Multi-echo segmented k-space imaging: an optimized hybrid sequence for ultrafast cardiac imaging

Multi-echo balanced SSFP with a sequential phase-encoding order for functional MR imaging at 7T

PURPOSE

To develop a 2D multi-echo passband balanced SSFP (bSSFP) sequence using an echo-train readout with a sequential phase-encoding order (sequential multi-echo bSSFP), and evaluate its performance in fast functional brain imaging at 7 T.

METHODS

As images of sequential multi-echo bSSFP exhibit multiple ghosts due to periodic k-space modulations, a GRAPPA-based reconstruction method was proposed to eliminate ghosting artifacts. MRI experiments were performed to compare the image quality of multi-echo bSSFP and conventional single-echo bSSFP. Submillimeter-resolution fMRI using a checkerboard visual stimulus was conducted to compare the activation characteristics of multi-echo bSSFP, conventional single-echo bSSFP and standard gradient-echo EPI (GE-EPI).

RESULTS

A higher mean structural similarity index was found between images of single-echo bSSFP and multi-echo bSSFP with a shorter echo train length (ETL). Multi-echo bSSFP (ETL = 3) showed higher temporal SNR (tSNR) values than GRAPPA-accelerated single-echo bSSFP (R = 2). In submillimeter-resolution fMRI experiments, multi-echo bSSFP (ETL = 3) approached the imaging speed of GRAPPA-accelerated single-echo bSSFP (R = 2), but without tSNR penalty and reduced activation due to acceleration. The median t-value and the number of significantly activated voxels were comparable between GE-EPI and multi-echo bSSFP (ETL = 3) that provides virtually distortion-free functional images and inherits the activation patterns of conventional bSSFP.

CONCLUSION

Sequential multi-echo bSSFP (ETL = 3) is suitable for fast fMRI with submillimeter in-plane resolution, and offers an option to accelerate bSSFP imaging without tSNR penalty like parallel imaging.

Optimized decoupling schemes in ultrafast HSQC experiments

Ultrafast (UF) 2D NMR, which enables the acquisition of 2D spectra within a single scan, is nowadays used in a large array of applications. However, the use of UF-HSQC experiments is still limited by the need to compromise between spectral widths and resolution. Interleaved acquisitions can overcome this limitation, albeit at the cost of strong artifacts. We show that the occurrence of these artifacts is mainly related to the discontinuous decoupling scheme which is generally used in UF-HSQC. In this paper, four continuous decoupling schemes using adiabatic pulses are optimized for this kind of experiments, giving access to full-range UF-HSQC spectra.

Quantification of regional myocardial wall motion by cardiovascular magnetic resonance

Cardiovascular magnetic resonance (CMR) is a versatile tool that also allows comprehensive and accurate measurement of both global and regional myocardial contraction. Quantification of regional wall motion parameters, such as strain, strain rate, twist and torsion, has been shown to be more sensitive to early-stage functional alterations. Since the invention of CMR tagging by magnetization saturation in 1988, several CMR techniques have been developed to enable the measurement of regional myocardial wall motion, including myocardial tissue tagging, phase contrast mapping, displacement encoding with stimulated echoes (DENSE), and strain encoded (SENC) imaging. These techniques have been developed with their own advantages and limitations. In this review, two widely used and closely related CMR techniques, i.e., tissue tagging and DENSE, will be discussed from the perspective of pulse sequence development and image-processing techniques. The clinical and preclinical applications of tissue tagging and DENSE in assessing wall motion mechanics in both normal and diseased hearts, including coronary artery diseases, hypertrophic cardiomyopathy, aortic stenosis, and Duchenne muscular dystrophies, will be discussed.

Cost-effectiveness of initial stress cardiovascular MR, stress SPECT or stress echocardiography as a gate-keeper test, compared with upfront invasive coronary angiography in the investigation and management of patients with stable chest pain: mid-term outcomes from the CECaT randomised controlled trial

OBJECTIVES

To compare outcomes and cost-effectiveness of various initial imaging strategies in the management of stable chest pain in a long-term prospective randomised trial.

SETTING

Regional cardiothoracic referral centre in the east of England.

PARTICIPANTS

898 patients (69% man) entered the study with 869 alive at 2 years of follow-up. Patients were included if they presented for assessment of stable chest pain with a positive exercise test and no prior history of ischaemic heart disease. Exclusion criteria were recent infarction, unstable symptoms or any contraindication to stress MRI.

PRIMARY OUTCOME MEASURES

The primary outcomes of this follow-up study were survival up to a minimum of 2 years post-treatment, quality-adjusted survival and cost-utility of each strategy.

RESULTS

898 patients were randomised. Compared with angiography, mortality was marginally higher in the groups randomised to cardiac MR (HR 2.6, 95% CI 1.1 to 6.2), but similar in the single photon emission CT-methoxyisobutylisonitrile (SPECT-MIBI; HR 1.0, 95% CI 0.4 to 2.9) and ECHO groups (HR 1.6, 95% CI 0.6 to 4.0). Although SPECT-MIBI was marginally superior to other non-invasive tests there were no other significant differences between the groups in mortality, quality-adjusted survival or costs.

CONCLUSIONS

Non-invasive cardiac imaging can be used safely as the initial diagnostic test to diagnose coronary artery disease without adverse effects on patient outcomes or increased costs, relative to angiography. These results should be interpreted in the context of recent advances in imaging technology.

TRIAL REGISTRATION

ISRCTN 47108462, UKCRN 3696.

