High Field Cardiac Magnetic Resonance Imaging
2017; Lippincott Williams & Wilkins; Volume: 10; Issue: 6 Linguagem: Inglês
10.1161/circimaging.116.005460
ISSN1942-0080
AutoresThoralf Niendorf, Jeanette Schulz‐Menger, Katharina Paul, Till Huelnhagen, Victor A. Ferrari, Russell Hodge,
Tópico(s)Atomic and Subatomic Physics Research
ResumoHomeCirculation: Cardiovascular ImagingVol. 10, No. 6High Field Cardiac Magnetic Resonance Imaging Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissionsDownload Articles + Supplements ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toSupplemental MaterialFree AccessReview ArticlePDF/EPUBHigh Field Cardiac Magnetic Resonance ImagingA Case for Ultrahigh Field Cardiac Magnetic Resonance Thoralf Niendorf, PhD, Jeanette Schulz-Menger, MD, Katharina Paul, PhD, Till Huelnhagen, Dipl Ing, Victor A. Ferrari, MD, PhD and Russell Hodge, MA Thoralf NiendorfThoralf Niendorf From the Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (T.N., K.P., T.H., R.H.); DZHK (German Centre for Cardiovascular Research), partner site Berlin (T.N., J.S.-M.); Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (J.S.-M.); Department for Cardiology and Nephrology, HELIOS Clinic Berlin-Buch, Germany (J.S.-M.); and Division of Cardiovascular Medicine and Penn Cardiovascular Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia (V.A.F.). , Jeanette Schulz-MengerJeanette Schulz-Menger From the Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (T.N., K.P., T.H., R.H.); DZHK (German Centre for Cardiovascular Research), partner site Berlin (T.N., J.S.-M.); Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (J.S.-M.); Department for Cardiology and Nephrology, HELIOS Clinic Berlin-Buch, Germany (J.S.-M.); and Division of Cardiovascular Medicine and Penn Cardiovascular Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia (V.A.F.). , Katharina PaulKatharina Paul From the Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (T.N., K.P., T.H., R.H.); DZHK (German Centre for Cardiovascular Research), partner site Berlin (T.N., J.S.-M.); Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (J.S.-M.); Department for Cardiology and Nephrology, HELIOS Clinic Berlin-Buch, Germany (J.S.-M.); and Division of Cardiovascular Medicine and Penn Cardiovascular Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia (V.A.F.). , Till HuelnhagenTill Huelnhagen From the Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (T.N., K.P., T.H., R.H.); DZHK (German Centre for Cardiovascular Research), partner site Berlin (T.N., J.S.-M.); Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (J.S.-M.); Department for Cardiology and Nephrology, HELIOS Clinic Berlin-Buch, Germany (J.S.-M.); and Division of Cardiovascular Medicine and Penn Cardiovascular Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia (V.A.F.). , Victor A. FerrariVictor A. Ferrari From the Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (T.N., K.P., T.H., R.H.); DZHK (German Centre for Cardiovascular Research), partner site Berlin (T.N., J.S.-M.); Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (J.S.-M.); Department for Cardiology and Nephrology, HELIOS Clinic Berlin-Buch, Germany (J.S.-M.); and Division of Cardiovascular Medicine and Penn Cardiovascular Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia (V.A.F.). and Russell HodgeRussell Hodge From the Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (T.N., K.P., T.H., R.H.); DZHK (German Centre for Cardiovascular Research), partner site Berlin (T.N., J.S.-M.); Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany (J.S.-M.); Department for Cardiology and Nephrology, HELIOS Clinic Berlin-Buch, Germany (J.S.-M.); and Division of Cardiovascular Medicine and Penn Cardiovascular Institute, Perelman School of Medicine, University of Pennsylvania, Philadelphia (V.A.F.). Originally published13 Jun 2017https://doi.org/10.1161/CIRCIMAGING.116.005460Circulation: Cardiovascular Imaging. 