 degrees. Imaging studies in volunteers document the feasibility of real-time prospective correction of rotational motion in vivo. Using a modified receiver operating characteristic method, motion-corrected functional MRI sensorimotor studies incorporating deliberate head rotations are shown to be superior to functional MRI time series acquired under similar conditions but without motion correction.

The shoulder: adaptive motion correction of MR images

PURPOSE

To evaluate an adaptive-motion-correction technique to reduce global motion in shoulder magnetic resonance (MR) images.

MATERIALS AND METHODS

In the adaptive-motion-correction technique, interleaved navigator echoes are used to provide a measure of view-to-view displacement along the craniocaudal direction for each image echo in the acquisition. The information is then retrospectively applied to the k-space data to correct for global shoulder motion. This algorithm was evaluated in a series of 143 consecutive patient shoulder examinations by comparing the original image set for each patient with the same image set after retrospective correction by means of this algorithm.

RESULTS

The average amplitude of craniocaudal motion was 1.4 mm. Image degradation due to motion was apparent in 100 (70%) of the 143 examinations. Application of the adaptive-motion-correction technique improved image quality in 73 (73%) of these 100 examinations or 51% of all 143 examinations.

CONCLUSION

Adaptive motion correction improved image quality in approximately three-quarters of the examinations in which motion was present.

An orthogonal correlation algorithm for ghost reduction in MRI

Ghosting in MRI due to modulation of k-space data can be caused by motion of the subject or characteristics of the sequence. A general solution for 2DFT MRI that reduces ghosting without causal modeling is presented. Separate image data sets are acquired in which the phase and frequency directions are swapped. In these two data sets, the image signal is correlated, whereas the ghost signals are not. By taking a correlation of these two data sets, an image with greatly reduced ghosting is obtained. The reduction is shown to depend both on the correct signal intensity of the image, as well as the ghost intensity in the ghosted region. The reduction approaches 100% in regions of low image signal, and is more moderate in regions of higher image signal. The process was applied to conventional spin-echo, fast-spin-echo, and gradient echo imaging of volunteers and a phantom. Results of a reader study of the volunteer images reflected a significant overall reduction of ghosting artifacts in all volunteer experiments.

3D MR angiography of pulmonary arteries using real-time navigator gating and magnetization preparation

An ECG-triggered magnetization-prepared segmented 3D fast gradient echo sequence was developed to perform pulmonary arterial MR angiography. A selective inversion recovery pulse was used in the magnetization preparation to suppress venous vasculature. A real-time gating technique based on navigator echoes was implemented to reduce respiration effects. Pencil-beam navigator echoes were acquired immediately before and after the readout train and processed in real-time to dynamically measure the diaphragm position, which was used to control data acquisition with an accept-or-reject-reacquire logic. In a study of 10 volunteers, a gated 3D acquisition with 28 slices required on average approximately 4 min of acquisition time, and six to seven segmental arteries related to the interlobar trunk of the pulmonary artery were depicted. The use of SIR pulse reduced venous signal by 99%. The gated acquisitions were superior to the ungated acquisitions (n = 10, P < 0.005). The real-time navigator gating technique is effective for reduction of respiration effects and thereby makes high resolution 3D MRA of the pulmonary arteries feasible.

Cardiac magnetic resonance fluoroscopy

A technique is described for high speed interactive imaging of the heart with either white or black blood contrast. Thirty-two views of a segmented, magnetization-prepared gradient echo sequence are acquired during diastole. Using three-quarter partial Fourier sampling, data for a complete 128 x 128 image are acquired in three cardiac cycles. High speed reconstruction provides an image update of each cardiac cycle 159 ms after measurement. An independent graphical user interface facilitates interactive control of section localization and contrast by permitting pulse sequence parameter modification during scanning. The efficiency and image quality of the cardiac MR fluoroscopy technique were evaluated in 11 subjects. Compared with the conventional graphic prescription method, the cardiac fluoroscopy technique provides an approximate eightfold reduction in the time required to obtain subject-specific double oblique sections. Image quality for these scout acquisitions performed during free breathing was sufficient to identify small cardiac structures.

