Gerhard Laub

Self‐gated cardiac cine MRI

The need for ECG gating presents many difficulties in cardiac magnetic resonance imaging (CMRI). Real‐time imaging techniques eliminate the need for ECG gating in cine CMRI, but they cannot offer the spatial and temporal resolution provided by segmented acquisition techniques. Previous MR signal‐based techniques have demonstrated an ability to provide cardiac gating information; however, these techniques result in decreased imaging efficiency. The purpose of this work was to develop a new “self‐gated” (SG) acquisition technique that eliminates these efficiency deficits by extracting the motion synchronization signal directly from the same MR signals used for image reconstruction. Three separate strategies are proposed for deriving the SG signal from data acquired using radial k‐space sampling: echo peak magnitude, kymogram, and 2D correlation. The SG techniques were performed on seven normal volunteers. A comparison of the results showed that they provided cine image series with no significant differences in image quality compared to that obtained with conventional ECG gating techniques. SG techniques represent an important practical advance in clinical MRI because they enable the acquisition of high temporal and spatial resolution cardiac cine images without the need for ECG gating and with no loss in imaging efficiency. Magn Reson Med 51:93–102, 2004.

3D magnetization-prepared true-FISP: A new technique for imaging coronary arteries

The purpose of this work was to develop an ECG‐triggered, segmented 3D true‐FISP (fast imaging with steady‐state precession) technique to improve the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) of breath‐hold coronary artery imaging. The major task was to optimize an appropriate magnetization preparation scheme to permit saturation of the epicardial fat signal. An α/2 preparation pulse was used to speed up the approach to steady‐state following a frequency‐selective fat‐saturation pulse in each heartbeat. The application of dummy cycles was found to reduce the oscillation of the magnetization during data acquisition. The fat saturation and magnetization preparation scheme was validated with simulations and phantom studies. Volunteer studies demonstrated substantially increased SNR (55%) and CNR (178%) for coronary arteries compared to FLASH (fast low‐angle shot) with the same imaging time. In conclusion, true‐FISP is a promising technique for coronary artery imaging. Magn Reson Med 46:494–502, 2001.

3D Time-Resolved MR Angiography (MRA) of the Carotid Arteries with Time-Resolved Imaging with Stochastic Trajectories: Comparison with 3D Contrast-Enhanced Bolus-Chase MRA and 3D Time-Of-Flight MRA

BACKGROUND AND PURPOSE: Time-resolved MR angiography (MRA) offers the combined advantage of large anatomic coverage and hemodynamic flow information. We applied parallel imaging and time-resolved imaging with stochastic trajectories (TWIST), which uses a spiral trajectory to undersample k-space, to perform time-resolved MRA of the extracranial internal carotid arteries and compare it to time-of-flight (TOF) and high-resolution contrast-enhanced (HR) MRA. MATERIALS AND METHODS: A retrospective review of 31 patients who underwent carotid MRA at 1.5T using TOF, time-resolved and HR MRA was performed. Images were evaluated for the presence and degree of ICA stenosis, reader confidence, and number of pure arterial frames attained with the TWIST technique. RESULTS: With a consensus interpretation of all sequences as the reference standard, accuracy for identifying stenosis was 90.3% for TWIST MRA, compared with 96.0% and 88.7% for HR MRA and TOF MRA, respectively. HR MRA was significantly more accurate than the other techniques (P < .05). TWIST MRA yielded datasets with high in-plane spatial resolution and distinct arterial and venous phases. It provided dynamic information not otherwise available. Mean diagnostic confidence was satisfactory or greater for TWIST in all patients. CONCLUSION: The TWIST technique consistently obtained pure arterial phase images while providing dynamic information. It is rapid, uses a low dose of contrast, and may be useful in specific circumstances, such as in the acute stroke setting. However, it does not yet have spatial resolution comparable with standard contrast-enhanced MRA.

Noninvasive detection of coronary artery stenosis using contrast-enhanced three-dimensional breath-hold magnetic resonance coronary angiography

OBJECTIVES The purpose of this study was to evaluate a contrast-enhanced three-dimensional (3D) breath-hold magnetic resonance (MR) technique for detection of coronary artery stenoses. BACKGROUND The accuracy of previously published MR coronary angiography protocols varies widely. Recently, coronary artery imaging using T1-shortening contrast agent has become possible, but so far there are no data concerning its clinical application. METHODS Magnetic resonance coronary angiography was performed in 50 patients with suspected coronary artery disease. Magnetic resonance data acquisition using an ultrafast 3D gradient-echo sequence lasted over 32 heartbeats within one single breath-hold. Twenty milliliters of gadopentetate dimeglumine was injected at a flow rate of 1 ml/s for two successive studies covering the main coronary arteries in single-oblique planes. Stenosis assessment by MR was compared with significant (diameter stenosis > 50%) stenoses on X-ray angiography. Evaluation was limited to the proximal and mid-coronary artery segments. RESULTS Two hundred sixty-eight of 350 artery segments (76.6%) could be evaluated. Left circumflex coronary artery was only evaluable in 50% of cases by MR. In the evaluable segments, 48 of 56 stenoses and 193 of 212 nonstenotic segments were correctly classified by MR. On a patient basis, MR correctly identified 34 of 36 patients with and 8 of 14 patients without significant coronary stenoses as demonstrated by X-ray angiography (sensitivity 94.4%, specificity 57.1%). CONCLUSIONS Oblique projection contrast-enhanced MR coronary angiograms obtained within one single breath-hold permit identification of patients with coronary stenoses in the proximal and mid segments of the major coronary arteries with satisfactory accuracy.

