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  • ItemOpen Access
    31P magnetization transfer measurements of Pi→ATP flux in exercising human muscle
    (American Physiological Society, 2016-03-15) Sleigh, Alison; Savage, David B; Williams, Guy B; Porter, David; Carpenter, T Adrian; Brindle, Kevin M; Kemp, Graham J; Savage, David [0000-0002-7857-7032]; Williams, Guy [0000-0001-5223-6654]; Carpenter, Adrian [0000-0002-2939-8222]; Brindle, Kevin [0000-0003-3883-6287]
    Fundamental criticisms have been made over the use of (31)P magnetic resonance spectroscopy (MRS) magnetization transfer estimates of inorganic phosphate (Pi)→ATP flux (VPi-ATP) in human resting skeletal muscle for assessing mitochondrial function. Although the discrepancy in the magnitude of VPi-ATP is now acknowledged, little is known about its metabolic determinants. Here we use a novel protocol to measure VPi-ATP in human exercising muscle for the first time. Steady-state VPi-ATP was measured at rest and over a range of exercise intensities and compared with suprabasal oxidative ATP synthesis rates estimated from the initial rates of postexercise phosphocreatine resynthesis (VATP). We define a surplus Pi→ATP flux as the difference between VPi-ATP and VATP. The coupled reactions catalyzed by the glycolytic enzymes GAPDH and phosphoglycerate kinase (PGK) have been shown to catalyze measurable exchange between ATP and Pi in some systems and have been suggested to be responsible for this surplus flux. Surplus VPi-ATP did not change between rest and exercise, even though the concentrations of Pi and ADP, which are substrates for GAPDH and PGK, respectively, increased as expected. However, involvement of these enzymes is suggested by correlations between absolute and surplus Pi→ATP flux, both at rest and during exercise, and the intensity of the phosphomonoester peak in the (31)P NMR spectrum. This peak includes contributions from sugar phosphates in the glycolytic pathway, and changes in its intensity may indicate changes in downstream glycolytic intermediates, including 3-phosphoglycerate, which has been shown to influence the exchange between ATP and Pi catalyzed by GAPDH and PGK.
  • ItemOpen Access
    The Cambridge MRI database for animal models of Huntington disease.
    (Elsevier BV, 2016-01-01) Sawiak, Stephen J; Morton, A Jennifer; Sawiak, Stephen [0000-0003-4210-9816]; Morton, Jennifer [0000-0003-0181-6346]
    We describe the Cambridge animal brain magnetic resonance imaging repository comprising 400 datasets to date from mouse models of Huntington disease. The data include raw images as well as segmented grey and white matter images with maps of cortical thickness. All images and phenotypic data for each subject are freely-available without restriction from (http://www.dspace.cam.ac.uk/handle/1810/243361/). Software and anatomical population templates optimised for animal brain analysis with MRI are also available from this site.
  • ItemPublished versionOpen Access
    MR fingerprinting with simultaneous B1 estimation.
    (Wiley, 2016-10) Buonincontri, Guido; Sawiak, Stephen J; Buonincontri, Guido [0000-0002-8386-639X]
    PURPOSE: MR fingerprinting (MRF) can be used for quantitative estimation of physical parameters in MRI. Here, we extend the method to incorporate B1 estimation. METHODS: The acquisition is based on steady state free precession MR fingerprinting with a Cartesian trajectory. To increase the sensitivity to the B1 profile, abrupt changes in flip angle were introduced in the sequence. Slice profile and B1 effects were included in the dictionary and the results from two- and three-dimensional (3D) acquisitions were compared. Acceleration was demonstrated using retrospective undersampling in the phase encode directions of 3D data exploiting redundancy between MRF frames at the edges of k-space. RESULTS: Without B1 estimation, T2 and B1 were inaccurate by more than 20%. Abrupt changes in flip angle improved B1 maps. T1 and T2 values obtained with the new MRF methods agree with classical spin echo measurements and are independent of the B1 field profile. When using view sharing reconstruction, results remained accurate (error <10%) when sampling under 10% of k-space from the 3D data. CONCLUSION: The methods demonstrated here can successfully measure T1, T2, and B1. Errors due to slice profile can be substantially reduced by including its effect in the dictionary or acquiring data in 3D. Magn Reson Med 76:1127-1135, 2016. © 2015 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
  • ItemOpen Access
    In vivo γ-aminobutyric acid measurement in rats with spectral editing at 4.7T.