2017;10:e005460Where Are We Coming From?For a new technology, high-field cardiac magnetic resonance (CMR; 3.0 T≤B0≤7.0 T) has come a long way. Not only has the number of reports referencing clinical applications risen,1,2 but prices for 3.0T systems have dropped almost 50% since the beginning of the millennium, and operational and service costs are virtually identical to those of 1.5T machines. New 3.0T installations make up about 25% of the total market share and are increasing at an annual growth rate of ≈10%. High field comprised ≈25% of the abstracts in CMR submitted to the most recent annual meetings of the Society for Cardiac Magnetic Resonance and the International Society for Magnetic Resonance in Medicine. Large-population studies targeting the heart are now being conducted at 3.0 T, including imaging within the framework of the German National Cohort.3 The advances in CMR at high magnetic field strengths foreshadow some of the potential benefits to be expected as the technology moves to ultrahigh fields (UHF; B0≥7.0 T).Where Do We Stand?The future of high-field CMR has obviously not ended at 3.0 T and is moving higher. Technical barriers are being addressed almost as fast as they appear. Clinical CMR at ultrahigh field strengths is rapidly underway.4–6 About 15% of sites already equipped with a 7.0 T magnetic resonance (MR) system have begun exploring UHF-CMR, with ≈50 peer-reviewed publications on human imaging. Early UHF-CMR applications include imaging and spectroscopy of the heart and large vessels.7–22 This can be achieved because of the inherent relationship between the signal-to-noise ratio (SNR) and magnetic field strength. In parallel, the field is making remarkable progress with respect to novel radiofrequency technologies20,23,24 and MR methods that should make clinical UHF-CMR feasible as 7.0 T MR machines become more widely available.Ultimately, we may see a solution to practical and technical issues that need to be resolved before UHF-CMR could move into clinical settings,25 but these challenges have always faced new technologies. As is customary, we can expect more pioneering research institutions, university hospitals, and large clinics to become early adopters of CMR at 7.0 T and start harvesting knowledge and know-how that will benefit us all. Streamlining hardware and software platforms and making them more robust should permit feasibility studies into CMR on which future applications will be built. These research instruments and activities are dedicated to exploring opportunities for discovery; at some point, the knowledge gained could surely justify the cost. This has been the case in the past, and the investment and operating costs of a 7.0 T MR facility can expect to draw on the same types of private–public partnerships, core facility reimbursement models, institutional funding, and extramural grants that are traditional mechanisms supporting high-end facilities in many institutes today.Where Can UHF-CMR Expect to Harvest New Discoveries?It has been extremely challenging to connect the molecular and cellular defects that characterize many cardiac diseases to the level of major organ systems at which they manifest themselves. Here, magnetic resonance imaging (MRI) has a capacity that is nearly unique among instruments: to simultaneously measure many levels of physiological structure, linking the scales of biologists to those of clinicians, in vivo. UHF-CMR stands to bridge a crucial gap in spatiotemporal resolution at a mesoscopic level above that of the cell, a terrain that is crucial to myocardial and pathological processes but has been difficult to explore.Cardiovascular Morphology and Cardiac Chamber QuantificationUHF-MR may help extend the boundaries of CMR by facilitating high spatial resolution imaging of cardiovascular morphology, as demonstrated in Figure 1, derived from black blood imaging of the human heart at 7.0T. A spatial resolution of 0.9×0.9×4.0 mm3 was applied, representing an enhancement by an order of magnitude over the standardized CMR protocols practiced in clinics today.26Download figureDownload PowerPointFigure 