Prospective adaptive navigator correction for breath-hold MR coronary angiography

Current MR coronary angiography (MRCA) methods use breath-holding to minimize respiratory motion. A major limitation to this technique is misregistration between imaging slices due to breath-hold variability. Prospective adaptive correction of image location using real-time navigator measurement of diaphragm position is a potential method for improving slice registration in breath-hold MRCA. Ten subjects underwent MRCA using an ECG-gated, fat-suppressed, segmented k-space, gradient-echo sequence. Transverse and coronal images were acquired using standard breath-holding with and without prospective navigator correction. Breath-hold MRCA with prospective navigator correction resulted in a 47% reduction in craniocaudal slice registration error compared to standard breath-holding (0.9 ± 0.2 mm versus 1.7 2 0.4 mm, P = 0.04). Prospective adaptive navigator correction of image location significantly improves slice registration for breath-hold MRCA and is a promising motion correction technique for cardiac MR.     

New respiratory gating technique for whole heart cine imaging: integration of a navigator slice in steady state free precession sequences

Purpose:

To evaluate the performance of a slice navigator sequence integrated into a b‐SSFP sequence for obtaining real time respiratory self‐gated whole heart cine imaging.

Materials and Methods:

In this work, we present a novel and robust approach for respiratory motion detection by integrating a slice navigator sequence into a balanced steady state free precession (b‐SSFP) sequence, while maintaining the steady state. The slice navigator sequence is integrated into consecutive repetition times (TRs) of a b‐SSFP sequence to excite and read out a navigator slice. We performed several phantom experiments to test the performance of the slice navigator sequence. Additionally, the method was evaluated in five volunteers and compared with breathing signals obtained from conventional pencil beam navigator sequence. Finally, the navigator slice was used to obtain whole heart MR cine images.

Results:

The breathing signals detected by the proposed method showed an excellent agreement with those obtained from pencil beam navigators. Moreover, the technique was capable of removing respiratory motion artifacts with minimal distortion of the steady state. Image quality comparison showed a statistical significant improvement from a quality score of 2.1 obtained by the nonrespiratory gated images, compared to a quality score of 3.4 obtained by the respiratory gated images.

Conclusion:

This novel method represents a robust approach to estimate breathing motion during SSFP imaging. The technique was successfully applied to acquire whole heart artifact‐free cine images. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Fourier registration of three-dimensional brain MR images: Exploiting the axis of rotation

We have developed a novel algorithm to register three-dimensional MR images that have undergone rigid body motion. The most interesting feature of the algorithm is that it reduces a general three-dimensional rotation to a simple planar rotation by finding the axis of rotation. The algorithm, which is a nontrivial three-dimensional extension of existing Fourier registration algorithms, has been tested on 30 artificially misaligned MR images of a phantom, four artificially misaligned MR images of a brain, and one case of actual patient motion. The algorithm successfully registered every image. The registration error for a voxel 10 cm from the origin for the artificially misaligned phantom images was 2.8 mm at most and had a mean of 1.2 mm and standard deviation of .7 mm. The registration parameters for the images contaminated by actual patient motion were similar to that from an established image registration algorithm. The results indicate that the algorithm is accurate, reliable, and fast. The rigid body model requires the brain to be segmented from MR images of the head before registration.     

Correction of CSF motion artifact on MR images of the brain and spine by pulse sequence modification: clinical evaluation

A modification of the standard spin-echo pulse sequence designed to suppress motion artifacts was clinically evaluated on T2-weighted MR images of the cervicocranial region. A retrospective study involving 40 patients, half of whom were examined with a standard T2-weighted multislice spin-echo sequence and half of whom were examined with a gradient waveform modification of the same sequence, uniformly demonstrated restoration of CSF signal intensity on images obtained with the gradient modified sequence. The cervical subarachnoid spaces, cisterna magna, medullary cistern, pontine cistern, fourth ventricle, and aqueduct were more consistently and brightly represented. However, the phase-encoding artifacts arising from CSF motion were not significantly reduced by using the gradient waveform modified pulse sequence. Digital subtraction of an image obtained with the standard sequence from an image of the same slice with the gradient modified sequence provides a direct image representation of CSF flow.     