    (Wiley, 2016-06) Sawiak, Stephen J; Jupp, Bianca; Taylor, Tom; Caprioli, Daniele; Carpenter, T Adrian; Dalley, Jeffrey W; Sawiak, Stephen [0000-0003-4210-9816]; Carpenter, Adrian [0000-0002-2939-8222]; Dalley, Jeffrey [0000-0002-2282-3660]
    PURPOSE: To evaluate the feasibility of spectral editing for quantification of γ-aminobutyric acid (GABA) in the rat brain and to determine whether altered GABA concentration in the ventral striatum is a neural endophenotype associated with trait-like impulsive behavior. MATERIALS AND METHODS: Spectra were acquired at 4.7T for 23 male Lister-hooded rats that had been previously screened for extremely low and high impulsivity phenotypes on an automated behavioral task (n = 11 low-impulsive; n = 12 high-impulsive). Voxels of 3 × 7 × 4 mm(3) (84 μL) centered bilaterally across the ventral striatum were used to evaluate GABA concentration ratios. RESULTS: Quantifiable GABA signals in the ventral striatum were obtained for all rats. Mean-edited GABA to n-acetyl aspartate (NAA) ratios in the ventral striatum were 0.22 (95% confidence interval [CI] [0.18, 0.25]). Mean GABA/NAA ratios in this region were significantly decreased by 28% in high-impulsive rats compared to low-impulsive rats (P = 0.02; 95% CI [-53%, -2%]). CONCLUSION: These findings demonstrate that spectral editing at 4.7T is a feasible method to assess in vivo GABA concentrations in the rat brain. The results show that diminished GABA content in the ventral striatum may be a neural endophenotype associated with impulsivity. J. Magn. Reson. Imaging 2016;43:1308-1312.
  • ItemOpen Access
    Simple and effective exercise design for assessing in vivo mitochondrial function in clinical applications using (31)P magnetic resonance spectroscopy.
    (Springer Science and Business Media LLC, 2016-01-11) Sleigh, Alison; Lupson, Victoria; Thankamony, Ajay; Dunger, David B; Savage, David B; Carpenter, T Adrian; Kemp, Graham J; Dunger, David [0000-0002-2566-9304]; Savage, David [0000-0002-7857-7032]; Carpenter, Adrian [0000-0002-2939-8222]
    The growing recognition of diseases associated with dysfunction of mitochondria poses an urgent need for simple measures of mitochondrial function. Assessment of the kinetics of replenishment of the phosphocreatine pool after exercise using (31)P magnetic resonance spectroscopy can provide an in vivo measure of mitochondrial function; however, the wider application of this technique appears limited by complex or expensive MR-compatible exercise equipment and protocols not easily tolerated by frail participants or those with reduced mental capacity. Here we describe a novel in-scanner exercise method which is patient-focused, inexpensive, remarkably simple and highly portable. The device exploits an MR-compatible high-density material (BaSO4) to form a weight which is attached directly to the ankle, and a one-minute dynamic knee extension protocol produced highly reproducible measurements of post-exercise PCr recovery kinetics in both healthy subjects and patients. As sophisticated exercise equipment is unnecessary for this measurement, our extremely simple design provides an effective and easy-to-implement apparatus that is readily translatable across sites. Its design, being tailored to the needs of the patient, makes it particularly well suited to clinical applications, and we argue the potential of this method for investigating in vivo mitochondrial function in new cohorts of growing clinical interest.
  • ItemPublished versionOpen Access
    Additional sampling directions improve detection range of wireless radiofrequency probes.
    (Wiley, 2016-09) Hoffmann, Malte; Mada, Marius; Carpenter, T Adrian; Sawiak, Stephen J; Williams, Guy B; Hoffmann, Malte [0000-0002-5511-0739]
    PURPOSE: While MRI is enhancing our knowledge about the structure and function of the human brain, subject motion remains a problem in many clinical applications. Recently, the use of wireless radiofrequency markers with three one-dimensional (1D) navigators for prospective correction was demonstrated. This method is restricted in the range of motion that can be corrected, however, because of limited information in the 1D readouts. METHODS: Here, the limitation of techniques for disambiguating marker locations was investigated. It was shown that including more sampling directions extends the tracking range for head rotations. The efficiency of trading readout resolution for speed was explored. RESULTS: Tracking of head rotations was demonstrated from -19.2 to 34.4°, -2.7 to 10.0°, and -60.9 to 70.9° in the x-, y-, and z-directions, respectively. In the presence of excessive head motion, the deviation of marker estimates from SPM8 was reduced by 17.1% over existing three-projection methods. This was achieved by using an additional seven directions, extending the time needed for readouts by a factor of 3.3. Much of this increase may be circumvented by reducing resolution, without compromising accuracy. CONCLUSION: Including additional sampling directions extends the range in which markers can be used, for patients who move a lot. Magn Reson Med 76:913-918, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
  • ItemAccepted versionOpen Access
    Combining MRI with PET for partial volume correction improves image-derived input functions in mice.