1. Left, Examples of high spatial resolution imaging of cardiac morphology at 7.0 T. These short-axis views of the heart were derived from fast spin-echo imaging using a spatial resolution of 0.9×0.9×4.0 mm3 (top) and 0.8×0.8×3.0 mm3 (bottom). Right, Full field-of-view and zoomed images of the carotid artery vessel wall acquired with 2D dark blood fast spin-echo imaging at 7.0 T using an in-plane spatial resolution of 0.55×0.55 mm2. For imaging a 4-channel transceiver radiofrequency building block (Figure 2D) comprising 4 loop elements was used for each carotid. The phase setting for each loop element was tailored for enhancing transmission field uniformity around the carotids.Plaque imaging, arterial wall imaging, and carotid wall imaging are applications that perhaps demand highest spatial resolution. Figure 1 depicts preliminary results obtained from 2-dimensional (2D) dark blood fast spin-echo imaging of the carotid artery wall at 7.0T using an in-plane spatial resolution of 0.6×0.6 mm2. A spatial resolution of 0.4×0.4×1.5 mm3 was demonstrated for the normal carotid vessel wall using T1-weighted dark blood fast spin-echo imaging at 7.0T.27 A quantitative comparison revealed a 2-fold average gain in SNR (Table).27 T1 relaxation time of the normal carotid vessel wall was 1628 ms at 7.0T (3.0T: T1=1227 ms), whereas a T2 relaxation time of 46 ms was observed at 7.0T (3.0T: T2=55 ms).27Table. Synopsis of Gains in SNR Obtained for Cardiovascular MR Applications at 7.0 T vs Lower Field StrengthsApplicationImaging TechniqueSNR GainNoteCarotid vessel wall imaging272D T1-weighted black blood prepared fast spin-echo2.0 (vs 3.0 T)RF coil designs used for reception were optimized for the 7.0- and 3.0T setup, 3.0- and 7.0T RF coil sensitivity similar at targetCardiac chamber quantification122D CINE fast gradient echo2.1 (vs 1.5 T)Different RF coil designs used for signal reception at 7.0 and 1. 5 T, RF coil sensitivity in favor of 1.5T setupNoncontrast-enhanced, time-resolved phase velocity 4D aortic flow imaging213D free-breathing, navigator-gated, velocity-encoded gradient echo2.2 (vs 3.0 T) 3.8 (vs 1. 5 T)Different RF coil designs used for signal reception at 7.0 and 3.0/1.5 T, RF coil sensitivity in favor of 3.0/1.5 T setupCoronary MRA103D free-breathing, navigator–gated, and fat-suppressed 3D k-space segmented fast-gradient echo1.63 (vs 3.0 T)Different RF coil designs used for signal reception at 7.0 and 3.0 T, RF coil sensitivity in favor of 3.0 T setup31P spectroscopy of the septum19UTE-CSI2.6–2.8 (PCr signal; vs 3.0 T)Same RF coil design used at 7.0 and 3.0 T2D indicates 2-dimensional; 3D, 3-dimensional; 4D, 4-dimensional; MR, magnetic resonance; MRA, magnetic resonance angiography; PCr, phosphocreatine; RF, radiofrequency; SNR, signal-to-noise ratio; and UTE-CSI, ultrashort echo time chemical shift imaging.Early studies confirmed that UHF-CMR can be used for cardiac chamber quantification of the left8,12,13 and the right ventricle16 using high-density arrays of radiofrequency antennae (Figure 2).20,23,24,28–30 These studies reported a 2.1 times SNR increase at 7.0 versus 1.5T.12 This favorable effect produces blood/myocardial contrast, which is competitive with that obtained for 2D CINE SSFP imaging at 1.5T, high-quality images with a uniform signal intensity and high blood/myocardial contrast over the entire heart as illustrated in Figure 3 and Movie I in the Data Supplement. The latter depicts short-axis views covering the heart from the apex to the base and demonstrates rather uniform signal intensity across the heart facilitated by a modular 32-channel transceiver loop antenna radiofrequency array at 7.0 T.20Figure 3 depicts 4-chamber and short-axis views of the heart obtained with a standardized CMR protocol26 and compares them with a protocol that enhances spatial resolution. The latter reduces the voxel size from 19.4 to ≈3.0 mm3, a 6-fold improvement in spatial resolution over a standardized clinical CMR protocol.26 This fidelity approaches the relative anatomic spatial