Keyhole method for high-speed human cardiac cine MR imaging.

Although electrocardiographic (ECG)-gated magnetic resonance (MR) imaging is widely used for cardiac imaging, it has several disadvantages, such as long imaging time, respiratory artifacts, and motion artifacts induced by arrhythmia. An MR image can be acquired within about 0.3 seconds by using a fast gradient-echo imaging method. When this method is continuously applied, only two to three images can be obtained during a single cardiac cycle. The goal of this study is to obtain cine MR images in a single cardiac cycle using fast gradient-echo imaging combined with the "keyhole" method. The optimal conditions for the keyhole method for cardiac cine imaging were obtained by computer simulation based on a simplified cardiac model. When the read-out direction was set parallel to the cardiac short axis, left ventricular motion was almost correctly reproduced by the keyhole method with acquisition time reduced to one-fourth. J. Magn. Reson. Imaging 1999;10:778-783.     

Isotropic CT examination of abdomen and pelvis diagnostic quality of reformat

RATIONALE AND OBJECTIVES: To evaluate the image quality of axial and coronal reformats obtained from isotropic resolution abdomino-pelvic computed tomography (CT) examinations. MATERIALS AND METHODS: Thirty consecutive patients with intravenous contrast-enhanced abdomino-pelvic CT examinations (Brilliance 40, Philips Medical Systems, Cleveland, OH) were enrolled for the study. The raw data were reconstructed into two sets of source axial images: 0.9-mm slice widths with 0.45-mm reconstruction interval (isotropic resolution) and 4-mm slice widths with 3-mm reconstruction interval (anisotropic resolution: group A). Isotropic data set was reformatted into axial and coronal stacks (groups B and C, respectively) with 4-mm slice width and 3-mm interval. Three independent readers evaluated stacks A to C using a 3-point scale for resolution of hepatic vessels, edge sharpness of kidneys, respiratory motion artifact, reconstruction artifact, noise, and overall image quality. RESULTS: There was no statistical difference among the groups A to C for vessel resolution, motion artifact, noise, and overall quality. The scores given to group C were significantly lower than those to groups A and B for reconstruction artifacts. There was no difference among groups A to C for overall impression of image quality. The interreader agreements were excellent for axial images (groups A and B) and moderate for coronal reformats. CONCLUSION: Isotropic scanning of the abdomen and pelvis allows creation of reformats with similar image quality as similar thickness axial source images. These reformats are of sufficient quality to form the basis of clinical interpretation.     

Reduction in flow artifacts by using interleaved data acquisition in segmented balanced steady-state free precession cardiac MRI.

Balanced steady-state free precession (SSFP) magnetic resonance (MR) imaging is feasible for cine cardiac images because of the high contrast between myocardium and blood pool and robustness to rapid blood flow. Nonetheless, the flow artifacts are often observed because of off-resonance effects and to in-flow effects of the blood flow. Although reshimming the gradients or readjusting the center frequency reduces the artifacts, the technique can be susceptible for respiratory and cardiac motion and operator-dependent. The purpose of this study is to use another MR imaging technique for the reduction in the flow artifacts in the heart: odd-even interleaved data acquisition in segmented balanced SSFP imaging. The flow artifacts in the ventricle, ghost outside the heart, and visualization of the myocardial border were visually compared between sequential and odd-even interleaved k-space data acquisitions in cine balanced SSFP cardiac MR imaging. The odd-even interleaved k-space data acquisition significantly reduced dark flow artifacts in the left ventricle, improved the visualization of the myocardial border, and was easily installed. This imaging technique should be applied to cine segmented balanced SSFP cardiac MR imaging.     

Motion correction in fMRI via registration of individual slices into an anatomical volume.

An automated retrospective image registration based on mutual information is adapted to a multislice functional magnetic resonance imaging (fMRI) acquisition protocol to provide accurate motion correction. Motion correction is performed by mapping each slice to an anatomic volume data set acquired in the same fMRI session to accommodate inter-slice head motion. Accuracy of the registration parameters was assessed by registration of simulated MR data of the known truth. The widely used rigid body volume registration approach based on stacked slices from the time series data may hinder statistical accuracy by introducing inaccurate assumptions of no motion between slices for multislice fMRI data. Improved sensitivity and specificity of the fMRI signal from mapping-each-slice-to-volume method is demonstrated in comparison with a stacked-slice correction method by examining functional data from two normal volunteers. The data presented in a standard anatomical coordinate system suggest the reliability of the mapping-each-slice-to-volume method to detect the activation signals consistent between the two subjects.     