Myocardial tagging by cardiovascular magnetic resonance: evolution of techniques--pulse sequences, analysis algorithms, and applications

Cardiovascular magnetic resonance (CMR) tagging has been established as an essential technique for measuring regional myocardial function. It allows quantification of local intramyocardial motion measures, e.g. strain and strain rate. The invention of CMR tagging came in the late eighties, where the technique allowed for the first time for visualizing transmural myocardial movement without having to implant physical markers. This new idea opened the door for a series of developments and improvements that continue up to the present time. Different tagging techniques are currently available that are more extensive, improved, and sophisticated than they were twenty years ago. Each of these techniques has different versions for improved resolution, signal-to-noise ratio (SNR), scan time, anatomical coverage, three-dimensional capability, and image quality. The tagging techniques covered in this article can be broadly divided into two main categories: 1) Basic techniques, which include magnetization saturation, spatial modulation of magnetization (SPAMM), delay alternating with nutations for tailored excitation (DANTE), and complementary SPAMM (CSPAMM); and 2) Advanced techniques, which include harmonic phase (HARP), displacement encoding with stimulated echoes (DENSE), and strain encoding (SENC). Although most of these techniques were developed by separate groups and evolved from different backgrounds, they are in fact closely related to each other, and they can be interpreted from more than one perspective. Some of these techniques even followed parallel paths of developments, as illustrated in the article. As each technique has its own advantages, some efforts have been made to combine different techniques together for improved image quality or composite information acquisition. In this review, different developments in pulse sequences and related image processing techniques are described along with the necessities that led to their invention, which makes this article easy to read and the covered techniques easy to follow. Major studies that applied CMR tagging for studying myocardial mechanics are also summarized. Finally, the current article includes a plethora of ideas and techniques with over 300 references that motivate the reader to think about the future of CMR tagging.

Two-dimensional phase correction method for single and multi-shot echo planar imaging

Ghost artifacts are a serious issue in single and multi-shot echo planar imaging. Because of these coherent artifacts, it is essential to consistently suppress the ghosts. In this article, we present a phase correction algorithm that achieves excellent ghost suppression for single and multi-shot echo planar imaging. The phase correction is performed along both the x (read) direction and y (phase) direction. To this end, we apply a double field of view prescan and compute the phase required for ghost suppression. This phase is fitted to a 2D polynomial. The fitted phase is used to correct the echo planar imaging images. The correction algorithm can be used with any readout gradient polarities and any number of shots. A flow chart of the correction method is provided to better clarify the full process. Finally, phantom and volunteer images demonstrate the improvement of artifact suppression obtained with this algorithm over conventional phase correction methods.

Whole-heart contrast-enhanced coronary magnetic resonance angiography using gradient echo interleaved EPI

Whole-heart coronary MR angiography (MRA) is a promising method for detecting coronary artery disease. However, the imaging time is relatively long (on the order of 10-15 min). Such a long imaging time may result in patient discomfort and compromise the robustness of whole-heart coronary MRA due to increased respiratory and cardiac motion artifacts. The goal of this study was to optimize a gradient echo interleaved echo planar imaging (GRE-EPI) acquisition scheme for reducing the imaging time of contrast-enhanced whole-heart coronary MRA. Numerical simulations and phantom studies were used to optimize the GRE-EPI sequence parameters. Healthy volunteers were scanned with both the proposed GRE-EPI sequence and a 3D TrueFISP sequence for comparison purposes. Slow infusion (0.5 cc/sec) of Gd-DTPA was used to enhance the signal-to-noise ratio (SNR) of the GRE-EPI acquisition. Whole-heart images with the GRE-EPI technique were acquired with a true resolution of 1.0 x 1.1 x 2.0 mm(3) in an average scan time of 4.7 +/- 0.7 min with an average navigator efficiency of 44 +/- 6%. The GRE-EPI acquisition showed excellent delineation of all the major coronary arteries with scan time reduced by a factor of 2 compared with the TrueFISP acquisition.

Improved cine displacement-encoded MRI using balanced steady-state free precession and time-adaptive sensitivity encoding parallel imaging at 3 T

Cine displacement-encoded MRI is a promising modality for quantifying regional myocardial function. However, it has two major limitations: low signal-to-noise ratio (SNR) and data acquisition efficiency. The purpose of this study was to incrementally improve the SNR and the data acquisition efficiency of cine displacement-encoded MRI through the combined use of balanced steady-state free precession (b-SSFP) imaging, 3T imaging, echo-combination image reconstruction, and time-adaptive sensitivity encoding (TSENSE) parallel imaging. Phantom experiments were performed to empirically determine the optimal excitation angle (alpha) and to estimate the measurement errors in the presence of 130 Hz peak-to-peak static magnetic field (B0) variation. The optimal alpha was determined to be 20 degrees . The intrinsic phase correction in the echo-combination effectively reduced the phase error, which produced small displacement errors (0.11 versus 0.11 mm) and negligible strain errors (-0.001 versus -0.002). Six healthy volunteers were imaged in three short-axis levels of the heart to evaluate the SNR and the relative accuracy of strain calculations. Compared with the 24-heartbeat cine echo-planar imaging acquisition, the 24-heartbeat non-accelerated b-SSFP acquisition yielded approximately 65% higher SNR, and the 12-heartbeat twofold accelerated b-SSFP acquisition yielded approximately 28% higher SNR. The 12-heartbeat twofold accelerated b-SSFP acquisition yielded functional maps with spatial resolution of 3.6 x 3.6 mm, temporal resolution of 35 ms, and relatively high SNR (31.2 +/- 5.4 at end diastole; 19.9 +/- 3.6 at end systole; 10.3 +/- 1.1 at late diastole; mean +/- SD). The left ventricular strain values between the non-accelerated and twofold accelerated b-SSFP acquisitions correlated strongly (slope = 0.99; bias = 0.00; R2 = 0.91) and were in excellent agreement. The combined implementation of b-SSFP imaging, 3T imaging, echo-combination image reconstruction, and TSENSE parallel imaging can be used to incrementally improve the cine displacement-encoded MRI pulse sequence.

