Feasibility of cardiac gating free of interference with electro-magnetic fields at 1.5 Tesla, 3.0 Tesla and 7.0 Tesla using an MR-stethoscope. Invest Radiol. 2009;44:539-547. [PMID: 19652614 DOI: 10.1097/rli.0b013e3181b4c15e]

Progress in Imaging the Human Torso at the Ultrahigh Fields of 7 and 10.5 T

Especially after the launch of 7 T, the ultrahigh magnetic field (UHF) imaging community achieved critically important strides in our understanding of the physics of radiofrequency interactions in the human body, which in turn has led to solutions for the challenges posed by such UHFs. As a result, the originally obtained poor image quality has progressed to the high-quality and high-resolution images obtained at 7 T and now at 10.5 T in the human torso. Despite these tremendous advances, work still remains to further improve the image quality and fully capitalize on the potential advantages UHF has to offer.

Doppler ultrasound cardiac gating of intracranial flow at 7T

BACKGROUND

Ultra-high field magnetic resonance imaging (MR) may be used to improve intracranial blood flow measurements. However, standard cardiac synchronization methods tend to fail at ultra-high field MR. Therefore, this study aims to investigate an alternative synchronization technique using Doppler ultrasound.

METHODS

Healthy subjects (n = 9) were examined with 7T MR. Flow was measured in the M1-branch of the middle cerebral artery (MCA) and in the cerebral aqueduct (CA) using through-plane phase contrast (2D flow). Flow in the circle of Willis was measured with three-dimensional, three-directional phase contrast (4D flow). Scans were gated with Doppler ultrasound (DUS) and electrocardiogram (ECG), and pulse oximetry data (POX) was collected simultaneously. False negative and false positive trigger events were counted for ECG, DUS and POX, and quantitative flow measures were compared.

RESULTS

There were fewer false positive triggers for DUS compared to ECG (5.3 ± 11 vs. 25 ± 31, p = 0.031), while no other measured parameters differed significantly. Net blood flow in M1 was similar between DUS and ECG for 2D flow (1.5 ± 0.39 vs. 1.6 ± 0.41, bias ± 1.96SD: - 0.021 ± 0.36) and 4D flow (1.8 ± 0.48 vs. 9 ± 0.59, bias ± 1.96SD: - 0.086 ± 0.57 ml). Net CSF flow per heart beat in the CA was also similar for DUS and ECG (3.6 ± 2.1 vs. 3.0 ± 5.8, bias ± 1.96SD: 0.61 ± 13.6 μl).

CONCLUSION

Gating with DUS produced fewer false trigger events than using ECG, with similar quantitative flow values. DUS gating is a promising technique for cardiac synchronization at 7T.

Cardiac functional magnetic resonance imaging at 7T: Image quality optimization and ultra-high field capabilities

BACKGROUND

7T cardiac magnetic resonance imaging (MRI) introduces several advantages, as well as some limitations, compared to lower-field imaging. The capabilities of ultra-high field (UHF) MRI have not been fully exploited in cardiac functional imaging.

AIM

To optimize 7T cardiac MRI functional imaging without the need for conducting B1 shimming or subject-specific tuning, which improves scan efficiency. In this study, we provide results from phantom and in vivo scans using a multi-channel transceiver modular coil.

METHODS

We investigated the effects of adding a dielectric pad at different locations next to the imaged region of interest on improving image quality in subjects with different body habitus. We also investigated the effects of adjusting the imaging flip angle in cine and tagging sequences on improving image quality, B1 field homogeneity, signal-to-noise ratio (SNR), blood-myocardium contrast-to-noise ratio (CNR), and tagging persistence throughout the cardiac cycle.

RESULTS

The results showed the capability of achieving improved image quality with high spatial resolution (0.75 mm × 0.75 mm × 2 mm), high temporal resolution (20 ms), and increased tagging persistence (for up to 1200 ms cardiac cycle duration) at 7T cardiac MRI after adjusting scan set-up and imaging parameters. Adjusting the imaging flip angle was essential for achieving optimal SNR and myocardium-to-blood CNR. Placing a dielectric pad at the anterior left position of the chest resulted in improved B1 homogeneity compared to other positions, especially in subjects with small chest size.

CONCLUSION

Improved regional and global cardiac functional imaging can be achieved at 7T MRI through simple scan set-up adjustment and imaging parameter optimization, which would allow for more streamlined and efficient UHF cardiac MRI.

Evolution of UHF Body Imaging in the Human Torso at 7T: Technology, Applications, and Future Directions

The potential value of ultrahigh field (UHF) magnetic resonance imaging (MRI) and spectroscopy to biomedical research and in clinical applications drives the development of technologies to overcome its many challenges. The increased difficulties of imaging the human torso compared with the head include its overall size, the dimensions and location of its anatomic targets, the increased prevalence and magnitude of physiologic effects, the limited availability of tailored RF coils, and the necessary transmit chain hardware. Tackling these issues involves addressing notoriously inhomogeneous transmit B1 (B1) fields, limitations in peak B1, larger spatial variations of the static magnetic field B0, and patient safety issues related to implants and local RF power deposition. However, as research institutions and vendors continue to innovate, the potential gains are beginning to be realized. Solutions overcoming the unique challenges associated with imaging the human torso are reviewed as are current studies capitalizing on the benefits of UHF in several anatomies and applications. As the field progresses, strategies associated with the RF system architecture, calibration methods, RF pulse optimization, and power monitoring need to be further integrated into the MRI systems making what are currently complex processes more streamlined. Meanwhile, the UHF MRI community must seize the opportunity to build upon what have been so far proof of principle and feasibility studies and begin to further explore the true impact in both research and the clinic.

Vital Sign Monitoring and Cardiac Triggering at 1.5 Tesla: A Practical Solution by an MR-Ballistocardiography Fiber-Optic Sensor

This article presents a solution for continuous monitoring of both respiratory rate (RR) and heart rate (HR) inside Magnetic Resonance Imaging (MRI) environments by a novel ballistocardiography (BCG) fiber-optic sensor. We designed and created a sensor based on the Fiber Bragg Grating (FBG) probe encapsulated inside fiberglass (fiberglass is a composite material made up of glass fiber, fabric, and cured synthetic resin). Due to this, the encapsulation sensor is characterized by very small dimensions (30 × 10 × 0.8 mm) and low weight (2 g). We present original results of real MRI measurements (conventionally most used 1.5 T MR scanner) involving ten volunteers (six men and four women) by performing conventional electrocardiography (ECG) to measure the HR and using a Pneumatic Respiratory Transducer (PRT) for RR monitoring. The acquired sensor data were compared against real measurements using the objective Bland⁻Altman method, and the functionality of the sensor was validated (95.36% of the sensed values were within the ±1.96 SD range for the RR determination and 95.13% of the values were within the ±1.96 SD range for the HR determination) by this means. The accuracy of this sensor was further characterized by a relative error below 5% (4.64% for RR and 4.87% for HR measurements). The tests carried out in an MRI environment demonstrated that the presence of the FBG sensor in the MRI scanner does not affect the quality of this imaging modality. The results also confirmed the possibility of using the sensor for cardiac triggering at 1.5 T (for synchronization and gating of cardiovascular magnetic resonance) and for cardiac triggering when a Diffusion Weighted Imaging (DWI) is used.