Prospective registration of human head magnetic resonance images for reproducible slice positioning using localizer images

PURPOSE: To facilitate assessing brain tumor growth and progression of stroke lesions by reproducible slice positioning in human head magnetic resonance (MR) images, a method for prospective registration is proposed that adjusts the image slice position without moving the patient and with no additional scans. MATERIALS AND METHODS: The gradient reference frame of follow-up examinations was adjusted to achieve the same image slice positioning relative to the patient as in the previous examination. The three-dimensional geometrical transformation parameters for the gradients were determined using two-dimensional image registration of three orthogonal localizer images. The method was developed and evaluated using a phantom with arbitrarily adjustable position. Feasibility for in vivo applications was demonstrated with brain MR imaging (MRI) of healthy volunteers. RESULTS: Standard retrospective registration was used for assessing the quality of the method. The accuracy of the realignment was 0.0 mm +/- 1.2 mm and -0.2 degrees +/- 0.9 degrees (mean +/- SD) in phantom experiments. In 10 examinations of volunteers, misalignments up to 49.2 mm and 21 degrees were corrected. The accuracy of the realignment after prospective registration was 0.1 mm +/- 1.5 mm and 0.2 degrees +/- 1.5 degrees. CONCLUSION: Image-based prospective registration using localizer images of the pre- and postexaminations is a robust method for reproducible slice positioning.     

Noise-adaptive nonlinear diffusion filtering of MR images with spatially varying noise levels.

Anisotropic diffusion filtering is widely used for MR image enhancement. However, the anisotropic filter is nonoptimal for MR images with spatially varying noise levels, such as images reconstructed from sensitivity-encoded data and intensity inhomogeneity-corrected images. In this work, a new method for filtering MR images with spatially varying noise levels is presented. In the new method, a priori information regarding the image noise level spatial distribution is utilized for the local adjustment of the anisotropic diffusion filter. Our new method was validated and compared with the standard filter on simulated and real MRI data. The noise-adaptive method was demonstrated to outperform the standard anisotropic diffusion filter in both image error reduction and image signal-to-noise ratio (SNR) improvement. The method was also applied to inhomogeneity-corrected and sensitivity encoding (SENSE) images. The new filter was shown to improve segmentation of MR brain images with spatially varying noise levels.     

Co-registration of x-ray and MR fields of view in a hybrid XMR system

PURPOSE: To validate one possible function of a real-time x-ray/MR (XMR) interface in a hybrid XMR system using x-ray images as "scouts" to prescribe the MR slices. MATERIALS AND METHODS: The registration process consists of two steps: 1) calibration, in which the system's geometric parameters are found from fiducial-based registration; and 2) application, in which the x-ray image of a target structure and the estimated geometric parameters are used to prescribe an MR slice to observe the target structure. Errors from the noise in the location of the fiducial markers, and MR gradient nonlinearity were studied. Computer simulations were used to provide guidelines for fiducial marker placement and tolerable error estimation. A least-squares-based correction method was developed to reduce errors from gradient nonlinearity. RESULTS: In simulations with both sources of errors and the correction for gradient nonlinearity, the use of 16 fiducial markers yielded a mean error of about 0.4 mm over a 7200 cm(3) volume. Phantom scans showed that the prescribed target slice hit most of the target line, and that the length visualized was improved with the least-squares correction. CONCLUSION: The use of 16 fiducial markers to co-register XMR FOVs can offer satisfactory accuracy in both simulations and experiments.     