Phased array ghost elimination

Parallel imaging may be applied to cancel ghosts caused by a variety of distortion mechanisms, including distortions such as off-resonance or local flow, which are space variant. Phased array combining coefficients may be calculated that null ghost artifacts at known locations based on a constrained optimization, which optimizes SNR subject to the nulling constraint. The resultant phased array ghost elimination (PAGE) technique is similar to the method known as sensitivity encoding (SENSE) used for accelerated imaging; however, in this formulation is applied to full field-of-view (FOV) images. The phased array method for ghost elimination may result in greater flexibility in designing acquisition strategies. For example, in multi-shot EPI applications ghosts are typically mitigated by the use of an interleaved phase encode acquisition order. An alternative strategy is to use a sequential, non-interleaved phase encode order and cancel the resultant ghosts using PAGE parallel imaging. Cancellation of ghosts by means of phased array processing makes sequential, non-interleaved phase encode acquisition order practical, and permits a reduction in repetition time, TR, by eliminating the need for echo-shifting. Sequential, non-interleaved phase encode order has benefits of reduced distortion due to off-resonance, in-plane flow and EPI delay misalignment. Furthermore, the use of EPI with PAGE has inherent fat-water separation and has been used to provide off-resonance correction using a technique referred to as lipid elimination with an echo-shifting N/2-ghost acquisition (LEENA), and may further generalized using the multi-point Dixon method. Other applications of PAGE include cancelling ghosts which arise due to amplitude or phase variation during the approach to steady state. Parallel imaging requires estimates of the complex coil sensitivities. In vivo estimates may be derived by temporally varying the phase encode ordering to obtain a full k-space dataset in a scheme similar to the autocalibrating TSENSE method. This scheme is a generalization of the UNFOLD method used for removing aliasing in undersampled acquisitions. The more general scheme may be used to modulate each EPI ghost image to a separate temporal frequency as described in this paper.

MRI of Myocardial Perfusion

An overwhelming number of myocardial perfusion studies are done by nuclear isotope imaging. Magnetic resonance imaging during the first pass of an injected, contrast bolus has some significant advantages for detection of blood flow deficits, namely higher spatial resolution, absence of ionizing radiation, and speed of the test. Previous clinical studies have demonstrated that excellent sensitivity and specificity can be achieved with MR myocardial perfusion imaging for detecting coronary artery disease, and assessment of patients with acute chest pain. Furthermore, an absolute quantification of myocardial blood flow is feasible, as was demonstrated by comparison of MR perfusion imaging, to measurements with isotope labeled microspheres in experimental models. An integrated assessment of perfusion, function, and viability, is thus feasible by MRI to answer important clinical challenges such as the identification of stunned or hibernating, but viable myocardium.

Myocardial first pass perfusion: steady-state free precession versus spoiled gradient echo and segmented echo planar imaging

The imaging sequences used in first pass (FP) perfusion to date have important limitations in contrast-to-noise ratio (CNR), temporal and spatial resolution, and myocardial coverage. As a result, controversy exists about optimal imaging strategies for FP myocardial perfusion. Since imaging performance varies from subject to subject, it is difficult to form conclusions without direct comparison of different sequences in the same subject. The purpose of this study was to directly compare the saturation recovery SSFP technique to other more commonly used myocardial first pass perfusion techniques, namely spoiled GRE and segmented EPI. Differences in signal-to-noise ratio (SNR), CNR, relative maximal upslope (RMU) of signal amplitude, and artifacts at comparable temporal and spatial resolution among the three sequences were investigated in computer simulation, contrast agent doped phantoms, and 16 volunteers. The results demonstrate that SSFP perfusion images exhibit an improvement of approximately 77% in SNR and 23% in CNR over spoiled GRE and 85% SNR and 50% CNR over segmented EPI. Mean RMU was similar between SSFP and spoiled GRE, but there was a 58% increase in RMU with SSFP versus segmented EPI.

Tagged magnetic resonance imaging of the heart: a survey

Magnetic resonance imaging (MRI) of the heart with magnetization tagging provides a potentially useful new way to assess cardiac mechanical function, through revealing the local motion of otherwise indistinguishable portions of the heart wall. While still an evolving area, tagged cardiac MRI is already able to provide novel quantitative information on cardiac function. Exploiting this potential requires developing tailored methods for both imaging and image analysis. In this paper, we review some of the progress that has been made in developing such methods for tagged cardiac MRI, as well as some of the ways these methods have been applied to the study of cardiac function.

Time-resolved undersampled projection reconstruction magnetic resonance imaging of the peripheral vessels using multi-echo acquisition

The hybrid projection reconstruction (PR) imaging provides high temporal resolution through an undersampled PR acquisition for the in-plane dimensions and Cartesian slice encoding for the through-plane dimension. The undersampling of projection data introduces streak artifact, which may severely compromise image quality. This study reports on a combination of multi-echo acquisition with time-resolved undersampled PR imaging and its application to peripheral magnetic resonance angiography. Multi-echo acquisition improved imaging speed effectively, thereby reducing the undersampling streak artifact and improving the temporal resolution. The gradient distortion was reduced through gradient calibration and accurate k-space trajectory measurement.