Highly Accelerated Breath-Hold Noncontrast Electrocardiographically- and Pulse-Gated Balanced Steady-State Free Precession Magnetic Resonance Angiography of the Thoracic Aorta: Comparison With Electrocardiographically-Gated Computed Tomographic Angiography

OBJECTIVE

The aim of this study was to evaluate agreement of measured thoracic aortic caliber in patients with aortic disease, using electrocardiographically-(ECG) and pulse-gated breath-hold noncontrast balanced steady-state free precession MRA (ECG-MRA, P-MRA) at 1.5 T, compared with ECG-gated computed tomographic angiography (CTA).

METHODS

Thirty-one patients underwent ECG-MRA, P-MRA, and CTA. Two readers independently measured aortic caliber in 7 segments, with agreement between techniques and readers evaluated. Image quality was qualitatively assessed.

RESULTS

There was overall excellent agreement among ECG-MRA, P-MRA, and CTA for measured aortic caliber (Lin's concordance correlation coefficient ≥0.94, all comparisons); however, lower concordance was noted at the annulus (Lin's concordance correlation coefficient <0.6) at segmental assessment. There was excellent interreader agreement for aortic caliber for all 3 techniques (intraclass correlation coefficient >0.94). Image quality was poorer for both MRA techniques compared with CTA, particularly at the aortic root.

CONCLUSIONS

Electrocardiographically-gated MRA and P-MRA at 1.5 T achieve comparable thoracic aortic measurements to gated CTA in clinical patients, despite inferior image quality.

Feasibility of aortic valve planimetry at 7 T ultrahigh field MRI: Comparison to aortic valve MRI at 3 T and 1.5 T

Introduction

This study examined the feasibility of aortic valve planimetry at 7 T ultrahigh field MRI in intraindividual comparison to 3 T and 1.5 T MRI.

Material and methods

Aortic valves of eleven healthy volunteers (mean age, 26.4 years) were examined on a 7 T, 3 T, and 1.5 T MR system using FLASH and TrueFISP sequences. Two experienced radiologists evaluated overall image quality, the presence of artefacts, tissue contrast ratios, identifiability, and image details of the aortic valve opening area (AVOA). Furthermore, AVOA was quantified twice by reader 1 and once by reader 2. Correlation analysis between artefact severity and employed magnetic field strength was performed by modified Fisher’s exact-test. Paired t-test was used to analyse for AVOA differences, and Bland-Altman plots were used to analyse AVOA intra-rater and inter-rater variability.

Results

Aortic valve imaging at 7 T, 3 T, and 1.5 T with using FLASH was less hampered by artefacts than TrueFISP imaging at 3 T and 1.5 T. Tissue contrast and image details were rated best at 7 T. AVOA was measured slightly smaller at 7 T compared to 3 T (TrueFISP, p-value = 0.057; FLASH, p-value = 0.016) and 1.5 T (TrueFISP, p-value = 0.029; FLASH, p-value = 0.018). Intra-rater and inter-rater variability of AVOA tended to be slightly smaller at 7 T than at 3 T and 1.5 T.

Conclusion

Aortic valve planimetry at 7 T ultrahigh field MRI is technically feasible and in healthy volunteers offers an improved tissue contrast and a slightly better reproducibility than MR planimetry at 1.5 T and 3 T.

Local Multi-Channel RF Surface Coil versus Body RF Coil Transmission for Cardiac Magnetic Resonance at 3 Tesla: Which Configuration Is Winning the Game?

INTRODUCTION

The purpose of this study was to demonstrate the feasibility and efficiency of cardiac MR at 3 Tesla using local four-channel RF coil transmission and benchmark it against large volume body RF coil excitation.

METHODS

Electromagnetic field simulations are conducted to detail RF power deposition, transmission field uniformity and efficiency for local and body RF coil transmission. For both excitation regimes transmission field maps are acquired in a human torso phantom. For each transmission regime flip angle distributions and blood-myocardium contrast are examined in a volunteer study of 12 subjects. The feasibility of the local transceiver RF coil array for cardiac chamber quantification at 3 Tesla is demonstrated.

RESULTS

Our simulations and experiments demonstrate that cardiac MR at 3 Tesla using four-channel surface RF coil transmission is competitive versus current clinical CMR practice of large volume body RF coil transmission. The efficiency advantage of the 4TX/4RX setup facilitates shorter repetition times governed by local SAR limits versus body RF coil transmission at whole-body SAR limit. No statistically significant difference was found for cardiac chamber quantification derived with body RF coil versus four-channel surface RF coil transmission. Our simulation also show that the body RF coil exceeds local SAR limits by a factor of ~2 when driven at maximum applicable input power to reach the whole-body SAR limit.

CONCLUSION

Pursuing local surface RF coil arrays for transmission in cardiac MR is a conceptually appealing alternative to body RF coil transmission, especially for patients with implants.

W(h)ither human cardiac and body magnetic resonance at ultrahigh fields? technical advances, practical considerations, applications, and clinical opportunities

The objective of this study was to document and review advances and groundbreaking progress in cardiac and body MR at ultrahigh fields (UHF, B0 ≥ 7.0 T) with the goal to attract talent, clinical adopters, collaborations and resources to the biomedical and diagnostic imaging communities. This review surveys traits, advantages and challenges of cardiac and body MR at 7.0 T. The considerations run the gamut from technical advances to clinical opportunities. Key concepts, emerging technologies, practical considerations, frontier applications and future directions of UHF body and cardiac MR are provided. Examples of UHF cardiac and body imaging strategies are demonstrated. Their added value over the kindred counterparts at lower fields is explored along with an outline of research promises. The achievements of cardiac and body UHF-MR are powerful motivators and enablers, since extra speed, signal and imaging capabilities may be invested to overcome the fundamental constraints that continue to hamper traditional cardiac and body MR applications. If practical obstacles, concomitant physics effects and technical impediments can be overcome in equal measure, sophisticated cardiac and body UHF-MR will help to open the door to new MRI and MRS approaches for basic research and clinical science, with the lessons learned at 7.0 T being transferred into broad clinical use including diagnostics and therapy guiding at lower fields. Copyright © 2015 John Wiley & Sons, Ltd.

Myocardial effective transverse relaxation time T2* Correlates with left ventricular wall thickness: A 7.0 T MRI study

PURPOSE

Myocardial effective relaxation time T2* is commonly regarded as a surrogate for myocardial tissue oxygenation. However, it is legitimate to assume that there are multiple factors that influence T2*. To this end, this study investigates the relationship between T2* and cardiac macromorphology given by left ventricular (LV) wall thickness and left ventricular radius, and provides interpretation of the results in the physiological context.