resolution—in terms of number of voxels with respect to anatomy—demonstrated for animal models.31 This achievement is translatable into opportunities for discovery, including the ability to detect subtle myocardial crypts (Figure 4) that often appear in conjunction with hypertrophic cardiomyopathy.32 High spatial fidelity gradient echo 2D CINE imaging at 7.0T permits the identification of such subtle morphological details in myocardial regions of extended hypertrophy and fibrosis.5 These structures have not been detectable in this particular myocardial region at current clinical field strengths in the same patient and may yield new insights into developmental aspects of myocardial remodeling.5 These preliminary results warrant further research in human subjects and animal models to link and validate the UHF-CMR mesoscopic imaging observations with pathological findings, HCM phenotypes, and molecular mechanisms of the disease.Download figureDownload PowerPointFigure 2. Four-chamber views of the heart of a healthy subject acquired at 7.0T using (A) a 4-channel loop, (B) an 8-channel loop, (C) a 16-channel loop, (D) a 32-channel loop, and (E) a 16-channel bow tie antenna transceiver RF array. For the CINE FLASH acquisitions of the 4-chamber views shown above, a spatial resolution of 1.4×1.4×4.0 mm3 was used for all radiofrequency array configurations. Reprinted from Niendorf et al25 with permission of the publisher. Copyright © 2015, John Wiley and Sons. Authorization for this adaptation has been obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.Download figureDownload PowerPointFigure 3. Left, End-diastolic short-axis views covering the heart from the apex to the base showing rather uniform signal intensity and no major signal voids. Images were derived from 2-dimensional (2D) CINE FLASH imaging using a spatial resolution of 1.1×1.1×2.5 mm3 and a 16-channel transceiver bow tie antenna RF array at 7.0 T. (Movie I in the Data Supplement shows a video encompassing all cardiac phases.) Right, Four-chamber views (left) and short-axis views (right) of the heart derived from 2D CINE FLASH acquisitions at 7.0 T using a standardized clinical protocol with a spatial resolution of 1.8×1.8×6.0 mm3 (top) and an enhanced spatial resolution of 1.1×1.1×2.5 mm3 (bottom). The latter constitutes a 6-fold improvement in spatial resolution over the standardized cardiac magnetic resonance protocol. Reprinted from Niendorf et al25 with permission of the publisher. Copyright © 2015, John Wiley and Sons. Authorization for this adaptation has been obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.Download figureDownload PowerPointFigure 4. A 3-chamber view (left) and a short-axis view (right) of the heart of a hypertrophic cardiomyopathy patient showing myocardial crypts (white arrows) in the anteroseptal region of the heart.5 Images were acquired at 7.0T using 2D CINE gradient echo imaging with a spatial resolution of 1.4×1.4×4.0 mm3.Real-Time MRI of the HeartUHF-CMR provides an unprecedented potential for real-time imaging and addressing some of the shortcomings and physiological constraints of traditional assessments of left ventricular and right ventricular structure and function. The accelerated imaging capabilities of free-breathing real-time imaging of the heart at 7.0 T are demonstrated in Figure 5. The spatial resolution of 1.2×1.2×6.0 mm3 and the frame rate of 30 frames per second surpass the requirements established by current clinical protocols for standardized assessments of left ventricular structure and function.26 The findings from such studies may be extrapolated to clinical CMR studies at lower spatial and temporal resolutions, which may enhance their clinical use.Download figureDownload PowerPointFigure 5. Examples derived from free-breathing real-time imaging of the heart at 7.0 T: a 4-chamber view (left), a midventricular short-axis view (center), and a 2-chamber view of the heart (right). Images were acquired at a rate of 30 frames per second and a spatial resolution of 