Signal-to-noise ratio behavior of steady-state free precession

Steady-state free precession (SSFP) is a rapid gradient-echo imaging technique that has recently gained popularity and is used in a variety of applications, including cardiac and real-time imaging, because of its high signal and favorable contrast between blood and myocardium. The purpose of this work was to examine the signal-to-noise ratio (SNR) behavior of images acquired with SSFP, and the dependence of SNR on imaging parameters such as TR, bandwidth, and image resolution, and the use of multi-echo sequences. In this work it is shown that the SNR of SSFP sequences is dependent only on pulse sequence efficiency, voxel dimensions, and relaxation parameters (T1 and T2). Notably, SNR is insensitive to bandwidth unless increases in bandwidth significantly decrease efficiency. Finally, we examined the relationship between pulse sequence performance (TR and efficiency) and gradient performance (maximum gradient strength and slew rate) for several imaging scenarios, including multi-echo sequences, to determine the optimum matching of maximum gradient strength and slew rate for gradient hardware designs. For standard modern gradient hardware (40 mT/m and 150 mT/m/ms), we found that the maximum gradient strength is more than adequate for the imaging resolution that is commonly encountered with rapid scouting (3 mm x 4 mm x 10 mm voxel). It is well matched for typical CINE and real-time cardiac imaging applications (1.5 mm x 2 mm x 6 mm voxel), and is inadequate for optimal matching with slew rate for high-resolution applications such as musculoskeletal imaging (0.5 x 0.8 x 3 mm voxel). For the lower-resolution methods, efficiency could be improved with higher slew rates; this provokes interest in designing methods for limiting dB/dt peripherally while achieving high switching rates in the imaging field of view. The use of multi-echo SSFP acquisitions leads to substantial improvements in sequence performance (i.e., increased efficiency and shorter TR).

Role of MRI in clinical cardiology

Rapid progress has been made in cardiac MRI (CMRI) over the past decade, which has firmly established it as a reliable and clinically important technique for assessment of cardiac structure, function, perfusion, and myocardial viability. Its versatility and accuracy is unmatched by any other individual imaging modality. CMRI is non-invasive and has high spatial resolution and avoids use of potentially nephrotoxic contrast agent or radiation. It has been extensively studied against other established non-invasive imaging modalities and has been shown to be superior in many scenarios, particularly with respect to assessment of cardiac and great vessel morphology and left ventricular function. Furthermore, its clinical use continues to expand with increasing experience and proliferation of CMRI centres. As worldwide prevalence of cardiovascular disease continues to rise, CMRI provides opportunity for improved and cost-effective non-invasive assessment. Continued progress in CMRI technology promises to further widen its clinical application in coronary imaging, myocardial perfusion, comprehensive assessment of valves, and plaque characterisation.

Auto-SENSE perfusion imaging of the whole human heart

PURPOSE

To show the application of auto-sensitivity encoding (SENSE)-a self-calibrating parallel imaging technique-to first pass perfusion imaging of the whole human heart.

MATERIALS AND METHODS

The self-calibrating parallel imaging method auto-SENSE was implemented for a saturation recovery turbo-fast low-angle shot (FLASH) sequence on a 1.5-T scanner using a standard four-element body phased array coil. By reducing the acquisition time per slice by a factor of two compared to conventional turbo FLASH imaging, the number of imaged slices could be doubled to six to ten with an unchanged temporal resolution of one image per heartbeat. This technique has been tested in eight healthy volunteers for contrast-enhanced heart perfusion imaging.

RESULTS

Auto-SENSE heart perfusion imaging with improved coverage of the human heart could be performed successfully in all volunteers. A first quantitative comparison of perfusion values between the auto-SENSE and the non-SENSE techniques shows good agreement.

CONCLUSION

Auto-SENSE allows perfusion imaging of the whole human heart without gaps.

Importance of k-space trajectory in echo-planar myocardial tagging at rest and during dobutamine stress

Hybrid fast gradient echo/echo-planar imaging (FGRE-EPI) can be used to increase temporal resolution, enhance tag contrast, and/or decrease scan time for breathhold myocardial tagging. However, off-resonance effects and motion can lead to local phase discontinuities in FGRE-EPI raw data when a conventional interleaved bottom-up k-space trajectory is used. These discontinuities can be particularly problematic for myocardial tagging, where the image energy is not only concentrated near the k-space origin, but is also concentrated in multiple spectral peaks centered throughout k-space. In this study, tag distortion artifacts in FGRE-EPI tagging due to off-resonance and velocity-induced phase discontinuities were characterized at rest and dobutamine stress, and the flyback and gradient moment smoothing (GMS) methods were shown to reduce these artifacts. For the specific parameters used in this study, flyback and GMS resulted in improved image quality at rest and stress, increased myocardium-tag contrast-to-noise ratio (11.4 +/- 2.1 vs. 10.0 +/- 2.9, P < 0.01 at rest; 11.1 +/- 1.8 vs. 8.1 +/- 2.4, P < 0.01 at stress), and reduced full width at half maximum of the tag profile (3.6 vs. 3.8 pixels at rest; 4.0 vs. 5.1 pixels at stress) compared to the conventional trajectory. A limitation of the improved trajectory is a parameter-dependent decrease in data acquisition efficiency. For the specific imaging protocol used, the repetition time of the improved trajectory increased by 36% compared to the conventional trajectory.

