Reproducibility of cardiac 31P MRS at 7 T

Synopsis Cardiac PCr/ATP ratios measured by P MRS change in cardiovascular disease giving them value as a biomarker. We scanned 13 healthy volunteers at 7T, assessing their PCr/ATP with 6 ½ min P CSI scans. These data have better reproducibility than a 30min 3T protocol previously published by our centre. Repeated PCr/ATP measurements from subjects in this study were not significantly (P=0.83) different. Measurements were significantly different (P<0.001) from DCM patient data acquired in a previous 7T study using the same coil and pulse sequence. This data will allow us to plan future 7T P-MRS clinical studies.Funded by a Sir Henry Dale Fellowship from the Wellcome Trust and the Royal Society (Grant No. 098436/Z/12/Z). JE receives a DPhil (PhD) studentship from the Medical Research Council (UK)

Using a whole-body 31P birdcage transmit coil and 16-element receive array for human cardiac metabolic imaging at 7T.

PURPOSE: Cardiac phosphorus magnetic resonance spectroscopy (31P-MRS) provides unique insight into the mechanisms of heart failure. Yet, clinical applications have been hindered by the restricted sensitivity of the surface radiofrequency-coils normally used. These permit the analysis of spectra only from the interventricular septum, or large volumes of myocardium, which may not be meaningful in focal disease. Löring et al. recently presented a prototype whole-body (52 cm diameter) transmit/receive birdcage coil for 31P at 7T. We now present a new, easily-removable, whole-body 31P transmit radiofrequency-coil built into a patient-bed extension combined with a 16-element receive array for cardiac 31P-MRS. MATERIALS AND METHODS: A fully-removable (55 cm diameter) birdcage transmit coil was combined with a 16-element receive array on a Magnetom 7T scanner (Siemens, Germany). Electro-magnetic field simulations and phantom tests of the setup were performed. In vivo maps of B1+, metabolite signals, and saturation-band efficiency were acquired across the torsos of eight volunteers. RESULTS: The combined (volume-transmit, local receive array) setup increased signal-to-noise ratio 2.6-fold 10 cm below the array (depth of the interventricular septum) compared to using the birdcage coil in transceiver mode. The simulated coefficient of variation for B1+ of the whole-body coil across the heart was 46.7% (surface coil 129.0%); and the in vivo measured value was 38.4%. Metabolite images of 2,3-diphosphoglycerate clearly resolved the ventricular blood pools, and muscle tissue was visible in phosphocreatine (PCr) maps. Amplitude-modulated saturation bands achieved 71±4% suppression of phosphocreatine PCr in chest-wall muscles. Subjects reported they were comfortable. CONCLUSION: This easy-to-assemble, volume-transmit, local receive array coil combination significantly improves the homogeneity and field-of-view for metabolic imaging of the human heart at 7T

Feasibility of absolute quantification for 31 P MRS at 7 T.

PURPOSE: Phosphorus spectroscopy can differentiate among liver disease stages and types. To quantify absolute concentrations of phosphorus metabolites, sensitivity calibration and transmit field ( B1+ ) correction are required. The trend toward ultrahigh fields (7 T) and the use of multichannel RF coils makes this ever more challenging. We investigated the constraints on reference phantoms, and implemented techniques for the absolute quantification of human liver phosphorus spectra acquired using a 10-cm loop and a 16-channel array at 7 T. METHODS: The effect of phantom conductivity was assessed at 25.8 MHz (1.5 T), 49.9 MHz (3 T), and 120.3 MHz (7 T) by electromagnetic modeling. Radiofrequency field maps ( B1± ) were measured in phosphate phantoms (18 mM and 40 mM) at 7 T. These maps were used to assess the correction of 4 phantom 3D-CSI data sets using 3 techniques: phantom replacement, explicit normalization, and simplified normalization. In vivo liver spectra acquired with a 10-cm loop were corrected with all 3 methods. Simplified normalization was applied to in vivo 16-channel array data sets. RESULTS: Simulations show that quantification errors of less than 3% are achievable using a uniform electrolyte phantom with a conductivity of 0.23-0.86 S.m-1 at 1.5 T, 0.39-0.58 S.m-1 at 3 T, and 0.34-0.42 S.m-1 (16-19 mM KH2 PO4(aq) ) at 7 T. The mean γ-ATP concentration quantified in vivo at 7 T was 1.39 ± 0.30 mmol.L-1 to 1.71 ± 0.35 mmol.L-1 wet tissue for the 10-cm loop and 1.88 ± 0.25 mmol.L-1 wet tissue for the array. CONCLUSION: It is essential to select a calibration phantom with appropriate conductivity for quantitative phosphorus spectroscopy at 7 T. Using an 18-mM phosphate phantom and simplified normalization, human liver phosphate metabolite concentrations were successfully quantified at 7 T.Funded by a Sir Henry Dale Fellowship from the Wellcome Trust and the Royal Society (Grant Number 098436/Z/12/B). LABP received a DPhil studentship from the Medical Research Council (UK). The support of Slovak grant agency VEGA (Grant Number 2/0001/17) and APVV (Grant Number 15-0029) is also gratefully acknowledged