1.2×1.2×6.0 mm3.23Myocardial Tissue Characterization and PhenotypingUHF-CMR opens new avenues into myocardial tissue characterization and phenotyping by mapping the effective transverse relaxation time T2*. The linear relationship between magnetic field strength and microscopic susceptibility effects makes 7.0 T an appealing avenue for T2* mapping.17 This will permit observing enhanced susceptibility effects by lowering the detection level for abnormal tissue as compared with low-field MR and extending the dynamic range of sensitivity for monitoring T2* changes.By reducing the in-phase interecho time from 4.8 ms at 1.5T to 1.02 ms at 7.0T, UHF-CMR can acquire multiple gradient echoes.14 Unlike single cardiac phase T2* mapping at lower fields, UHF-CMR permits 2D CINE T2* mapping across the entire cardiac cycle. Phase-resolved, high-resolution T2* maps have been made of the human heart in vivo at 7.0T.14,22 Temporal changes in T2* across the cardiac cycle are not assessable at 1.5 or 3.0T because of scan time constraints. CINE T2* mapping at 7.0T accomplishes this, reporting cyclic changes in septal T2* with a mean increase of end-systolic T2* of ≈10% compared with end diastole.22 The periodic changes in interventricular septal myocardial T2* correlate well with changes in the thickness of the septal wall and the left ventricular radius throughout the cardiac cycle (Figure 6). Cyclic T2* variations were attributed to changes in myocardial blood volume fraction rather than oxygenation, an example of the unique information that temporally resolved MR relaxation mapping can reveal about cardiac (patho)physiology in vivo.22Download figureDownload PowerPointFigure 6. Ultrahigh field cardiac magnetic resonance (UHF-CMR) permits 2-dimensional (2D) CINE T2* mapping across the entire cardiac cycle. Top, A magnitude image and a T2* map of an end-diastolic short-axis view of the heart obtained for a healthy subject at 7.0 T. Center, A magnitude image and a T2* map of an end-systolic short-axis view of the heart obtained for a patient with hypertrophic cardiomyopathy at 7.0T. Bottom, Phase-resolved, high-resolution T2* mapping at 7.0T revealed that septal T2* correlates with myocardial wall thickness (left) and left ventricular inner radius (right) in healthy volunteers at 7.0T.22 Error bars in scatter plots indicate SEM.The first cardiac T2* maps made at 7.0T of hypertrophic cardiomyopathy demonstrate that septal T2* is increased in patients compared with healthy controls4 as illustrated in Figure 6. A mean septal T2* of 17.5 ms for HCM patients and of 13.7 ms for healthy subjects matched by body mass index and age were reported.4 These findings suggest that T2* mapping at UHF could provide an important imaging-based biomarker in support of diagnoses and risk stratification in cardiomyopathies.Heteronuclear MRI for Metabolic and Nanomolecular ProbingThe greater spectral resolution and sensitivity gain of UHF-MR helps clinicians and researchers to move beyond conventional 1H imaging to study other MR nuclei that are more relevant to cardiac metabolism. The higher magnetic fields permit imaging phosphorous (31P), sodium (23Na), fluorine (19F), carbon (13C), oxygen (17O), potassium (39K), chlorine (35Cl), and other nuclei. Each new substance adds greater dimension to our view of cardiac metabolism, bioenergetics, and tissue function in ways that should help us integrate our knowledge of events at the molecular scale and their effects on a higher level.Sodium MRI (23Na MRI), for example, can provide clearer insights into ion homeostasis. Currently, clinical applications are limited by the rapidly decaying 23Na signal and the low sensitivity of 23Na MRI versus 1H MR. The gain of sensitivity at 7.0 T, permitted 3-dimensional 23Na imaging of the entire heart with a spatial resolution of 6×6×6 mm3 in clinically acceptable acquisition times of <10 minutes.33 Information from high fields will likely be applicable in the reverse direction—new associations revealed by 23Na will shed light on conventionally