Myocardial tissue tracking with two-dimensional cine displacement-encoded MR imaging: development and initial evaluation

A breath-hold two-dimensional cine magnetic resonance (MR) pulse sequence based on displacement encoding with stimulated echoes (DENSE) for quantitative myocardial motion tracking was developed and evaluated. In the sequence, complementary spatial modulation of magnetization was used for time-independent artifact suppression, and echo-planar imaging was used for rapid data sampling. Twelve healthy volunteers underwent cine DENSE MR imaging, and six of them also underwent conventional MR imaging myocardial tagging. The circumferential shortening component of strain (E(cc)) was measured on cine DENSE MR images and conventional tagged MR images. With complementary spatial modulation of magnetization, 10% or less of the total cine DENSE MR image energy was attributed to an artifact-generating echo during systolic imaging. Two-dimensional intramyocardial displacement and strain were measured at cine DENSE MR imaging with spatial resolution and temporal resolution of 2.7 x 2.7 mm and 60 msec, respectively. E(cc) measured at cine DENSE MR imaging correlated well with that measured at conventional MR imaging tagging (slope = 0.88, intercept = 0.00, R = 0.87).

Assessment of regional systolic and diastolic dysfunction in familial hypertrophic cardiomyopathy using MR tagging

Diastolic and systolic left ventricular (LV) dysfunction often significantly contribute to disabling symptoms in familial hypertrophic cardiomyopathy (FHC). This study compares regional LV function (midwall circumferential strain) during systole and diastole in eight FHC patients and six normal volunteers (NVs) using MR tagging. A prospectively-gated fast gradient-echo sequence with an echo-train readout was modified to support complementary spatial modulation of magnetization (CSPAMM) tagging and full cardiac cycle data acquisition using the cardiac phase to order reconstruction (CAPTOR), thus providing tag persistence and data acquisition during the entire cardiac cycle. Total systolic strains in FHC patients were significantly reduced in septal and inferior regions (both P < 0.01). Early-diastolic strain rates were reduced in all regions of the FHC group (all P < 0.03). The combination of CSPAMM and CAPTOR allows regional indices of myocardial function to be quantified throughout the cardiac cycle. This technique reveals regional differences in systolic and diastolic impairment in FHC patients.

Magnetic resonance image-guided transcatheter closure of atrial septal defects

BACKGROUND

Recent developments in cardiac MRI have extended the potential spectrum of diagnostic and interventional applications. The purpose of this study was to test the ability of MRI to perform transcatheter closures of secundum type atrial septal defects (ASD) and to assess ASD size and changes in right cardiac chamber volumes in the same investigation.

METHODS AND RESULTS

In 7 domestic swine (body weight, 38+/-13 kg), an ASD (Q(p):Q(s)=1.7+/-0.2) was created percutaneously by balloon dilation of the fossa ovalis. The ASD was imaged and sized by both conventional radiography and MRI. High-resolution MRI of the ASD diameters correlated well with postmortem examination (r=0.97). Under real-time MR fluoroscopy, the introducer sheath was tracked toward the left atrium with the use of novel miniature MR guide wires. The defect was then closed with an Amplatzer Septal Occluder. In all animals, it was possible to track and interactively control the position of the guide wire within the vessels and the heart, including the successful deployment of the Amplatzer Septal Occluder. Right atrial and ventricular volumes were calculated before and after the intervention by using cine-MRI. Both volumes were found to be significantly reduced after ASD closure (P<0.005).

CONCLUSIONS

These in vivo studies demonstrate that catheter tracking and ASD device closure can be performed under real-time MRI guidance with the use of intravascular antenna guide wires. High-resolution imaging allows accurate determination of ASD size before the intervention, and immediate treatment effects such as changes in right cardiac volumes can also be measured.

FIESTA-ET: high-resolution cardiac imaging using echo-planar steady-state free precession

This work describes a technique that combines multishot echo-planar imaging (EPI) with steady-state free precession (SSFP, also known as TrueFISP, FIESTA, and balanced FFE) for multislice, cine MR imaging of the heart. Unlike recently reported methods, the technique presented here (FIESTA-ET) is high-resolution and does not require offline reconstruction or postprocessing. It is therefore suitable for use on standard clinical scanners. FIESTA-ET was compared with conventional FIESTA imaging in 10 volunteers and quantitative analyses of myocardial signal-to-noise ratios (SNR) and ventricular volumes were performed. While providing comparable image quality, FIESTA-ET required half the acquisition time per slice of conventional FIESTA. Because multiple slices could be imaged in a single breathhold, the entire heart could be scanned in less than 2 min. Although the FIESTA-ET images exhibited an unexpected increase (P < 0.0005) in myocardial SNR of 16% over FIESTA, the volumetric measurements showed excellent correlation.

Cardiac MRI: recent progress and continued challenges

Cardiac MRI continues to develop and advance. MRI accurately depicts cardiac structure, function, perfusion, and myocardial viability with an overall capacity unmatched by any other single imaging modality. MRI is an accepted and widely utilized tool for cardiovascular research. Its clinical use has been limited, but is increasing because of its proven clinical efficacy, the proliferation of cardiac-capable MRI systems, and the development of improved pulse sequences. The following article reviews the landmark developments in this field, with an emphasis on recent progress in the evaluation of ischemic or acquired heart disease.

Multislice first-pass cardiac perfusion MRI: validation in a model of myocardial infarction

The purpose of this study was to validate a first-pass MRI method for imaging myocardial perfusion with multislice coverage and relatively small analyzable regions of interest (ROIs). A fast gradient-echo (FGRE) sequence with an echo-train (ET) readout was used to achieve multislice coverage, and a high dose of a contrast agent (CA) was used to achieve a high signal-to-noise ratio (SNR). Dogs (N = 6) were studied 1 day after reperfused myocardial infarction, and fluorescent microspheres were used as a standard for perfusion. First-pass MRI correlated well vs. microsphere flow, achieving mean R values of 0.87 (range = 0.82-0.93), 0.71 (range = 0.46-0.85), and 0.72 (range = 0.49-0.95) for subendocardial ROIs, transmural ROIs, and the endocardial-epicardial ratio, respectively. Additionally, analysis of myocardial time-intensity curves (TICs) indicated that 15.8 +/- 6 sectors, corresponding to 260 microl of endocardium, can be analyzed (R(2) > 0.95).