METHODS

High spatio-temporally resolved myocardial CINE T2* mapping was performed in 10 healthy volunteers using a 7.0 Tesla (T) full-body MRI system. Ventricular septal wall thickness, left ventricular inner radius, and T2* were analyzed. Macroscopic magnetic field changes were elucidated using cardiac phase-resolved magnetic field maps.

RESULTS

Ventricular septal T2* changes periodically over the cardiac cycle, increasing in systole and decreasing in diastole. Ventricular septal wall thickness and T2* showed a significant positive correlation, whereas the inner LV radius and T2* were negatively correlated. The effect of macroscopic magnetic field gradients on T2* can be considered minor in the ventricular septum.

CONCLUSION

Our findings suggest that myocardial T2* is related to tissue blood volume fraction. Temporally resolved T2* mapping could be beneficial for myocardial tissue characterization and for understanding cardiac (patho)physiology in vivo. Magn Reson Med 77:2381-2389, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Doppler Ultrasound Triggering for Cardiovascular MRI at 3T in a Healthy Volunteer Study

Purpose:

Electrocardiogram (ECG) triggering for cardiac magnetic resonance (CMR) may be influenced by electromagnetic interferences with increasing magnetic field strength. The aim of this study was to evaluate the performance of Doppler ultrasound (DUS) as an alternative trigger technique for CMR in comparison to ECG and pulse oximetry (POX) at 3T and using different sequence types.

Methods:

Balanced turbo field echo two-dimensional (2D) short axis cine CMR and 2D phase-contrast angiography of the ascending aorta was performed in 11 healthy volunteers at 3T using ECG, DUS, and POX for cardiac triggering. DUS and POX triggering were compared to the reference standard of ECG in terms of trigger quality (trigger detection and temporal variability), image quality [endocardial blurring (EB)], and functional measurements [left ventricular (LV) volumetry and aortic blood flow velocimetry].

Results:

Trigger signal detection and temporal variability did not differ significantly between ECG/DUS (I = 0.6) and ECG/POX (P = 0.4). Averaged EB was similar for ECG, DUS, and POX (p_ECG/DUS = 0.4, p_ECG/POX = 0.9). Diastolic EB was significantly decreased for DUS in comparison to ECG (P = 0.02) and POX (P = 0.04). The LV function assessment and aortic blood flow were not significantly different.

Conclusion:

This study demonstrated the feasibility of DUS for gating human CMR at 3T. The magnetohydrodynamic effect did not significantly disturb ECG triggering in this small healthy volunteer study. DUS showed a significant improvement in diastolic EB but could not be identified as a superior trigger method. The potential benefit of DUS has to be evaluated in a larger clinical patient population.

Sodium MRI of the human heart at 7.0 T: preliminary results

The objective of this work was to examine the feasibility of three-dimensional (3D) and whole heart coverage (23)Na cardiac MRI at 7.0 T including single-cardiac-phase and cinematic (cine) regimes. A four-channel transceiver RF coil array tailored for (23)Na MRI of the heart at 7.0 T (f = 78.5 MHz) is proposed. An integrated bow-tie antenna building block is used for (1)H MR to support shimming, localization and planning in a clinical workflow. Signal absorption rate simulations and assessment of RF power deposition were performed to meet the RF safety requirements. (23) Na cardiac MR was conducted in an in vivo feasibility study. 3D gradient echo (GRE) imaging in conjunction with Cartesian phase encoding (total acquisition time T(AQ)  = 6 min 16 s) and whole heart coverage imaging employing a density-adapted 3D radial acquisition technique (T(AQ)  = 18 min 20 s) were used. For 3D GRE-based (23)Na MRI, acquisition of standard views of the heart using a nominal in-plane resolution of (5.0 × 5.0) mm(2) and a slice thickness of 15 mm were feasible. For whole heart coverage 3D density-adapted radial (23)Na acquisitions a nominal isotropic spatial resolution of 6 mm was accomplished. This improvement versus 3D conventional GRE acquisitions reduced partial volume effects along the slice direction and enabled retrospective image reconstruction of standard or arbitrary views of the heart. Sodium cine imaging capabilities were achieved with the proposed RF coil configuration in conjunction with 3D radial acquisitions and cardiac gating. Cardiac-gated reconstruction provided an enhancement in blood-myocardium contrast of 20% versus the same data reconstructed without cardiac gating. The proposed transceiver array enables (23)Na MR of the human heart at 7.0 T within clinical acceptable scan times. This capability is in positive alignment with the needs of explorations that are designed to examine the potential of (23)Na MRI for the assessment of cardiovascular and metabolic diseases.

16-channel bow tie antenna transceiver array for cardiac MR at 7.0 tesla

PURPOSE

To design, evaluate, and apply a bow tie antenna transceiver radiofrequency (RF) coil array tailored for cardiac MRI at 7.0 Tesla (T).

METHODS

The radiofrequency (RF) coil array comprises 16 building blocks each containing a bow tie shaped λ/2-dipole antenna. Numerical simulations were used for transmission field homogenization and RF safety validation. RF characteristics were examined in a phantom study. The array's suitability for high spatial resolution two-dimensional (2D) CINE imaging and for real time imaging of the heart was examined in a volunteer study.

RESULTS

The arrays transmission fields and RF characteristics are suitable for cardiac MRI at 7.0T. The coil performance afforded a spatial resolution as good as (0.8 × 0.8 × 2.5) mm(3) for segmented 2D CINE MRI at 7.0T which is by a factor of 12 superior versus standardized protocols used in clinical practice at 1.5T. The proposed transceiver array supports 1D acceleration factors of up to R = 6 without impairing image quality significantly.

CONCLUSION

The 16-channel bow tie antenna transceiver array supports accelerated and high spatial resolution cardiac MRI. The array is compatible with multichannel transmission and provides a technological basis for future clinical assessment of parallel transmission techniques at 7.0 Tesla. Magn Reson Med 75:2553-2565, 2016. © 2015 Wiley Periodicals, Inc.

On the subjective acceptance during cardiovascular magnetic resonance imaging at 7.0 Tesla

PURPOSE

This study examines the subjective acceptance during UHF-CMR in a cohort of healthy volunteers who underwent a cardiac MR examination at 7.0T.

METHODS

Within a period of two-and-a-half years (January 2012 to June 2014) a total of 165 healthy volunteers (41 female, 124 male) without any known history of cardiac disease underwent UHF-CMR. For the assessment of the subjective acceptance a questionnaire was used to examine the participants experience prior, during and after the UHF-CMR examination. For this purpose, subjects were asked to respond to the questionnaire in an exit interview held immediately after the completion of the UHF-CMR examination under supervision of a study nurse to ensure accurate understanding of the questions. All questions were answered with "yes" or "no" including space for additional comments.