    (Institute of Electrical and Electronics Engineers (IEEE), 2015-06-01) Evans, Eleanor; Buonincontri, Guido; Izquierdo, David; Methner, Carmen; Hawkes, Rob C; Ansorge, Richard E; Krieg, Thomas; Carpenter, T Adrian; Sawiak, Stephen J; Ansorge, Richard [0000-0002-5064-3640]; Krieg, Thomas [0000-0002-5192-580X]; Carpenter, Adrian [0000-0002-2939-8222]; Sawiak, Stephen [0000-0003-4210-9816]
    Accurate kinetic modelling using dynamic PET requires knowledge of the tracer concentration in plasma, known as the arterial input function (AIF). AIFs are usually determined by invasive blood sampling, but this is prohibitive in murine studies due to low total blood volumes. As a result of the low spatial resolution of PET, image-derived input functions (IDIFs) must be extracted from left ventricular blood pool (LVBP) ROIs of the mouse heart. This is challenging because of partial volume and spillover effects between the LVBP and myocardium, contaminating IDIFs with tissue signal. We have applied the geometric transfer matrix (GTM) method of partial volume correction (PVC) to 12 mice injected with 18F-FDG affected by a Myocardial Infarction (MI), of which 6 were treated with a drug which reduced infarction size [1]. We utilised high resolution MRI to assist in segmenting mouse hearts into 5 classes: LVBP, infarcted myocardium, healthy myocardium, lungs/body and background. The signal contribution from these 5 classes was convolved with the point spread function (PSF) of the Cambridge split magnet PET scanner and a non-linear fit was performed on the 5 measured signal components. The corrected IDIF was taken as the fitted LVBP component. It was found that the GTM PVC method could recover an IDIF with less contamination from spillover than an IDIF extracted from PET data alone. More realistic values of Ki were achieved using GTM IDIFs, which were shown to be significantly different (p<0.05) between the treated and untreated groups.
  • ItemOpen Access
    Comparison of first pass bolus AIFs extracted from sequential 18F-FDG PET and DSC-MRI of mice.
    (Elsevier BV, 2014-01-11) Evans, Eleanor; Sawiak, Stephen J; Ward, Alexander O; Buonincontri, Guido; Hawkes, Robert C; Carpenter, T Adrian; Sawiak, Stephen [0000-0003-4210-9816]; Carpenter, Adrian [0000-0002-2939-8222]
    Accurate kinetic modelling of in vivo physiological function using positron emission tomography (PET) requires determination of the tracer time-activity curve in plasma, known as the arterial input function (AIF). The AIF is usually determined by invasive blood sampling methods, which are prohibitive in murine studies due to low total blood volumes. Extracting AIFs from PET images is also challenging due to large partial volume effects (PVE). We hypothesise that in combined PET with magnetic resonance imaging (PET/MR), a co-injected bolus of MR contrast agent and PET ligand can be tracked using fast MR acquisitions. This protocol would allow extraction of a MR AIF from MR contrast agent concentration-time curves, at higher spatial and temporal resolution than an image-derived PET AIF. A conversion factor could then be applied to the MR AIF for use in PET kinetic analysis. This work has compared AIFs obtained from sequential DSC-MRI and PET with separate injections of gadolinium contrast agent and 18F-FDG respectively to ascertain the technique's validity. An automated voxel selection algorithm was employed to improve MR AIF reproducibility. We found that MR and PET AIFs displayed similar character in the first pass, confirmed by gamma variate fits (p<0.02). MR AIFs displayed reduced PVE compared to PET AIFs, indicating their potential use in PET/MR studies.
  • ItemOpen Access
    MRI-derived arterial input functions for PET kinetic modelling in rats
    (Elsevier, 2013) Evans, E; Sawiak, SJ; Adrian Carpenter, T; Sawiak, Stephen [0000-0003-4210-9816]
    Simultaneous PET–MR acquisition provides the high temporal and spatial resolution of MRI with the specificity of PET. In PET, accurate modelling of physiological function in vivo requires the time-activity curve of tracer in blood plasma, known as the arterial input function (AIF). As the gold standard method of blood sampling is inherently prohibitive in the small animal case, here we discuss how we prepare to rapidly sample MRI signals from gadolinium-doped tracer to obtain the tracer input functions from a simultaneous PET-MR measurement. ΔR2⁎ measurements taken from EPI images were used to obtain first pass bolus AIFs in the rat brain from DSC-MRI datasets of 5 rats. AIFs obtained using our automatic algorithm were found to be consistent between animals and compared well with manual methods without need for a priori voxel selection. A variable flip angle FLASH sequence used for T1 mapping was successfully tested in a phantom study, providing accurate measurements of Gd concentration.