High temporal resolution phase contrast MRI with multiecho acquisitions

Velocity imaging with phase contrast (PC) MRI is a noninvasive tool for quantitative blood flow measurement in vivo. A shortcoming of conventional PC imaging is the reduction in temporal resolution as compared to the corresponding magnitude imaging. For the measurement of velocity in a single direction, the temporal resolution is halved because one must acquire two differentially flow-encoded images for every PC image frame to subtract out non-velocity-related image phase information. In this study, a high temporal resolution PC technique which retains both the spatial resolution and breath-hold length of conventional magnitude imaging is presented. Improvement by a factor of 2 in the temporal resolution was achieved by acquiring the differentially flow-encoded images in separate breath-holds rather than interleaved within a single breath-hold. Additionally, a multiecho readout was incorporated into the PC experiment to acquire more views per unit time than is possible with the single gradient-echo technique. A total improvement in temporal resolution by approximately 5 times over conventional PC imaging was achieved. A complete set of images containing velocity data in all three directions was acquired in four breath-holds, with a temporal resolution of 11.2 ms and an in-plane spatial resolution of 2 mm x 2 mm.

Real-time volume rendered MRI for interventional guidance

Volume renderings from magnetic resonance imaging data can be created and displayed in real-time with user interactivity. This can provide continuous 3D feedback to assist in guiding an interventional procedure. A system is presented which can produce real-time volume renderings from 2D multi-slice or 3D MR pulse sequences. Imaging frame rates up to 30 per second have been demonstrated with a latency of approximately one-third of a second, depending on the image matrix size. Several interactive capabilities have been implemented to enhance visualization such as cut planes, individual channel scaling and color highlighting, view sharing, saturation preparation, complex subtraction, gating control, and choice of alpha blending or MIP rendering. The system is described and some interventional application examples are shown. To view movies of some of the examples, enter the following address into a web browser: http://nhlbi.nih.gov/labs/papers/lce/guttman/rtvolmri/index/htm.

Real-time cardiac cine imaging with SPIDER: steady-state projection imaging with dynamic echo-train readout

Steady-state projection imaging with dynamic echo-train readout (SPIDER) is a multiecho radial k-space trajectory TrueFISP sequence developed for real-time cine imaging of the heart. This new pulse sequence combines the superior SNR and blood-to-myocardium contrast of TrueFISP with the increased scan time efficiency of EPI and undersampled projection reconstruction. SPIDER sequence RF repetition time (TR) was minimized by limiting the echo-train to a length of three while acquiring the first and third echoes asymmetrically. A temporal resolution of 45 ms was achieved with TR/TE1/TE2/TE3 of 3.24/0.6/1.6/2.6 ms and a factor of 2 view sharing scheme. Phantom experiments showed little difference between the weighting of the signals acquired at each of the echo times but did show considerable off-resonance modulation between them. In vivo experiments demonstrated the feasibility of using the SPIDER sequence for real-time imaging in the cardiac short axis orientation.

T1 measurements in cell cultures: a new tool for characterizing contrast agents at 1.5T

The objective of this work was to assess the feasibility and accuracy of T1 and relaxivity measurements in cell cultures using 1.5T magnetic resonance imaging (MRI) with the long-term goal to develop a tool for evaluation of novel paramagnetic agents in a realistic macromolecular environment. This initial study was carried out using MCF-7 cells treated with independently determined concentrations of Gd-DTPA. Two cell culture systems were evaluated: cell pellets and single layers of cells grown on microporous inserts. High-resolution T1 measurements of cell cultures were acquired with two dimensional Inversion Recovery Fast Spin Echo (2D-IR-FSE), three dimensional Inversion Recovery Fast Spin Echo (3D-IR-FSE), and 3D-SPGR sequences. The T1 and relaxivity accuracy of these sequences was confirmed with aqueous Gd-DTPA samples of known concentration. Relaxivities of 1.71 +/- 0.15 [mM(-1)second(-1)] and 1.55 +/- 0.50 [mM(-1)second(-1)] were measured in the cell pellets and cell monolayers, respectively, and were different from the value of 4.3 [mM(-1)second(-1)] for Gd-DTPA in water. Both cell pellets and monolayers are suitable for initial assessment of novel MR contrast agents.

Measurements of left ventricular dimensions using real-time acquisition in cardiac magnetic resonance imaging: comparison with conventional gradient echo imaging

This study investigates the use of real-time acquisition in cardiac magnetic resonance imaging (MRI) for measurements of left ventricular dimensions in comparison with conventional gradient echo acquisition. Thirty-one subjects with a variety of left ventricular morphologies to represent a typical clinical population were studied. Short-axis data sets of the left ventricle (LV) were acquired using a conventional turbo-gradient echo and an ultrafast hybrid gradient echo/echo planar sequence with acquisition in real-time. End-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF) and left ventricular mass (LV mass) were measured. The agreement between the two acquisitions and interobserver, intraobserver and interstudy variabilities were determined. The bias between the two methods was 5.86 ml for EDV, 0.23 ml for ESV and 0.94% for EF. LV mass measurements were significantly lower with the real-time method (mean bias 14.38 g). This is likely to be the result of lower spatial resolution and chemical shift artefacts with the real-time method. Interobserver, intraobserver and interstudy variabilities were low for all parameters. In conclusion, real time acquisition in MRI can provide accurate and reproducible measurements of LV dimensions in subjects with normal as well as abnormal LV morphologies, but LV mass measurements were lower than with conventional gradient echo imaging.