acquired 1H imaging data and help clinicians interpret observations at lower field strengths. The signal intensity gain at 7.0 T has recently enabled CINE 23Na imaging of the beating heart at a temporal resolution of 100 ms34 as illustrated in Figure 7. Feasibility studies of 23Na MRI at 7.0T should reveal the diagnostic value of this tool in distinguishing viable from nonviable tissue in ischemic heart disease patients. It may also offer deeper insights into hypertrophic cardiomyopathy, because of reports that this condition is accompanied by a 33% decrease in Na+, K+-ATPase activity and a 40% increase in intracellular Na+ concentrations.Download figureDownload PowerPointFigure 7. A CINE series of sodium images of the heart covering the entire cardiac cycle with a temporal resolution of 0.1 s. For data acquisition and reconstruction of the transversal view of the heart, a nominal isotropic spatial resolution of 6 mm3 was used.34Potassium ions (K+) play a vital role in myocardial function. Although extracellular K+ concentrations can easily be estimated from laboratory analysis of blood samples, a method for measuring the intracellular K+ content in vivo is greatly needed and would permit novel insights into pathophysiological processes of cardiac diseases, including arrhythmias. Potassium MRI is challenging because the sensitivity is ≈6 orders of magnitude less than that of 1H MRI. The sensitivity gain of 7.0 T will enable for the first time quantitative in vivo assessment of the cellular potassium content in the myocardium. This approach opens an entirely new research field of MRI-driven phenotyping as a link to personalized medicine.Phosphorus MR at higher fields will permit new observations of energy metabolism in vivo. The sensitivity gain at ultrahigh magnetic fields has been demonstrated in comparisons between cardiac 31P-MRS at 7.0 and 3.0T,19 which highlighted the marked superiority of cardiac 31P spectra at 7.0T as shown in Figure 8. SNR improvements of 2.6 to 2.8 (Table) for phosphocreatine together with a reduced SD for the phosphocreatine/ATP ratio were observed, permitting enhanced quantification of the phosphocreatine/ATP concentration ratio.19 7.0T should quickly become the field strength of choice for probing myocardial energetics with cardiac 31P MR spectroscopy.19 This conclusion is supported by the first study of this type, performed in a cohort of 25 patients with dilated cardiomyopathy.6 The phosphocreatine/ATP ratio was found to be significantly lower in dilated cardiomyopathy patients than in healthy control subjects (Figure 8). These results are a precursor to broader clinical studies aiming to assess the effect of energy sparing drugs in patients with dilated cardiomyopathy.6Download figureDownload PowerPointFigure 8. Examples for phosphorous (31P) magnetic resonance (MR) spectroscopy in healthy subjects19 (top) and patients with dilated cardiomyopathy6 (bottom) at 7.0 T (red lines) and at 3.0 T (blue lines). The 31P MR spectra were derived from a myocardial region located in the middle of the interventricular septum of the heart. 31P MR spectroscopy at 7.0 T provided a substantial signal-to-noise ratio (SNR) advantage (SNRgain,PCr=2.6–2.8) over 31P MR spectroscopy at 3.0 T.19 Printed with permission of Christopher Rodgers, Radcliffe Department of Medicine, University of Oxford, Oxford, UK. DPG indicates 2,3-diphosphoglycerate; PCr, phosphocreatine; and PDE, phosphodiesters.Fluorine magnetic resonance (19F MR) is a valuable tool for in vivo tracking and quantification of fluorine-containing exogenous agents, such as emulsified perfluorcarbon tracer or 19F-labeled cells. Natural fluorine is virtually absent in body tissue, so 19MRI yields background-free images with complete signal selectivity and specificity. 