Highly accelerated real‐time cardiac cine MRI using k–t SPARSE‐SENSE

For patients with impaired breath‐hold capacity and/or arrhythmias, real‐time cine MRI may be more clinically useful than breath‐hold cine MRI. However, commercially available real‐time cine MRI methods using parallel imaging typically yield relatively poor spatio‐temporal resolution due to their low image acquisition speed. We sought to achieve relatively high spatial resolution (～2.5 × 2.5 mm²) and temporal resolution (～40 ms), to produce high‐quality real‐time cine MR images that could be applied clinically for wall motion assessment and measurement of left ventricular function. In this work, we present an eightfold accelerated real‐time cardiac cine MRI pulse sequence using a combination of compressed sensing and parallel imaging (k‐t SPARSE‐SENSE). Compared with reference, breath‐hold cine MRI, our eightfold accelerated real‐time cine MRI produced significantly worse qualitative grades (1–5 scale), but its image quality and temporal fidelity scores were above 3.0 (adequate) and artifacts and noise scores were below 3.0 (moderate), suggesting that acceptable diagnostic image quality can be achieved. Additionally, both eightfold accelerated real‐time cine and breath‐hold cine MRI yielded comparable left ventricular function measurements, with coefficient of variation <10% for left ventricular volumes. Our proposed eightfold accelerated real‐time cine MRI with k–t SPARSE‐SENSE is a promising modality for rapid imaging of myocardial function. J. Magn. Reson. Imaging 2013. © 2012 Wiley Periodicals, Inc.     

Forty-millisecond MR imaging of the abdomen at 2.0 T

The ability of an ultrafast magnetic resonance (MR) imaging technique to provide abdominal MR images free of motion artifacts was studied. Individual T2-weighted transverse MR images were acquired in as little as 40 msec on a whole-body system operating at 2.0 T. Clinical evaluation was undertaken with fat-suppressed images in which only protons of water molecules contributed to image signal intensity. The ultrafast MR images were compared with conventional MR images obtained at 0.6 T. In 22 patients and two healthy volunteers, ultrafast MR images were of diagnostic quality and free of motion artifacts. Images obtained at an echo time (TE) of 30 msec (imaging time, 40 msec) had liver signal-to-noise ratios of 56.3 +/- 22.6 (n = 19). Because of a smaller data matrix, ultrafast MR images had soft-tissue interfaces that were less sharp than those of the highest-quality conventional MR images in which no motion artifacts were present. However, ultrafast MR images demonstrated high T2-dependent soft-tissue contrast, and pathologic and normal anatomies were readily detected with both imaging techniques. This ultrafast imaging technique has significant promise in whole-body MR imaging, in which motion artifacts often degrade image quality.     

Automatic registration of MR and SPECT images for treatment planning in prostate cancer

RATIONALE AND OBJECTIVES: To aid in surgical and radiation therapy planning for prostate adenocarcinoma, a general-purpose automatic registration method that is based on mutual information was used to align magnetic resonance (MR) images and single photon emission computed tomographic (SPECT) images of the pelvis and prostate. MATERIALS AND METHODS: The authors assessed the effects of various factors on alignment between pairs of MR and SPECT images, including the use of particular pulse sequences in MR imaging, image voxel intensity scaling, the use of different regions on the MR-SPECT histogram, spatial masking of nonoverlapping visual data between images, and multiresolution optimization. A mutual information algorithm was used as the cost function for automatic registration. Automatic registration was deemed acceptable when it resulted in a transformation with less than 2 voxel units (6 mm) difference in translation and less than 2 degree difference in rotation from that obtained with manual registration performed independently by nuclear medicine radiologists. RESULTS: Paired sets of MR and SPECT image volumes from four of five patients were successfully registered. For successful registration, MR images must be optimal and registration must be performed at full spatial resolution and at the full intensity range. Masking, cropping, and the normalization of mutual information, used to register partially overlapping MR-SPECT volumes, were not successful. Multiresolution optimization had little effect on the accuracy and speed of the registration. CONCLUSION: Automatic registration between MR and SPECT images of the pelvis can be achieved when data acquisition and image processing are performed properly. It should prove useful for prostate cancer diagnosis, staging, and treatment planning.     