RESULTS

Transient muscular contraction was documented in 12.7% of the questionnaires. Muscular contraction was reported to occur only during periods of scanning with the magnetic field gradients being rapidly switched. Dizziness during the study was reported by 12.7% of the subjects. Taste of metal was reported by 10.1% of the study population. Light flashes were reported by 3.6% of the entire cohort. 13% of the subjects reported side effects/observations which were not explicitly listed in the questionnaire but covered by the question about other side effects. No severe side effects as vomiting or syncope after scanning occurred. No increase in heart rate was observed during the UHF-CMR exam versus the baseline clinical examination.

CONCLUSIONS

This study adds to the literature by detailing the subjective acceptance of cardiovascular magnetic resonance imaging examinations at a magnetic field strength of 7.0T. Cardiac MR examinations at 7.0T are well tolerated by healthy subjects. Broader observational and multi-center studies including patient cohorts with cardiac diseases are required to gain further insights into the subjective acceptance of UHF-CMR examinations.

A multi-purpose open-source triggering platform for magnetic resonance

Many MR scans need to be synchronised with external events such as the cardiac or respiratory cycles. For common physiological functions commercial trigger equipment exists, but for more experimental inputs these are not available. This paper describes the design of a multi-purpose open-source trigger platform for MR systems. The heart of the system is an open-source Arduino Due microcontroller. This microcontroller samples an analogue input and digitally processes these data to determine the trigger. The output of the microcontroller is programmed to mimic a physiological signal which is fed into the electrocardiogram (ECG) or pulse oximeter port of MR scanner. The microcontroller is connected to a Bluetooth dongle that allows wireless monitoring and control outside the scanner room. This device can be programmed to generate a trigger based on various types of input. As one example, this paper describes how it can be used as an acoustic cardiac triggering unit. For this, a plastic stethoscope is connected to a microphone which is used as an input for the system. This test setup was used to acquire retrospectively-triggered cardiac scans in ten volunteers. Analysis showed that this platform produces a reliable trigger (>99% triggers are correct) with a small average 8 ms variation between the exact trigger points.

Pre-clinical functional Magnetic Resonance Imaging Part II: The heart

One third of all deaths worldwide in 2008 were caused by cardiovascular diseases (CVD), and the incidence of CVD related deaths rises ever more. Thus, improved imaging techniques and modalities are needed for the evaluation of cardiac morphology and function. Cardiac magnetic resonance imaging (CMRI) is a minimally invasive technique that is increasingly important due to its high spatial and temporal resolution, its high soft tissue contrast and its ability of functional and quantitative imaging. It is widely accepted as the gold standard of cardiac functional analysis. In the short period of small animal MRI, remarkable progress has been achieved concerning new, fast imaging schemes as well as purpose-built equipment. Dedicated small animal scanners allow for tapping the full potential of recently developed animal models of cardiac disease. In this paper, we review state-of-the-art cardiac magnetic resonance imaging techniques and applications in small animals at ultra-high fields (UHF).

Accelerated fast spin-echo magnetic resonance imaging of the heart using a self-calibrated split-echo approach

Purpose

Design, validation and application of an accelerated fast spin-echo (FSE) variant that uses a split-echo approach for self-calibrated parallel imaging.

Methods

For self-calibrated, split-echo FSE (SCSE-FSE), extra displacement gradients were incorporated into FSE to decompose odd and even echo groups which were independently phase encoded to derive coil sensitivity maps, and to generate undersampled data (reduction factor up to R = 3). Reference and undersampled data were acquired simultaneously. SENSE reconstruction was employed.

Results

The feasibility of SCSE-FSE was demonstrated in phantom studies. Point spread function performance of SCSE-FSE was found to be competitive with traditional FSE variants. The immunity of SCSE-FSE for motion induced mis-registration between reference and undersampled data was shown using a dynamic left ventricular model and cardiac imaging. The applicability of black blood prepared SCSE-FSE for cardiac imaging was demonstrated in healthy volunteers including accelerated multi-slice per breath-hold imaging and accelerated high spatial resolution imaging.

Conclusion

SCSE-FSE obviates the need of external reference scans for SENSE reconstructed parallel imaging with FSE. SCSE-FSE reduces the risk for mis-registration between reference scans and accelerated acquisitions. SCSE-FSE is feasible for imaging of the heart and of large cardiac vessels but also meets the needs of brain, abdominal and liver imaging.

Magnetic resonance imaging at ultrahigh fields

Since the introduction of 4 T human systems in three academic laboratories circa 1990, rapid progress in imaging and spectroscopy studies in humans at 4 T and animal model systems at 9.4 T have led to the introduction of 7 T and higher magnetic fields for human investigation at about the turn of the century. Work conducted on these platforms has demonstrated the existence of significant advantages in SNR and biological information content at these ultrahigh fields, as well as the presence of numerous challenges. Primary difference from lower fields is the deviation from the near field regime; at the frequencies corresponding to hydrogen resonance conditions at ultrahigh fields, the RF is characterized by attenuated traveling waves in the human body, which leads to image nonuniformities for a given sample-coil configuration because of interferences. These nonuniformities were considered detrimental to the progress of imaging at high field strengths. However, they are advantageous for parallel imaging for signal reception and parallel transmission, two critical technologies that account, to a large extend, for the success of ultrahigh fields. With these technologies, and improvements in instrumentation and imaging methods, ultrahigh fields have provided unprecedented gains in imaging of brain function and anatomy, and started to make inroads into investigation of the human torso and extremities. As extensive as they are, these gains still constitute a prelude to what is to come given the increasingly larger effort committed to ultrahigh field research and development of ever better instrumentation and techniques.

Evaluation of highly accelerated real-time cardiac cine MRI in tachycardia

Electrocardiogram (ECG)-gated breath-hold cine MRI is considered to be the gold standard test for the assessment of cardiac function. However, it may fail in patients with arrhythmia, impaired breath-hold capacity and poor ECG gating. Although ungated real-time cine MRI may mitigate these problems, commercially available real-time cine MRI pulse sequences using parallel imaging typically yield relatively poor spatiotemporal resolution because of their low image acquisition efficiency. As an extension of our previous work, the purpose of this study was to evaluate the diagnostic quality and accuracy of eight-fold-accelerated real-time cine MRI with compressed sensing (CS) for the quantification of cardiac function in tachycardia, where it is challenging for real-time cine MRI to provide sufficient spatiotemporal resolution. We evaluated the performances of eight-fold-accelerated cine MRI with CS, three-fold-accelerated real-time cine MRI with temporal generalized autocalibrating partially parallel acquisitions (TGRAPPA) and ECG-gated breath-hold cine MRI in 21 large animals with tachycardia (mean heart rate, 104 beats per minute) at 3T. For each cine MRI method, two expert readers evaluated the diagnostic quality in four categories (image quality, temporal fidelity of wall motion, artifacts and apparent noise) using a Likert scale (1-5, worst to best). One reader evaluated the left ventricular functional parameters. The diagnostic quality scores were significantly different between the three cine pulse sequences, except for the artifact level between CS and TGRAPPA real-time cine MRI. Both ECG-gated breath-hold cine MRI and eight-fold accelerated real-time cine MRI yielded all four scores of ≥ 3.0 (acceptable), whereas three-fold-accelerated real-time cine MRI yielded all scores below 3.0, except for artifact (3.0). The left ventricular ejection fraction (LVEF) measurements agreed better between ECG-gated cine MRI and eight-fold-accelerated real-time cine MRI (mean difference, -1.6%) than between ECG-gated cine MRI and three-fold-accelerated real-time cine MRI (mean difference, -5.7%). Eight-fold-accelerated real-time cine MRI with CS yields acceptable diagnostic quality and relatively accurate LVEF measurements in the challenging setting of tachycardia.