19F detection is currently beyond the sensitivity of most clinical apparatus at lower field strengths because of signal-to-noise constraints related to reproducibility; this is reflected by the low number of reports in the literature to date. UHF-MR can perform 19F MRI in vivo through the design of dedicated radiofrequency antennae. Applications might include in vivo monitoring of the inflammatory response to acute myocardial infarction and early ventricular remodeling, as well as studies of the bioavailability and pharmacokinetics of 19F-containing drugs and 19F-labeled cells.Vascular UHF-MRIAnother common cardiovascular application of MR that stands to benefit from higher field strengths is magnetic resonance angiography (MRA) of the large vessels. The SNR gain and the reduction in the geometry factor (g-factor) can be used to counter noise amplification caused by the acceleration techniques necessary for real-time MRA, in order to achieve high spatiotemporal resolution MRA and for scan time shortening.UHF-MRA permits larger fields of view along the head–feet direction over current short-bore low-field MR scanners because of the longer magnet and the better B0-uniformity along the z-direction. Local transmit/receive radiofrequency coil arrays tailored for UHF-CMR support anatomic coverage required for MR of large vessels as demonstrated in Figure 9 (left, center) for a field of viewhead-feet=35 cm for free-breathing real-time imaging (spatial resolution=1.2×1.2×6.0 mm3) and segmented CINE imaging (spatial resolution=1.1×1.1×2.5 mm3) of the human aorta. Another potential application includes large volume coverage, time-resolved phase velocity MRA (4D flow), used to study flow patterns and wall shear stress in large vessels. Aortic 4D flow imaging has recently been demonstrated as feasible at 7.0T using 1.2-mm isotropic spatial resolution (Figure 9, right).35 SNR quantification of 4D aortic flow imaging at 7.0T revealed 2.2 times the SNR of 3.0T and 3.8 times the SNR of 1.5T (Table), which is in accordance with theoretical considerations.21 The baseline SNR advantage of UHF-MR also translates into better spatiotemporal resolution in contrast-enhanced MRA. Quantification of contrast enhancement of 4D aortic flow imaging showed that SNR gains achieved by contrast-enhanced versus nonenhanced MRA of the descending aorta were 1.8-fold at 1.5T, 1.7-fold at 3T, and 1.4-fold at 7T.21Download figureDownload PowerPointFigure 9. Examples of large field of view cardiovascular imaging derived from non–contrast-enhanced free-breathing real-time imaging (left), segmented 2-dimensional (2D) CINE FLASH imaging (center), and 4D magnetic resonance (MR) flow imaging (right) of the aorta at 7.0 T. The accelerated imaging capabilities at 7.0 T and the anatomic coverage of the 16-channel bow tie antenna array used for transmission and reception (Figure 2) supported free-breathing real-time imaging of the aorta at a rate of 30 frames per second using highly undersampled radial 2D FLASH with nonlinear inverse reconstruction at a spatial resolution of 1.2×1.2×6.0 mm3. For comparison, conventional, segmented 2D CINE FLASH imaging of the same slice was conducted with an in-plane spatial resolution of 1.1×1.1×2.5 mm3. The real-time and the 2D CINE FLASH images of the aorta demonstrate the 35-cm anatomic coverage of the 16-channel bow tie antenna array along the head–feet direction. Right, Pathline visualization of blood velocity within the human aorta at t=306 ms relative to R wave of the ECG signal. Data were obtained from 4D flow measurements at 7.0 T using 1.2-mm isotropic spatial and 40.8-ms temporal resolution. 4D flow image (right) printed with permission Sebastian Schmitter, Germany Metrology Institute, Berlin, Germany and Center for Magnetic Resonance Research, University of Minnesota Medical School, Minneapolis.The capabilities of noncontrast UHF-MRA were demonstrated for high spatial resolution (0.45×0.45×1.2 mm3) coronary artery imaging,18 which revealed enhancements in coronary vessel edge sharpness at 7.0T compared with state-of-the-art coronary artery imaging at 3.0T. SNR of the right coronary artery blood pool was found to be increased (163%; Table) at 7.0 versus 3.0T.10 These improvements should also benefit coronary vesse
Referência(s)