Real-time adaptive motion correction in functional MRI

Functional magnetic resonance imaging (fMRI) of the brain is often degraded by bulk head motion. Algorithms that address this by retrospective re-registration of images in an fMRI time series are all fundamentally limited by any motion that occurs through-plane. Here, a technique is described that can account for such motion by prospective correction in real time. A navigator echo is used before every image acquisition to detect superior/inferior displacements of the head. The displacement information is then used to adjust the plane of excitation of the ensuing single-shot echo-planar fMRI axial image. These correction updates can be completed in 100 mm with motion sensitivity at least as small as 0.5 mm. The efficacy of this method is documented in phantom and human studies.

Algorithms for extracting motion information from navigator echoes

Algorithms to reliably detect motion in navigator echoes are crucial to many MRI motion suppression techniques. The accuracy of these algorithms is affected by noise and deformation of navigator echo profile caused by physiologic motion. This study compared the performance of algorithms based on correlation and least squares for extracting displacement information from motion-monitoring navigator echoes, using computer simulation and in vivo imaging. The least squares algorithm was determined to be of higher accuracy than the correlation algorithm against errors caused by noise and profile deformation.

Multiple breathhold 3D time-of-flight MR angiography of the renal arteries

A technique is described for angiographic imaging of the renal arteries with acquisition performed over several periods of suspended respiration. The 3D Fourier transform (FT) gradient-echo angiographic sequence uses magnetization preparation and appropriately chosen delay times for background nulling and time-of-flight enhancement of the vasculature. The sequence was applied to 10 volunteers, each of whom was imaged in three ways: (i) over a series of breathholds in which feedback was provided to enable reproducible breathholding; (ii) over a series of breathholds with no feedback; and (iii) over continuous respiration. Results were evaluated by measuring the transverse extent of the well-delineated renal vasculature and by noting the distal extent of the vasculature branching (main, segmental, and interlobar branches). The transverse extent of renal vasculature visible with breathhold feedback, breathholding, and free breathing was 6.1 +/- 0.9 cm, 5.0 +/- 1.8 cm, and 4.0 +/- 1.4 cm, respectively (mean +/- SD). Breathhold feedback enabled visualization of segmental renal arteries bilaterally in all 10 volunteers.

T1-weighted MR imaging of the brain using a fast inversion recovery pulse sequence

The purpose of this paper was to develop and evaluate a fast inversion recovery (FIR) technique for T1-weighted MR imaging of contrast-enhancing brain pathology. The FIR technique was developed, capable of imaging 24 sections in approximately 7 minutes using two echoes per repetition and an alternating echo phase encoding assignment. Resulting images were compared with conventional T1-weighted spin echo (T1SE) images in 18 consecutive patients. Compared with corresponding T1SE images, FIR images were quantitatively comparable or superior for lesion-to-background contrast and contrast-to-noise ratio (CNR). Gray-to-white matter and cerebrospinal fluid (CSF)-to-white matter contrast and CNR were statistically superior in FIR images. Qualitatively, the FIR technique provided comparable lesion detection, improved lesion conspicuity, and superior image contrast compared with T1SE images. Although FIR images had greater amounts of image artifacts, there was not a statistically increased amount of interpretation-interfering image artifact. FIR provides T1-weighted images that are superior to T1SE images for a number of image quality criteria.

Navigator-echo-based real-time respiratory gating and triggering for reduction of respiration effects in three-dimensional coronary MR angiography

PURPOSE

To test the hypothesis that respiration effects in three-dimensional (3D) coronary magnetic resonance (MR) imaging can be reduced with navigator-echo-based gating or triggering according to the superior-inferior position of the diaphragm.

MATERIALS AND METHODS

Real-time respiratory gating and respiratory triggering (breath hold with feedback) were implemented with navigator echoes in a magnetization-prepared, segmented, 3D coronary imaging sequence. The two techniques were first tested with a motion phantom. An imaging protocol that compared real-time respiratory-gated acquisition, real-time respiratory-triggered acquisition, and continuous acquisition was then evaluated in six healthy subjects.