[Problems and chances of high field magnetic resonance imaging]

CLINICAL/METHODICAL ISSUE

The spatial, temporal and spectral resolution in magnetic resonance imaging (MRI) is in many cases currently not sufficient to detect submillimeter lesions or to image the dynamics of the beating heart.

STANDARD RADIOLOGICAL METHODS

At present MRI systems at 1.5 T and 3 T are the standard units for clinical imaging.

METHODICAL INNOVATIONS

The use of ultrahigh magnetic fields of 7 T and higher increases the signal-to-noise ratio, which holds promise for a significant improvement of the spatial and/or temporal resolution as well as for new contrast mechanisms.

PERFORMANCE

With 7 T MRI, images of the brain have been acquired routinely with a spatial resolution of 0.3 mm. The theoretical improvement of the signal-to-noise ratio is often not fully realized due to B1 inhomogeneities and contrast variations.

ACHIEVEMENTS

With MRI at 7 T a notable increase in spatial resolution can be achieved. Methods such as time-of-flight MR angiography and susceptibility-weighted imaging (e.g. neurofunctional MRI, fMRI) profit especially from the higher field strengths. Transmission field inhomogeneities are still a major challenge for ultrahigh field (UHF) MRI and are also a partially unsolved safety problem.

PRACTICAL RECOMMENDATIONS

The use of UHF MRI is currently limited to special applications and the expected gain of the high field must be weighed against technical limitations in both image acquisition and interpretation.

[Cardiovascular ultrahigh field magnetic resonance imaging : challenges, technical solutions and opportunities]

CLINICAL/METHODICAL ISSUE

This involves high spatial resolution cardiac imaging with ultrahigh magnetic fields (7 T) and clinically acceptable image quality.

STANDARD RADIOLOGICAL METHODS

Cardiovascular magnetic resonance imaging (MRI) at a field strength of 1.5 T using a spatial resolution of (2 × 2 × 6-8) mm(3).

METHODICAL INNOVATIONS

Cardiac MRI at ultrahigh field strength makes use of multitransmit/receive radiofrequency (RF) technology and development of novel technology that utilizes the traits of ultrahigh field MRI.

PERFORMANCE

Enhanced spatial resolution which is superior by a factor of 6-10 to what can be achieved by current clinical cardiac MRI. The relative spatial resolution (pixels per anatomical structure) comes close to what can be accomplished by current cardiac MRI in small rodents.

ACHIEVEMENTS

Feasibility studies demonstrate the gain in spatial resolution at 7.0 T due to the sensitivity advantage inherent to ultrahigh magnetic fields.

PRACTICAL RECOMMENDATIONS

Please stay tuned and please put further weight behind the solution of the remaining technical problems of cardiac MRI at 7.0 T.

Progress and promises of human cardiac magnetic resonance at ultrahigh fields: a physics perspective

A growing number of reports eloquently speak about explorations into cardiac magnetic resonance (CMR) at ultrahigh magnetic fields (B0≥7.0 T). Realizing the progress, promises and challenges of ultrahigh field (UHF) CMR this perspective outlines current trends in enabling MR technology tailored for cardiac MR in the short wavelength regime. For this purpose many channel radiofrequency (RF) technology concepts are outlined. Basic principles of mapping and shimming of transmission fields including RF power deposition considerations are presented. Explorations motivated by the safe operation of UHF-CMR even in the presence of conductive implants are described together with the physics, numerical simulations and experiments, all of which detailing antenna effects and RF heating induced by intracoronary stents at 7.0 T. Early applications of CMR at 7.0 T and their clinical implications for explorations into cardiovascular diseases are explored including assessment of cardiac function, myocardial tissue characterization, MR angiography of large and small vessels as well as heteronuclear MR of the heart and the skin. A concluding section ventures a glance beyond the horizon and explores future directions. The goal here is not to be comprehensive but to inspire the biomedical and diagnostic imaging communities to throw further weight behind the solution of the many remaining unsolved problems and technical obstacles of UHF-CMR with the goal to transfer MR physics driven methodological advancements into extra clinical value.

High spatial resolution and temporally resolved T2* mapping of normal human myocardium at 7.0 Tesla: an ultrahigh field magnetic resonance feasibility study

Myocardial tissue characterization using T(2)(*) relaxation mapping techniques is an emerging application of (pre)clinical cardiovascular magnetic resonance imaging. The increase in microscopic susceptibility at higher magnetic field strengths renders myocardial T(2)(*) mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T(2)(*) imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T(2)(*) mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T(2)(*) maps. In vivo studies showed that the peak-to-peak B(0) difference following volume selective shimming was reduced to approximately 80 Hz for the four chamber view and mid-ventricular short axis view of the heart and to 65 Hz for the left ventricle. No severe susceptibility artifacts were detected in the septum and in the lateral wall for T(2)(*) weighting ranging from TE = 2.04 ms to TE = 10.2 ms. For TE >7 ms, a susceptibility weighting induced signal void was observed within the anterior and inferior myocardial segments. The longest T(2)(*) values were found for anterior (T(2)(*) = 14.0 ms), anteroseptal (T(2)(*) = 17.2 ms) and inferoseptal (T(2)(*) = 16.5 ms) myocardial segments. Shorter T(2)(*) values were observed for inferior (T(2)(*) = 10.6 ms) and inferolateral (T(2)(*) = 11.4 ms) segments. A significant difference (p = 0.002) in T(2)(*) values was observed between end-diastole and end-systole with T(2)(*) changes of up to approximately 27% over the cardiac cycle which were pronounced in the septum. To conclude, these results underscore the challenges of myocardial T(2)(*) mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes.

Two-dimensional sixteen channel transmit/receive coil array for cardiac MRI at 7.0 T: design, evaluation, and application

PURPOSE

To design, evaluate, and apply a 2D 16-channel transmit/receive (TX/RX) coil array tailored for cardiac magnetic resonance imaging (MRI) at 7.0 T.

MATERIALS AND METHODS

The cardiac coil array consists of two sections each using eight elements arranged in a 2 × 4 array. Radiofrequency (RF) safety was validated by specific absorption rate (SAR) simulations. Cardiac imaging was performed using 2D CINE FLASH imaging, T 2 mapping, and fat-water separation imaging. The characteristics of the coil array were analyzed including parallel imaging performance, left ventricular chamber quantification, and overall image quality.