Dynamic 3D MR angiography of the pulmonary arteries in under four seconds.

Although 3D MRA has been shown to provide excellent depiction of the pulmonary arterial tree, its clinical use has been limited due to lengthy breath‐holding requirements. Employing the newest gradient generation (1.5 T MR system, amplitude of 40 mT/m and a slew rate of 200 mT/m/msec), we evaluated a technique permitting the dynamic acquisition of 3D data sets of the entire pulmonary tree in under 4 seconds. Coronal image sets were collected using a repetition time of 1.64 msec and an echo time of 0.6 msec, resulting in an acquisition time of 3.74 seconds. Three volunteers and eight dyspneic patients with known or suspected pulmonary embolism underwent MRI of the pulmonary arteries. The pulmonary arterial tree was visible to a subsegmental level in all examined subjects. Regarding the presence of pulmonary emboli in four patients, there was complete concordance between MR angiographic findings and those of corroborative studies. We conclude that diagnostic MRA of the pulmonary vasculature can be obtained even in patients with severe respiratory distress. J. Magn. Reson. Imaging 2001;13:372–377.

Optimal k-Space Sampling for Dynamic Contrast-Enhanced MRI with an Application to MR Renography

For time‐resolved acquisitions with k‐space undersampling, a simulation method was developed for selecting imaging parameters based on minimization of errors in signal intensity versus time and physiologic parameters derived from tracer kinetic analysis. Optimization was performed for time‐resolved angiography with stochastic trajectories (TWIST) algorithm applied to contrast‐enhanced MR renography. A realistic 4D phantom comprised of aorta and two kidneys, one healthy and one diseased, was created with ideal tissue time‐enhancement pattern generated using a three‐compartment model with fixed parameters, including glomerular filtration rate (GFR) and renal plasma flow (RPF). TWIST acquisitions with different combinations of sampled central and peripheral k‐space portions were applied to this phantom. Acquisition performance was assessed by the difference between simulated signal intensity (SI) and calculated GFR and RPF and their ideal values. Sampling of the 20% of the center and 1/5 of the periphery of k‐space in phase‐encoding plane and data‐sharing of the remaining 4/5 minimized the errors in SI (<5%), RPF, and GFR (both <10% for both healthy and diseased kidneys). High‐quality dynamic human images were acquired with optimal TWIST parameters and 2.4 sec temporal resolution. The proposed method can be generalized to other dynamic contrast‐enhanced MRI applications, e.g., MR angiography or cancer imaging. Magn Reson Med, 2009.

TrueFISP—technical considerations and cardiovascular applications

Magnetic resonance imaging (MRI) using steady-state free precession (SSFP) sequences was described in the early years of MR imaging. However, due to hardware imperfections, these techniques were not robust enough at the time to play any significant role in clinical MRI. More recently, significant hardware improvements became widely available, and the SSFP sequences such as TrueFISP (true fast imaging with steady-state precession) became very popular for a variety of clinical applications due to the distinct improvements in signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The purpose of this article is to give an overview on the basics of TrueFISP imaging and to demonstrate its potential for current clinical applications with a focus on cardiovascular MRI.

Contrast‐enhanced breath‐hold three‐dimensional magnetic resonance angiography in the evaluation of renal arteries: Optimization of technique and pitfalls

The authors describe the optimization of a contrast‐enhanced, breath‐held, three‐dimensional magnetic resonance angiography (CE‐BH‐3DMRA) technique in the assessment of the renal arteries and compare its utility with conventional x‐ray angiography (XRA). Signal optimization using specific pulse sequence parameters was based on the patients circulatory conditions, injection rate, and pulse sequence timing. Fifty‐one patients (27 M, 24 F; mean age 69.7 years) were evaluated with CE‐BH‐3DMRA and XRA. All patients had an MR angiogram 3 months either before or after XRA. A test bolus study was performed for accurate assessment of transit time in each patient. A total of 51 patients (115 vessels) were studied in which the sensitivity and specificity for all renal artery stenoses including the proximal and mid‐renal arterial segments were 96% and 92%, respectively. In‐stent stenosis could only be diagnosed by quantifying flow beyond the stent using an additional triggered phase contrast cine pulse sequence. A total of 11 accessory renal arteries were correctly identified. In addition, fibromuscular dysplasia in two patients and stents in three patients were correctly identified on MRA. J. Magn. Reson. Imaging 2000;12:912–923.