RESULTS

Real-time respiratory-gated and respiratory-triggered acquisition were superior to continuous acquisition with two signals averaged (P = .025). The performance of the gated acquisition was about the same as that of the triggered acquisition (P = .05).

CONCLUSION

Navigator-echo-based, real-time respiratory-gating and respiratory-triggering techniques are practical methods for effective reduction of respiration effects in coronary MR imaging.

Orbital navigator echoes for motion measurements in magnetic resonance imaging

A single "orbital" navigator echo, that has a circular k-space trajectory, is used to simultaneously measure in-plane rotational and multi-axis translational global motion. Rotation is determined from the shift in the magnitude profile of the echo with respect to a reference echo. Displacements are calculated from the phase difference between the current echo and a reference echo. Phantom studies show that this technique can accurately measure rotation and translations. Preliminary results from adaptive motion correction studies on phantom and human subjects indicate that the orbital navigator echo is an effective method for motion measurement in MRI.

3D coronary MR angiography in multiple breath-holds using a respiratory feedback monitor

To reduce respiratory blur and ghosts in 3D coronary imaging, a data acquisition scheme using consistent multiple breath-holds was implemented. A navigator echo was acquired and processed in real time to dynamically measure diaphragm position. This information was provided as a visual prompt to the patient to maintain consistency in breath-hold levels such that the variation range of diastolic heart position was less than 2 mm. Preliminary results indicate that this multiple breath-hold acquisition scheme, compared with acquisition under respiration, can significantly reduce blur and ghost artifacts in 3D coronary imaging.

New technical developments in magnetic resonance imaging of epilepsy

Within the last several years a number of technical developments have been made in magnetic resonance imaging (MRI) that can potentially impact clinical and research MR imaging application in epilepsy. These include developments in instrumentation and in pulse sequences. Advances in instrumentation include higher capacity gradient systems and multiple receiver coils as directed to brain imaging. Advances in pulse sequence include use of fast or turbo-spin-echo techniques, variants of echo-planar imaging, and sequences such as fluid-attenuation inversion recovery (FLAIR) targeted to specific applications of brain imaging. The purpose of this paper is to review several of these developments.

Initial clinical experience in MR imaging of the brain with a fast fluid-attenuated inversion-recovery pulse sequence

PURPOSE

To evaluate fast fluid-attenuated inversion-recovery (FLAIR) technique for imaging brain abnormalities.

MATERIALS AND METHODS

A fast FLAIR sequence was developed that provided 36 5-mm contiguous sections in 5 minutes 8 seconds. Resulting images were compared with dual-echo T2-weighted spin-echo images of 41 consecutive patients with brain abnormalities.

RESULTS

Contrast and contrast-to-noise ratios (C/Ns) (for contrast between the lesion and background and between the lesion and cerebrospinal fluid) for fast FLAIR exceeded the corresponding values for T2-weighted spin-echo images for all but the second-echo lesion-to-background C/N. Fast FLAIR provided equivalent or greater overall lesion conspicuity and enabled greater lesion detection in 98% and 100%, respectively, of the evaluations. Fast FLAIR images more often had image artifact, but this did not interfere with image interpretation in a significantly (P < or = .05) greater number of evaluations.

CONCLUSION

Fast FLAIR provides images that are superior to proton-density- and T2-weighted images for many image quality criteria.

Echo-planar imaging of the liver with a standard MR imaging system

PURPOSE

A multisection, whole-body echo-planar imaging (EPI) sequence was developed to obtain T2-weighted images of the liver in one 18-second breath hold with a standard magnetic resonance (MR) imaging system.

MATERIALS AND METHODS

This capability was achieved by dividing the data acquisition period into eight interleaved segments rather than one or two as implemented previously with EPI systems having high-power gradient subsystems.