RESULTS

RF characteristics were found to be appropriate for all subjects included in the study. The SAR values derived from the simulations fall well within the limits of legal guidelines. The baseline signal-to-noise ratio (SNR) advantage at 7.0 T was put to use to acquire 2D CINE images of the heart with a very high spatial resolution of (1 × 1 × 4) mm(3) . The proposed coil array supports 1D acceleration factors of up to R = 4 without significantly impairing image quality.

CONCLUSION

The 16-channel TX/RX coil has the capability to acquire high contrast and high spatial resolution images of the heart at 7.0 T.

Ultrasound-guided identification of cardiac imaging windows

PURPOSE

Currently, the use of cine magnetic resonance imaging (MRI) to identify cardiac quiescent periods relative to the electrocardiogram (ECG) signal is insufficient for producing submillimeter-resolution coronary MR angiography (MRA) images. In this work, the authors perform a time series comparison between tissue Doppler echocardiograms of the interventricular septum (IVS) and concurrent biplane x-ray angiograms. Our results indicate very close agreement between the diastasis gating windows identified by both the IVS and x-ray techniques.

METHODS

Seven cath lab patients undergoing diagnostic angiograms were simultaneously scanned during a breath hold by ultrasound and biplane x-ray for six to eight heartbeats. The heart rate of each patient was stable. Dye was injected into either the left or right-coronary vasculature. The IVS was imaged using color tissue Doppler in an apical four-chamber view. Diastasis was estimated on the IVS velocity curve. On the biplane angiograms, proximal, mid, and distal regions were identified on the coronary artery (CA). Frame by frame correlation was used to derive displacement, and then velocity, for each region. The quiescent periods for a CA and its subsegments were estimated based on velocity. Using Pearson's correlation coefficient and Bland-Altman analysis, the authors compared the start and end times of the diastasis windows as estimated from the IVS and CA velocities. The authors also estimated the vessel blur across the diastasis windows of multiple sequential heartbeats of each patient.

RESULTS

In total, 17 heartbeats were analyzed. The range of heart rate observed across patients was 47-79 beats per minute (bpm) with a mean of 57 bpm. Significant correlations (R > 0.99; p < 0.01) were observed between the IVS and x-ray techniques for the identification of the start and end times of diastasis windows. The mean difference in the starting times between IVS and CA quiescent windows was -12.0 ms. The mean difference in end times between IVS and CA quiescent windows was -3.5 ms. In contrast, the correlation between RR interval and both the start and duration of the x-ray gating windows were relatively weaker: R = 0.63 (p = 0.13) and R = 0.86 (p = 0.01). For IVS gating windows, the average estimated vessel blurs during single and multiple heartbeats were 0.5 and 0.66 mm, respectively. For x-ray gating windows, the corresponding values were 0.26 and 0.44 mm, respectively.

CONCLUSIONS

In this study, the authors showed that IVS velocity can be used to identify periods of diastasis for coronary arteries. Despite variability in mid-diastolic rest positions over multiple steady rate heartbeats, vessel blurring of 0.5-1 mm was found to be achievable using the IVS gating technique. The authors envision this leading to a new cardiac gating system that, compared with conventional ECG gating, provides better resolution and shorter scan times for coronary MRA.

Cardiac magnetic resonance: is phonocardiogram gating reliable in velocity-encoded phase contrast imaging?

OBJECTIVES

To assess the diagnostic accuracy of phonocardiogram (PCG) gated velocity-encoded phase contrast magnetic resonance imaging (MRI).

METHODS

Flow quantification above the aortic valve was performed in 68 patients by acquiring a retrospectively PCG- and a retrospectively ECG-gated velocity-encoded GE-sequence at 1.5 T. Peak velocity (PV), average velocity (AV), forward volume (FV), reverse volume (RV), net forward volume (NFV), as well as the regurgitant fraction (RF) were assessed for both datasets, as well as for the PCG-gated datasets after compensation for the PCG trigger delay.

RESULTS

PCG-gated image acquisition was feasible in 64 patients, ECG-gated in all patients. PCG-gated flow quantification overestimated PV (Δ 3.8 ± 14.1 cm/s; P = 0.037) and underestimated FV (Δ -4.9 ± 15.7 ml; P = 0.015) and NFV (Δ -4.5 ± 16.5 ml; P = 0.033) compared with ECG-gated imaging. After compensation for the PCG trigger delay, differences were only observed for PV (Δ 3.8 ± 14.1 cm/s; P = 0.037). Wide limits of agreement between PCG- and ECG-gated flow quantification were observed for all variables (PV: -23.9 to 31.4 cm/s; AV: -4.5 to 3.9 cm/s; FV: -35.6 to 25.9 ml; RV: -8.0 to 7.2 ml; NFV: -36.8 to 27.8 ml; RF: -10.4 to 10.2 %).

CONCLUSIONS

The present study demonstrates that PCG gating in its current form is not reliable enough for flow quantification based on velocity-encoded phase contrast gradient echo (GE) sequences.

KEY POINTS

Phonocardiogram gating is an alternative to ECG-gating in cardiac MRI. Phonocardiogram gating shows only limited reliability for velocity-encoded cardiac MRI. Further refinements of the post-processing algorithm are necessary.

Comparison of three multichannel transmit/receive radiofrequency coil configurations for anatomic and functional cardiac MRI at 7.0T: implications for clinical imaging

OBJECTIVES

To implement, examine, and compare three multichannel transmit/receive coil configurations for cardiovascular MR (CMR) at 7T.

METHODS

Three radiofrequency transmit-receive (TX/RX) coils with 4-, 8-, and 16-coil elements were used. Ten healthy volunteers (seven males, age 28 ± 4 years) underwent CMR at 7T. For all three RX/TX coils, 2D CINE FLASH images of the heart were acquired. Cardiac chamber quantification, signal-to-noise ratio (SNR) analysis, parallel imaging performance assessment, and image quality scoring were performed.

RESULTS

Mean total examination time was 29 ± 5 min. All images obtained with the 8- and 16-channel coils were diagnostic. No significant difference in ejection fraction (EF) (P > 0.09) or left ventricular mass (LVM) (P > 0.31) was observed between the coils. The 8- and 16-channel arrays yielded a higher mean SNR in the septum versus the 4-channel coil. The lowest geometry factors were found for the 16-channel coil (mean ± SD 2.3 ± 0.5 for R = 4). Image quality was rated significantly higher (P < 0.04) for the 16-channel coil versus the 8- and 4-channel coils.

CONCLUSIONS

All three coil configurations are suitable for CMR at 7.0T under routine circumstances. A larger number of coil elements enhances image quality and parallel imaging performance but does not impact the accuracy of cardiac chamber quantification.

KEY POINTS

• Cardiac chamber quantification using 7.0T magnetic resonance imaging is feasible. • Examination times for cardiac chamber quantification at 7.0T match current clinical practice. • Multichannel transceiver RF technology facilitates improved image quality and parallel imaging performance. • Increasing the number of RF channels does not influence cardiac chamber quantification.