Techniques for fast stereoscopic MRI

Stereoscopic MRI can impart 3D perception with only two image acquisitions. This economy over standard multiplanar 3D volume renderings allows faster frame rates, which are needed for real-time imaging applications. Real-time 3D perception may enhance the appreciation of complex anatomical structures, and may improve hand-eye coordination while manipulating a medical device during an image-guided interventional procedure. To this goal, a system is being developed to acquire and display stereoscopic MR images in real-time. A clinically used, fast gradient-recalled echo-train sequence has been modified to produce stereo image pairs. Features have been added for depth cueing, view sharing, and bulk signal suppression. A workstation was attached to a clinical MR scanner for fast data extraction, image reconstruction and stereoscopic image display.

Ghost artifact cancellation using phased array processing

In this article, a method for phased array combining is formulated which may be used to cancel ghosts caused by a variety of distortion mechanisms, including space variant distortions such as local flow or off-resonance. This method is based on a constrained optimization, which optimizes SNR subject to the constraint of nulling ghost artifacts at known locations. The resultant technique is similar to the method known as sensitivity encoding (SENSE) used for accelerated imaging; however, in this formulation it is applied to full field-of-view (FOV) images. The method is applied to multishot EPI with noninterleaved phase encode acquisition. A number of benefits, as compared to the conventional interleaved approach, are reduced distortion due to off-resonance, in-plane flow, and EPI delay misalignment, as well as eliminating the need for echo-shifting. Experimental results demonstrate the cancellation for both phantom as well as cardiac imaging examples.

Qualitative and quantitative analysis of regional left ventricular wall dynamics using real-time magnetic resonance imaging: comparison with conventional breath-hold gradient echo acquisition in volunteers and patients

A real-time magnetic resonance imaging (MRI) acquisition sequence was evaluated for the assessment of left ventricular wall motion (WM) and wall thickening (WT). Ten normal volunteers and 21 patients were studied. Short-axis cine images of the left ventricle (LV) were acquired with a fast gradient echo and an ultrafast segmented echo-planar imaging (EPI) sequence. Qualitative and quantitative analysis of WM and WT was performed on a segmental basis. Qualitative scores agreed between the two methods in 691 of 724 segments (95.4%) with good reproducibility. Quantitative measurements of WM and WT were significantly lower (P < 0.001) with the real-time method (WM: mean bias, 0.49 mm; WT: mean bias, 0.61 mm). The largest differences were observed in the anterior and lateral segments and in patients with dilated ventricles. The lower resolution of the real-time sequence and artifacts was probably responsible for these differences. In conclusion, real-time cardiac MRI can be used for qualitative assessment of wall dynamics but is presently insufficient for quantitative analysis.

Advanced cardiac MR imaging of ischemic heart disease

Important advances in rapid magnetic resonance (MR) imaging technology and its application to cardiovascular imaging have been made during the past decade. High-field-strength clinical magnets, high-performance gradient hardware, and ultrafast pulse sequence technology are rapidly making the vision of a comprehensive "one-stop shop" cardiac MR imaging examination a reality. This examination is poised to have a significant effect on the management of coronary artery disease by means of assessment of wall motion with tagging and pharmacologic stress testing, evaluation of the coronary microvasculature with perfusion imaging, and direct visualization of the coronary arteries with MR coronary angiography. This article reviews current state-of-the-art pulse sequence technology and its application to the evaluation of ischemic heart disease by means of MR tagging with dobutamine stress testing, MR perfusion imaging, and MR coronary angiography. Cutting edge areas of research in coil design and exciting new areas of metabolic and oxygen level-dependent imaging are also explored.

Visualization of flow patterns distal to aortic valve prostheses in humans using a fast approach for cine 3D velocity mapping

The fluid dynamic performance of mechanical heart valves differs from normal valves and thus is considered related to late clinical complications in patients. Since flow patterns evolving around heart valves are complex in space and time, flow visualization based on time-resolved 3D velocity data might add important information regarding the performance of specific valve designs in vivo. However, previous cine 3D techniques for three-directional phase-contrast velocity mapping suffer from long scan duration and therefore might hamper assessment in patients. A hybrid 3D phase-contrast sequence combining segmented k-space acquisition with short EPI readout trains is presented with its validation in vitro. The technique was applied to study flow patterns downstream from a bileaflet aortic prosthesis in six patients. Navigator echoes were incorporated for respiratory motion compensation. Before flow visualization, spurious phase errors due to concomitant gradient fields and eddy currents were corrected. Flow visualization was based on particle paths and animated velocity vector plots. Dedicated algorithms for particle path integration were implemented to account for the considerable motion of the ascending aorta during the cardiac cycle. A distinct flow pattern reflecting the valve design was observed closest to the valve during early flow acceleration. Reverse flow occurred adjacent to high velocity jets and above the hinge housings. Later in systole, flow became confined to the central vessel area and reverse flow along the inner aortic curvature developed. Further downstream from the valve, flow patterns varied considerably among patients, indicating the impact of varying aortic anatomy in vivo. It is concluded that MR velocity mapping is a potential tool for studying 3D flow patterns evolving around heart valve prostheses in humans. J. Magn. Reson. Imaging 2001;13:690-698.

Coronary artery imaging using contrast-enhanced 3D segmented EPI

The purpose of the work was to evaluate the effectiveness of extracellular contrast media in improving MR coronary angiography using breath-hold segmented echo-planar imaging (SEPI). Two protocols were designed to optimize the inversion recovery-prepared contrast-enhanced SEPI method. In 15 healthy volunteers, significant improvements in signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, and length of visualization were observed post-contrast. The method with two targeted scans to cover the left and right arteries, respectively, following separate 20-mL contrast injections, was found to yield thinner slices and longer right coronary artery (RCA) visualization than a single scan following a 40-mL contrast injection without compromising SNR and CNR. In conclusion, extracellular contrast media substantially improves the delineation of coronary arteries with SEPI. J. Magn. Reson. Imaging 2001;13:676-681.