RESULTS

The interleaved echo-planar images had excellent depiction of anatomy and no identifiable respiratory artifact. In 26 lesions in 12 patients, the eight-shot echo-planar images (2,000/66 [repetition time msec/echo time msec]) had superior contrast compared with conventional T2-weighted spin-echo (SE) images (2,500/60) by an average factor of 1.22 +/- 0.31 (standard deviation) and an average contrast-to-noise ratio relative to conventional T2-weighted SE images of 0.85 +/- 0.22.

CONCLUSION

With a conventional MR imaging system, breath-hold T2-weighted echo-planar images of the liver are comparable in diagnostic quality to conventional T2-weighted SE images.

A monitoring, feedback, and triggering system for reproducible breath-hold MR imaging

A technique is described that provides improved reproducibility of breath-holding for MR image acquisition by monitoring the superior-inferior (S/I) position of the diaphragm. The method incorporates detection of the level of inspiration using an MR signal, rapid display to the patient of diaphragm position to enable breath-hold adjustment, and triggering of image data acquisition once appropriate position is attained. The response time of the system is short, approximately 10 ms. Studies in six volunteers using this method demonstrate a considerable decrease in the S/I range of diaphragm position over 10 consecutive periods of suspended respiration. The mean range is 1.3 mm with the system, while it is 8.3 mm without using it. It is expected that this method will be of assistance in many abdominal and cardiothoracic studies that use breath-hold techniques.

Recursive RF excitation

We have investigated the properties of a recursive process in which the output signal from a given RF excitation pulse may be used as the input (excitation) pulse of a subsequent iteration. This recursive excitation technique increases contrast and improves feature segmentation for the purpose of motion tracking.

Respiratory kinematics of the upper abdominal organs: a quantitative study

Despite the fact that respiratory motion is a major factor limiting the image quality of MR examinations in the upper abdomen, little quantitative information is available about the kinematics of visceral motion during respiration. The objective of this study was to obtain a measure of the relative longitudinal and transverse displacements of the upper abdominal organs during breathing using an MR line scan technique.

Real-time interactive color flow MR imaging

Real-time interactive color flow magnetic resonance (MR) imaging is a combination of real-time MR imaging and color encoding of velocity-induced phase angle. Flow-compensated (FC) and flow-encoded (FE) images are acquired continuously by using gradient echoes and a 12-msec repetition time. Each image is reconstructed within 200 msec of acquisition, and the FC magnitude image is displayed in gray-scale format. The phase difference between the reconstructed FC and FE images, a difference proportional to velocity along the flow-encoding direction, is encoded in color and superimposed on the gray-scale FC image. Magnitude and phase information are thus presented simultaneously. The viewer may interactively adjust many acquisition parameters during data acquisition. Experimental results of phantom and in vivo human studies validate the method. Characteristics of the color flow MR imaging technique are compared with those of duplex color ultrasound.

T2-weighted spin-echo pulse sequence with variable repetition and echo times for reduction of MR image acquisition time

Use of intraacquisition modification of pulse-sequence parameters to reduce acquisition time for conventional T2-weighted spin-echo images was evaluated. With this technique (variable-rate spin-echo pulse sequence), the repetition time and echo time (TR msec/TE msec) were reduced during imaging as a function of the phase-encoding view. To maintain T2-based contrast, TR and TE for the low-spatial-frequency views were left at their prescribed values (eg, 2,000/80). TR and TE for the high-spatial-frequency views were progressively reduced during imaging (eg, to 1,000/20). Acquisition time was reduced by as much as 25%. In one pulse sequence, the duration of multisection imaging nominally performed at TR 2,000 and with 256 phase-encoding views was reduced from 9 minutes 30 seconds to 6 minutes 30 seconds. In all sequences, edges and small structures were enhanced, and T2 contrast was somewhat decreased in high spatial frequencies. Filtering of the raw data before reconstruction can suppress these effects and provide a net increase in contrast-to-noise ratio.

Real-time interactive magnetic resonance imaging

We describe a system for performing interactive MRI in real time. Using a TR/TE 7.1/3.5 ms sequence, the operator may alter a scan parameter and observe the effects of the alteration on the image within a few hundred milliseconds. With this system, we can interactively control the oblique scan slice orientation and, using inversion pulses, the image contrast.