7-T MR--from research to clinical applications?

Over 20,000 MR systems are currently installed worldwide and, although the majority operate at magnetic fields of 1.5 T and below (i.e. about 70%), experience with 3-T (in high-field clinical diagnostic imaging and research) and 7-T (research only) human MR scanners points to a future in functional and metabolic MR diagnostics. Complementary to previous studies, this review attempts to provide an overview of ultrahigh-field MR research with special emphasis on emerging clinical applications at 7 T. We provide a short summary of the technical development and the current status of installed MR systems. The advantages and challenges of ultrahigh-field MRI and MRS are discussed with special emphasis on radiofrequency inhomogeneity, relaxation times, signal-to-noise improvements, susceptibility effects, chemical shifts, specific absorption rate and other safety issues. In terms of applications, we focus on the topics most likely to gain significantly from 7-T MR, i.e. brain imaging and spectroscopy and musculoskeletal imaging, but also body imaging, which is particularly challenging. Examples are given to demonstrate the advantages of susceptibility-weighted imaging, time-of-flight MR angiography, high-resolution functional MRI, (1)H and (31)P MRSI in the human brain, sodium and functional imaging of cartilage and the first results (and artefacts) using an eight-channel body array, suggesting future areas of research that should be intensified in order to fully explore the potential of 7-T MR systems for use in clinical diagnosis.

Estimation of the timing of carotid artery flow using peripheral pulse wave-gated MRI

PURPOSE

To investigate the relationship between peripheral pulse wave (PPW)-gating and the carotid systolic pulse wave in a large clinical patient cohort, and to establish a process for correct estimation of delay time from PPW-gating to foot (ie, beginning) or peak times of carotid systolic pulse waves.

MATERIALS AND METHODS

Subjects comprised 209 patients scanned using 3T magnetic resonance imaging (MRI) for PPW-gated phase contrast images at the common carotid artery. Stepwise multiple regression analysis was conducted for the relationship between foot or peak times and the following factors after excluding correlated factors with coefficients ≥0.5: pulse rate (PR); systolic blood pressure; diastolic blood pressure; height; body weight; body mass index; Brinkman index; diabetes mellitus; hypertension; and hyperlipidemia.

RESULTS

PR showed significant correlation with foot (r = -0.86, P < 0.001) and peak (r = -0.87, P < 0.001) times. The following equations were derived: foot time (msec) = -8.55 × PR + 993.1 and peak time (msec) = -9.21 × PR + 1142.3. No other factors showed significant correlations.

CONCLUSION

PR was the only factor showing significant relationships to foot and peak times of carotid artery flow. The derived equations will facilitate various kinds of noncontrast MR acquisition with simple PPW-gating.

Cardiac MRI of the fetal heart using a novel triggering method: initial results in an animal model

PURPOSE

To investigate MRI of the fetal heart by way of a novel triggering method with the use of an MR-compatible cardiotocography (CTG) in an animal model.

MATERIALS AND METHODS

Fetal cardiac MRI was performed on four pregnant ewes on a 1.5 Tesla (T) MR system. A CTG was rendered MR compatible and its signal was used for the triggering of the fetal heart to perform cardiac cine MRI of the fetal heart with maternal free-breathing with cine steady-state free precession. The left ventricular volume and function were measured from the short-axis (view). The image quality of anatomical structures was assessed.

RESULTS

All cardiac valves and the foramen ovale could be visualized. Myocardial contraction was depicted in cine sequences. The average blood volume at the end systole was 1.7 mL (SD ± 0.12). The average volume at the end diastole was 4.6 mL (± 0.4); thus the average stroke volumes of the left ventricle were 2.87 mL (± 0.31) with ejection fractions of 60.53% (± 4.17).

CONCLUSION

The newly developed MR compatible CTG could be used as a tool for cardiac triggering method of the fetal heart. This novel device might help fetal cardiac MRI technology in the future.

7 Tesla (T) human cardiovascular magnetic resonance imaging using FLASH and SSFP to assess cardiac function: validation against 1.5 T and 3 T

We report the first comparison of cardiovascular magnetic resonance imaging (CMR) at 1.5 T, 3 T and 7 T field strengths using steady state free precession (SSFP) and fast low angle shot (FLASH) cine sequences. Cardiac volumes and mass measurements were assessed for feasibility, reproducibility and validity at each given field strength using FLASH and SSFP sequences. Ten healthy volunteers underwent retrospectively electrocardiogram (ECG) gated CMR at 1.5 T, 3 T and 7 T using FLASH and SSFP sequences. B1 and B0 shimming and frequency scouts were used to optimise image quality. Cardiac volume and mass measurements were not significantly affected by field strength when using the same imaging sequence (P > 0.05 for all parameters at 1.5 T, 3 T and 7 T). SSFP imaging returned larger end diastolic and end systolic volumes and smaller left ventricular masses than FLASH imaging at 7 T, and at the lower field strengths (P < 0.05 for each parameter). However, univariate general linear model analysis with fixed effects for sequence and field strengths found an interaction between imaging sequence and field strength (P = 0.03), with a smaller difference in volumes and mass measurements between SSFP and FLASH imaging at 7 T than 1.5 T and 3 T. SSFP and FLASH cine imaging at 7 T is technically feasible and provides valid assessment of cardiac volumes and mass compared with CMR imaging at 1.5 T and 3 T field strengths.

Pulse wave as an alternate signal for data synchronization during gated myocardial perfusion SPECT imaging

PURPOSE

Proper identification of the cardiac cycle is essential for gated SPECT myocardial perfusion imaging. We have developed an alternate method of ECG for gating, that is, using the peripheral pulse wave (PW) as the triggering signal for gated SPECT acquisition. The aim of this study is to compare the use of this method of gating with the standard ECG trigger.

METHODS

We tested the PW triggering by comparing it with the ECG trigger. We evaluated 33 patients (25 males, 8 females), average age of 61 years (39-80) referred for stress myocardial perfusion imaging. Data from all patients were acquired twice and were processed by CEqual and QGS software. We compared the left ventricular ejection fraction (LVEF), end-diastolic and end-systolic volumes (EDV, ESV). Paired t test and Pearson correlation coefficient were used for comparison.

RESULTS

The mean LVEF, EDV, and ESV calculated with the ECG trigger were 0.52, 120, and 64, respectively, those with the pulse-wave trigger were 0.48, 126, and 71, respectively. Mean paired difference for LVEF was -0.034 (P<0.001), for EDV 5.9 (P=0.012), and for ESV 7.9 (P<0.001). Pearson correlation coefficient for LVEF was 0.955, for EDV 0.987, and for ESV 0.991 (P<0.001 for all correlations).

CONCLUSION

Triggering of gated-data acquisition by the PW is feasible. Quantitative parameters of cardiac function correlate highly with those obtained from the ECG trigger and the absolute differences are not clinically significant across a wide range of values.