Image quality and diagnostic accuracy of unenhanced SSFP MR angiography compared with conventional contrast-enhanced MR angiography for the assessment of thoracic aortic diseases

ObjectivesThe purpose of this study was to determine the image quality and diagnostic accuracy of three-dimensional (3D) unenhanced steady state free precession (SSFP) magnetic resonance angiography (MRA) for the evaluation of thoracic aortic diseases.MethodsFifty consecutive patients with known or suspected thoracic aortic disease underwent free-breathing ECG-gated unenhanced SSFP MRA with non-selective radiofrequency excitation and contrast-enhanced (CE) MRA of the thorax at 1.5 T. Two readers independently evaluated the two datasets for image quality in the aortic root, ascending aorta, aortic arch, descending aorta, and origins of supra-aortic arteries, and for abnormal findings. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were determined for both datasets. Sensitivity, specificity, and diagnostic accuracy of unenhanced SSFP MRA for the diagnosis of aortic abnormalities were determined.ResultsAbnormal aortic findings, including aneurysm (n = 47), coarctation (n = 14), dissection (n = 12), aortic graft (n = 6), intramural hematoma (n = 11), mural thrombus in the aortic arch (n = 1), and penetrating aortic ulcer (n = 9), were confidently detected on both datasets. Sensitivity, specificity, and diagnostic accuracy of SSFP MRA for the detection of aortic disease were 100% with CE-MRA serving as a reference standard. Image quality of the aortic root was significantly higher on SSFP MRA (P < 0.001) with no significant difference for other aortic segments (P > 0.05). SNR and CNR values were higher for all segments on SSFP MRA (P < 0.01).ConclusionOur results suggest that free-breathing navigator-gated 3D SSFP MRA with non-selective radiofrequency excitation is a promising technique that provides high image quality and diagnostic accuracy for the assessment of thoracic aortic disease without the need for intravenous contrast material.

Noncontrast 3D steady-state free-precession magnetic resonance angiography of the whole chest using nonselective radiofrequency excitation over a large field of view: comparison with single-phase 3D contrast-enhanced magnetic resonance angiography.

Objectives:To evaluate the feasibility of three-dimensional (3D) steady-state free-precession (SSFP) magnetic resonance angiography (MRA) using nonselective radiofrequency excitation in the assessment of cardiac morphology, thoracic aorta, main pulmonary, and proximal coronary arteries. Material and Methods:Thirty consecutive patients (19 males; 11 females; age range, 20–74) with various cardiac and thoracic vascular diseases underwent free-breathing respiratory navigator-gated electrocardiogram-triggered noncontrast SSFP MRA and conventional high-resolution 3D contrast-enhanced MRA (CE-MRA) of the thorax at 1.5 T. Two readers evaluated both datasets for findings, vascular delineation and sharpness (from 0, not visualized to 3, excellent definition), artifacts, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) in 14 vascular segments including aorta, supra-aortic, pulmonary, and coronary arteries, and in cardiac chambers. Statistical analysis was performed using Wilcoxon test for vessel delineation, and [&kgr;] coefficient for interobserver variability. Results:3D SSFP and CE-MRA were successfully performed in all patients. Scan time for SSFP MRA ranged from 5 to 10 minutes (mean ± standard deviation, 7 ± 2 minutes). On SSFP MRA, readers 1 and 2 graded 233 (97.1%) and 234 (97.5%) coronary arterial segments and cardiac chambers, and 275 (91.7%) and 278 (92.7%) noncoronary arterial segments with diagnostic definition (grades 2 and 3) (k = 0.86). On conventional CE-MRA, readers 1 and 2 graded 10 (4.2%) and 12 (5%) coronary arterial segments and cardiac chambers, and 272 (90.7%) and 270 (90%) noncoronary arterial segments with diagnostic definition (grades 2 and 3) (k = 0.89). Segmental visibility was higher for aortic root, pulmonary trunk, proximal coronary arteries, and heart chambers (P < 0.001), and lower for supra-aortic arteries (P < 0.001) on SSFP MRA for each reader. SNR and CNR values were higher for aortic root and aorta on SSFP MRA (P < 0.001 for both). No significant difference existed between SNR and CNR values for the other vascular segments and cardiac chambers on SSFP and CE-MRA (P > 0.05 for all). The 2 readers demonstrated vascular stenosis and dilatation/aneurysm in 7 and 35 segments on both datasets, respectively. Conclusion:Noncontrast 3D SSFP MRA with nonselective radiofrequency excitation provides high image quality and sufficient SNR and CNR for confident assessment of cardiac and thoracic vascular diseases including congenital heart diseases. Our results suggest that noncontrast SSFP MRA outperforms CE-MRA in visualization of cardiac chambers, proximal coronary arteries, pulmonary trunk, and aortic root.