Myocardial wall tagging with undersampled projection reconstruction

Azimuthally undersampled projection reconstruction (PR) acquisition is investigated for use in myocardial wall tagging with MR using grid tags to provide increased temporal and spatial resolution. PR can provide the high-resolution images required for tagging with very few projections, at the expense of artifact. Insight is provided into the PR undersampling artifact, in the context of measuring myocardial motion with tags. For Fourier transform imaging, at least 112 phase-encodings must be collected to image tagging grids spaced 7 pixels apart. PR requires about 80 projections, a 1.4-fold reduction in scan time. Magn Reson Med 45:562-567, 2001. Published 2001 Wiley-Liss, Inc.

Ultrafast pulse sequence techniques for cardiac magnetic resonance imaging

Cardiac magnetic resonance imaging is a rapidly emerging field that has seen tremendous advances in the past decade. Central to the development of effective imaging strategies has been the advent of high-performance gradient hardware and the exploitation of their speed characteristics through specialized pulse sequences well suited for cardiac imaging. These advances have facilitated unprecedented acquisition times that now approach echocardiographic frame rates, while maintaining excellent image quality. This article provides a detailed overview of advanced pulse sequence technology and approaches currently taken to maximize speed performance and image quality. In particular, segmented K-space techniques that include single-echo and multiecho spoiled gradient-echo imaging as well as steady-state free precession imaging are discussed. Finally, spiral and fast spin-echo techniques are explored. Examples of common applications of these pulse sequences are presented.

Noninvasive imaging in congenital heart disease

Imaging algorithms in congenital heart disease, as in the patient with acquired heart diseases continue to evolve, with more and more information gleaned noninvasively. The emphasis will be on the newer aspects of imaging, not cross sectional echocardiography with color Doppler.

Optimization of fast cardiac imaging using an echo-train readout

Fast gradient-echo sequences that use an echo-train readout are becoming more widely used, particularly for imaging the heart. An important issue for these sequences involves determining the optimal duration for the echo-train readout. In normal volunteer scans and theoretically the echo-train readout duration was varied from 2.4 to 32.8 msec. Myocardial signal-to-noise ratio (SNR), myocardium-tag signal difference-to-noise ratio (SDNR), flow artifact-to-noise ratio (FNR), and geometric distortion were measured and/or calculated. Our results showed that to obtain high SNR, SDNR, and data acquisition efficiency while minimizing FNR and geometric distortion, the readout duration should be 10-15 msec at 1.5 T.

Magnetic resonance perfusion imaging in ischemic heart disease

This review explores the present status of contrast media available for myocardial perfusion studies, the magnetic resonance (MR) sequences adapted to multi-slice first-pass acquisitions, and the issue of myocardial perfusion quantification. To date, only low molecular weight paramagnetic gadolinium chelates have been used in clinical protocols for myocardial perfusion. With the availability of fast MR acquisition techniques to follow the first-pass distribution of the contrast agent in the myocardium, the bolus tracking technique represents the more widely used protocol in MR perfusion studies. On T1-weighted imaging, the ischemic zone appears with a delayed and lower signal enhancement compared with normally perfused myocardium. Visual analysis of the image series can be greatly improved by image post-processing to obtain relative myocardial perfusion maps. With an intravascular tracer, myocardial kinetics are in theory easier to analyze in terms of perfusion. In experimental studies, different intravascular or blood pool MR contrast agents have been tested to measure quantitative perfusion parameters. If a simple flow-limited kinetic model is developed with MR contrast agents, one important clinical application will be the evaluation of the functional consequence of coronary stenoses, ie, non-invasive evaluation of the coronary reserve.

Minimizing dead-periods in flow-encoded or -compensated pulse sequences while imaging in oblique planes

A technique is developed to minimize magnetic resonance pulse sequence dead-periods, with first and higher moment requirements, while imaging in oblique planes. Dead-period requirements of starting amplitude, ending amplitudes, area, and other moment requirements are transformed from the image coordinate system to the physical gradient coordinate system, where the waveforms are then designed. With this technique, the full capabilities of the gradient hardware are utilized. An online algorithm is presented to perform dead-period minimization and gradient waveform design when the first moment and area of the dead-period are specified. The algorithm is then used to implement three-axis flow-compensation in a fast spoiled gradient-echo sequence. This results in a minimal increase in TE and TR over an equivalent non-flow-compensated sequence, and little variation in the minimum TR over the entire range of oblique slice orientations. Applications of this algorithm extend to the optimization of any pulse sequence in which the first moment is important and oblique imaging is required. J. Magn. Reson. Imaging 1999;10:183-192.

Referenceless interleaved echo-planar imaging

Interleaved echo-planar imaging (EPI) is an ultrafast imaging technique important for applications that require high time resolution or short total acquisition times. Unfortunately, EPI is prone to significant ghosting artifacts, resulting primarily from system time delays that cause data matrix misregistration. In this work, it is shown mathematically and experimentally that system time delays are orientation dependent, resulting from anisotropic physical gradient delays. This analysis characterizes the behavior of time delays in oblique coordinates, and a new ghosting artifact caused by anisotropic delays is described. "Compensation blips" are proposed for time delay correction. These blips are shown to remove the effects of anisotropic gradient delays, eliminating the need for repeated reference scans and postprocessing corrections. Examples of phantom and in vivo images are shown.