Advances in diagnostic radiology

Recent advances in diagnostic radiology are discussed on the basis of current publications in Investigative Radiology. Publications in the journal during 2009 and 2010 are reviewed, evaluating developments by modality and anatomic region. Technological advances continue to play a major role in the evolution and clinical practice of diagnostic radiology, and as such constitute a major publication focus. In the past 2 years, this includes advances in both magnetic resonance and computed tomography (in particular, the advent of dual energy computed tomography). An additional major focus of publications concerns contrast media, and in particular continuing research involving nephrogenic systemic fibrosis, its etiology, and differentiation of the gadolinium chelates on the basis of in vivo stability.

Quantitative assessment of left ventricular function in humans at 7 T

The purpose of this study was to determine the ability of 7 T cardiac magnetic resonance imaging (MRI) to quantitatively assess left ventricular volumes, mass, and function from cine short-axis series and left ventricular diastolic filling from velocity-encoded MRI in 10 healthy volunteers. As comparative "gold standard," the corresponding measures obtained at 1.5 T were taken. Left ventricular volumes, function, and mass were obtained by manual image segmentation. Trans-mitral flow graphs were obtained from 2D one-directional through-plane velocity-encoded MRI planned at the mitral valve in end-systole. Imaging at 7 T MRI was successful in 80% of the examinations. Assessment of left ventricular volumes, function, and mass at 7 T showed good agreement with 1.5 T (no significant differences between variables describing volumes, function, and mass with intraclass correlation coefficients ranging from 0.77 to 0.96). Trans-mitral stroke volume and the ratio between early and atrial peak filling rate showed strong agreement at both field strengths (no significant differences between stroke volumes and filling ratios with intraclass correlation coefficients 0.92 for stroke volumes and 0.77 for peak filling ratios). In conclusion, this study shows that assessing left ventricular volumes, function, and flow is feasible at 7 T MRI and that standardized MRI protocols provide similar quantitative results when compared with 1.5 T MRI.

Acoustic cardiac triggering: a practical solution for synchronization and gating of cardiovascular magnetic resonance at 7 Tesla

BACKGROUND

To demonstrate the applicability of acoustic cardiac triggering (ACT) for imaging of the heart at ultrahigh magnetic fields (7.0 T) by comparing phonocardiogram, conventional vector electrocardiogram (ECG) and traditional pulse oximetry (POX) triggered 2D CINE acquisitions together with (i) a qualitative image quality analysis, (ii) an assessment of the left ventricular function parameter and (iii) an examination of trigger reliability and trigger detection variance derived from the signal waveforms.

RESULTS

ECG was susceptible to severe distortions at 7.0 T. POX and ACT provided waveforms free of interferences from electromagnetic fields or from magneto-hydrodynamic effects. Frequent R-wave mis-registration occurred in ECG-triggered acquisitions with a failure rate of up to 30% resulting in cardiac motion induced artifacts. ACT and POX triggering produced images free of cardiac motion artefacts. ECG showed a severe jitter in the R-wave detection. POX also showed a trigger jitter of approximately Δt = 72 ms which is equivalent to two cardiac phases. ACT showed a jitter of approximately Δt = 5 ms only. ECG waveforms revealed a standard deviation for the cardiac trigger offset larger than that observed for ACT or POX waveforms.Image quality assessment showed that ACT substantially improved image quality as compared to ECG (image quality score at end-diastole: ECG = 1.7 ± 0.5, ACT = 2.4 ± 0.5, p = 0.04) while the comparison between ECG vs. POX gated acquisitions showed no significant differences in image quality (image quality score: ECG = 1.7 ± 0.5, POX = 2.0 ± 0.5, p = 0.34).

CONCLUSIONS

The applicability of acoustic triggering for cardiac CINE imaging at 7.0 T was demonstrated. ACT's trigger reliability and fidelity are superior to that of ECG and POX. ACT promises to be beneficial for cardiovascular magnetic resonance at ultra-high field strengths including 7.0 T.

Right coronary MR angiography at 7 T: a direct quantitative and qualitative comparison with 3 T in young healthy volunteers

PURPOSE

To objectively compare quantitative parameters related to image quality attained at coronary magnetic resonance (MR) angiography of the right coronary artery (RCA) performed at 7 T and 3 T.

MATERIALS AND METHODS

Institutional review board approval was obtained, and volunteers provided signed informed consent. Ten healthy adult volunteers (mean age ± standard deviation, 25 years ± 4; seven men, three women) underwent navigator-gated three-dimensional MR angiography of the RCA at 7 T and 3 T. For 7 T, a custom-built quadrature radiofrequency transmit-receive surface coil was used. At 3 T, a commercial body radiofrequency transmit coil and a cardiac coil array for signal reception were used. Segmented k-space gradient-echo imaging with spectrally selective adiabatic fat suppression was performed, and imaging parameters were similar at both field strengths. Contrast-to-noise ratio between blood and epicardial fat; signal-to-noise ratio of the blood pool; RCA vessel sharpness, diameter, and length; and navigator efficiency were quantified at both field strengths and compared by using a Mann-Whitney U test.

RESULTS

The contrast-to-noise ratio between blood and epicardial fat was significantly improved at 7 T when compared with that at 3 T (87 ± 34 versus 52 ± 13; P = .01). Signal-to-noise ratio of the blood pool was increased at 7 T (109 ± 47 versus 67 ± 19; P = .02). Vessel sharpness obtained at 7 T was also higher (58% ± 9 versus 50% ± 5; P = .04). At the same time, RCA vessel diameter and length and navigator efficiency showed no significant field strength-dependent difference.

CONCLUSION

In our quantitative and qualitative study comparing in vivo human imaging of the RCA at 7 T and 3 T in young healthy volunteers, parameters related to image quality attained at 7 T equal or surpass those from 3 T.

Toward cardiovascular MRI at 7 T: clinical needs, technical solutions and research promises

OBJECTIVE

To consider potential clinical needs, technical solutions and research promises of ultrahigh-field strength cardiovascular MR (CMR).

METHODS

A literature review is given, surveying advantages and disadvantages of CMR at ultrahigh fields (UHF). Key concepts, emerging technologies, practical considerations and applications of UHF CMR are provided. Examples of UHF CMR imaging strategies and their added value are demonstrated, including the numerous unsolved problems. A concluding section explores future directions in UHF CMR.

RESULTS

UHF CMR can be regarded as one of the most challenging MRI applications. Image quality achievable at UHF is not always exclusively defined by signal-to-noise considerations. Some of the inherent advantages of UHF MRI are offset by practical challenges. But UHF CMR can boast advantages over its kindred lower field counterparts by trading the traits of high magnetic fields for increased temporal and/or spatial resolution.

CONCLUSIONS

CMR at ultrahigh-field strengths is a powerful motivator, since speed and signal may be invested to overcome the fundamental constraints that continue to hamper traditional CMR. If practical challenges can be overcome, UHF CMR will help to open the door to new approaches for basic science and clinical research.