Quantification of human cardiac inorganic phosphate content in vivo by 31P-MRSI at 7T

Synopsis: Determination of human cardiac Pi using 31P-MRS is challenging as the resonance frequency of Pi is concealed by a close resonating 2,3-DPG signal originating from blood. Long TR acquisition using adiabatic excitation at 7T can compensate for the rapid blood signal replacement in partially-saturated short TR scans. In order to quantify Pi concentration in vivo, knowledge about longitudinal relaxation of Pi is still required. We have measured the T1 of Pi in 4 healthy volunteers at 7T using dual-TR method and used this value to quantify cardiac Pi concentration in 8 healthy volunteers.We acknowledge financial support from a Sir Henry Dale Fellowship awarded by the Wellcome Trust and the Royal Society (098436/Z/12/Z), and from Slovak Grant Agencies VEGA (2/0001/17) and APVV (15-0029)

Whole-body 7T 31P birdcage transmit coil driven by a 35kW RF amplifier with an integrated 30-element 31P receive array and an 8-element 1H transmit/receive array

Synopsis: We describe our experiences implementing a whole-body transmit coil driven by a 35kW RF power amplifier, with a 30-element 31P receive array, and an 8-element 1H transmit/receive array, optimised for cardiac 31P-MRS at 7T. We describe an adaptation to the vendor’s standard SAR monitoring to monitor RF power levels up to the full 35kW output of the RFPA. This new hardware was found to achieve better 31P B1+ and SNR at the depth of the heart than other coils available in our institution. This setup promises to allow the first regionally-resolved, whole-heart 31P-MRSI studies at 7T in the near future.Funded by a Sir Henry Dale Fellowship from the Wellcome Trust and the Royal Society (098436/Z/12/Z); a Science Enhancement from the Wellcome Trust (Grant No. 098436/Z/12/A); the EPACephalosporin Fund (Grant No. CF 284);the Oxford BHFCentre of Research Excellence (Grant No.RE/13/1/30181); and Slovak Grant Agencies VEGA (2/0001/17) and APVV (15-0029). We gratefully acknowledge support from Iulius Dragonu, Karsten Wicklow and Ulrich Fontius at Siemens Healthcare

Using a whole-body 31P birdcage transmit coil and 16-element receive array for human cardiac metabolic imaging at 7T.

PURPOSE: Cardiac phosphorus magnetic resonance spectroscopy (31P-MRS) provides unique insight into the mechanisms of heart failure. Yet, clinical applications have been hindered by the restricted sensitivity of the surface radiofrequency-coils normally used. These permit the analysis of spectra only from the interventricular septum, or large volumes of myocardium, which may not be meaningful in focal disease. Löring et al. recently presented a prototype whole-body (52 cm diameter) transmit/receive birdcage coil for 31P at 7T. We now present a new, easily-removable, whole-body 31P transmit radiofrequency-coil built into a patient-bed extension combined with a 16-element receive array for cardiac 31P-MRS. MATERIALS AND METHODS: A fully-removable (55 cm diameter) birdcage transmit coil was combined with a 16-element receive array on a Magnetom 7T scanner (Siemens, Germany). Electro-magnetic field simulations and phantom tests of the setup were performed. In vivo maps of B1+, metabolite signals, and saturation-band efficiency were acquired across the torsos of eight volunteers. RESULTS: The combined (volume-transmit, local receive array) setup increased signal-to-noise ratio 2.6-fold 10 cm below the array (depth of the interventricular septum) compared to using the birdcage coil in transceiver mode. The simulated coefficient of variation for B1+ of the whole-body coil across the heart was 46.7% (surface coil 129.0%); and the in vivo measured value was 38.4%. Metabolite images of 2,3-diphosphoglycerate clearly resolved the ventricular blood pools, and muscle tissue was visible in phosphocreatine (PCr) maps. Amplitude-modulated saturation bands achieved 71±4% suppression of phosphocreatine PCr in chest-wall muscles. Subjects reported they were comfortable. CONCLUSION: This easy-to-assemble, volume-transmit, local receive array coil combination significantly improves the homogeneity and field-of-view for metabolic imaging of the human heart at 7T