Abdominal phase-contrast MR angiography: Breath-Hold versus non-breath-hold techniques

Breath-hold magnetic resonance (MR) imaging is now replacing many non-breath-hold pulse sequences in the upper abdomen because of faster imaging times and improved image quality. The authors compared non-breath-hold cine phase-contrast (PC) and breath hold 2D phase-contrast (2DPC) magnetic resonance (MR) angiograms of the main portal vein (MPV) and superior mesenteric artery (SMA) in 12 volunteers. All angiograms were graded in overall image quality, vessel conspicuity, and signal-to-noise ratios (SNR). In the MPV MR angiograms, the breath-hold 2DPC sequence produced better images than the non-breath-hold cine PC sequence as graded by overall image quality (P=.016) and SNR (P=.004). Conversely, in the SMA MR angiograms, the non-breath-hold cine PC sequence produced better images than the breath-hold sequence in terms of overall image quality (P=.008) and SNR (P=.008). By reducing the most significant cause of image artifacts, (ie, using a breath-hold 2DPC sequence to decrease respiratory misregistration of the MPV, and using a cardiac-gated cine PC sequence to minimize pulsatile artifacts of the SMA), one can clearly optimize the quality of MR angiography.     

Contrast-enhanced T1-weighted fluid-attenuated inversion-recovery BLADE magnetic resonance imaging of the brain: an alternative to spin-echo technique for detection of brain lesions in the unsedated pediatric patient?

RATIONALE AND OBJECTIVES: We compared contrast-enhanced T1-weighted magnetic resonance (MR) imaging of the brain using different types of data acquisition techniques: periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER, BLADE) imaging versus standard k-space sampling (conventional spin-echo pulse sequence) in the unsedated pediatric patient with focus on artifact reduction, overall image quality, and lesion detectability. MATERIALS AND METHODS: Forty-eight pediatric patients (aged 3 months to 18 years) were scanned with a clinical 1.5-T whole body MR scanner. Cross-sectional contrast-enhanced T1-weighted spin-echo sequence was compared to a T1-weighted dark-fluid fluid-attenuated inversion-recovery (FLAIR) BLADE sequence for qualitative and quantitative criteria (image artifacts, image quality, lesion detectability) by two experienced radiologists. Imaging protocols were matched for imaging parameters. Reader agreement was assessed using the exact Bowker test. RESULTS: BLADE images showed significantly less pulsation and motion artifacts than the standard T1-weighted spin-echo sequence scan. BLADE images showed statistically significant lower signal-to-noise ratio but higher contrast-to-noise ratios with superior gray-white matter contrast. All lesions were demonstrated on FLAIR BLADE imaging, and one false-positive lesion was visible in spin-echo sequence images. CONCLUSION: BLADE MR imaging at 1.5 T is applicable for central nervous system imaging of the unsedated pediatric patient, reduces motion and pulsation artifacts, and minimizes the need for sedation or general anesthesia without loss of relevant diagnostic information.     

POCS‐enhanced correction of motion artifacts in parallel MRI

A new method for correction of MRI motion artifacts induced by corrupted k‐space data, acquired by multiple receiver coils such as phased arrays, is presented. In our approach, a projections onto convex sets (POCS)‐based method for reconstruction of sensitivity encoded MRI data (POCSENSE) is employed to identify corrupted k‐space samples. After the erroneous data are discarded from the dataset, the artifact‐free images are restored from the remaining data using coil sensitivity profiles. The error detection and data restoration are based on informational redundancy of phased‐array data and may be applied to full and reduced datasets. An important advantage of the new POCS‐based method is that, in addition to multicoil data redundancy, it can use a priori known properties about the imaged object for improved MR image artifact correction. The use of such information was shown to improve significantly k‐space error detection and image artifact correction. The method was validated on data corrupted by simulated and real motion such as head motion and pulsatile flow. Magn Reson Med 63:1104–1110, 2010. © 2010 Wiley‐Liss, Inc.     

Free-breathing imaging of the heart using 2D cine-GRICS (generalized reconstruction by inversion of coupled systems) with assessment of ventricular volumes and function

Purpose:

To assess cardiac function by means of a novel free‐breathing cardiac magnetic resonance imaging (MRI) strategy.

Materials and Methods:

A stack of ungated 2D steady‐state free precession (SSFP) slices was acquired during free breathing and reconstructed as cardiac cine imaging based on the generalized reconstruction by inversion of coupled systems (GRICS). A motion‐compensated sliding window approach allows reconstructing cine movies with most motion artifacts cancelled. The proposed reconstruction uses prior knowledge from respiratory belts and electrocardiogram recordings and features a piecewise linear model that relates the electrocardiogram signal to cardiac displacements. The free‐breathing protocol was validated in six subjects against a standard breath‐held protocol.

Results:

Image sharpness, as assessed by the image gradient entropy, was comparable to that of breath‐held images and significantly better than in uncorrected images. Volumetric parameters of cardiac function in the left ventricle (LV) and right ventricle (RV) were similar, including end‐systolic volumes, end‐diastolic volumes and mass, stroke volumes, and ejection fractions (with differences of 3% ± 2.4 in the LV and 2.9% ± 4.4 in the RV). The duration of the free‐breathing protocol was nearly the same as the breath‐held protocol.

Conclusion:

Free‐breathing cine‐GRICS enables accurate assessment of volumetric parameters of cardiac function with efficient correction of motion. J. Magn. Reson. Imaging 2012;340‐351. © 2011 Wiley Periodicals, Inc.     

Automatic method to assess local CT-MR imaging registration accuracy on images of the head

BACKGROUND AND PURPOSE: Precise registration of CT and MR images is crucial in many clinical cases for proper diagnosis, decision making or navigation in surgical interventions. Various algorithms can be used to register CT and MR datasets, but prior to clinical use the result must be validated. To evaluate the registration result by visual inspection is tiring and time-consuming. We propose a new automatic registration assessment method, which provides the user a color-coded fused representation of the CT and MR images, and indicates the location and extent of poor registration accuracy. METHODS: The method for local assessment of CT-MR registration is based on segmentation of bone structures in the CT and MR images, followed by a voxel correspondence analysis. The result is represented as a color-coded overlay. The algorithm was tested on simulated and real datasets with different levels of noise and intensity non-uniformity. RESULTS: Based on tests on simulated MR imaging data, it was found that the algorithm was robust for noise levels up to 7% and intensity non-uniformities up to 20% of the full intensity scale. Due to the inability to distinguish clearly between bone and cerebro-spinal fluids in the MR image (T1-weighted), the algorithm was found to be optimistic in the sense that a number of voxels are classified as well-registered although they should not. However, nearly all voxels classified as misregistered are correctly classified. CONCLUSION: The proposed algorithm offers a new way to automatically assess the CT-MR image registration accuracy locally in all the areas of the volume that contain bone and to represent the result with a user-friendly, intuitive color-coded overlay on the fused dataset.     

Dynamic display of the temporomandibular joint meniscus by using "fast-scan" MR imaging

In order to display temporomandibular joint (TMJ) images as a dynamic or motion study, a protocol was developed to obtain MR images of the TMJ in multiple phases of opening by using the "fast-scanning" capabilities of the GE Signa MR scanner. To facilitate this procedure a prototype device was also developed to passively open the patient's mouth from resting (closed) to fully open in user-defined increments (minimum 1 mm). MR imaging (surface coil) was carried out at each successive station using the GRASS, pulse-sequence data base of the GE Signa system operating at 1.5 T. Image-acquisition parameters were optimized in studies of cadavers and volunteers to obtain the clearest delineation of the TMJ meniscus and to determine any potential tradeoffs between total imaging time per slice (image quality), patient tolerance, and other practical considerations. For viewing, the images were sequentially placed in the video memory of the operating console and displayed in a back-and-forth-closed cine loop or "movie" mode at variable (operator-selectable) speeds. The dynamic sequences in four individuals were compared with static open- and closed-mouth views obtained with routine pulse sequences. Any single image from the dynamic display lacked the high resolution of the routine static images because of technical limitations of the pulse-sequence data base. However, in the movie mode the pertinent joint structures (such as meniscus and condyle) were clearly delineated, as were several of the important muscles of mastication. The anterior motion (translation) of the meniscus during jaw opening is particularly evident and suggests great potential for functional evaluation. These results show the feasibility of dynamic TMJ imaging with MR. The added information of the cine display potentially complements the routine static images and may prove extremely valuable in the assessment of TMJ dysfunction.