15 research outputs found
Simultaneous 13N-Ammonia and gadolinium first-pass myocardial perfusion with quantitative hybrid PET-MR imaging: a phantom and clinical feasibility study
Background
Positron emission tomography (PET) is the non-invasive reference standard for myocardial blood flow (MBF) quantification. Hybrid PET-MR allows simultaneous PET and cardiac magnetic resonance (CMR) acquisition under identical experimental and physiological conditions. This study aimed to determine feasibility of simultaneous 13N-Ammonia PET and dynamic contrast-enhanced CMR MBF quantification in phantoms and healthy volunteers.
Methods
Images were acquired using a 3T hybrid PET-MR scanner. Phantom study: MBF was simulated at different physiological perfusion rates and a protocol for simultaneous PET-MR perfusion imaging was developed. Volunteer study: five healthy volunteers underwent adenosine stress. 13N-Ammonia and gadolinium were administered simultaneously. PET list mode data was reconstructed using ordered subset expectation maximisation. CMR MBF was quantified using Fermi function-constrained deconvolution of arterial input function and myocardial signal. PET MBF was obtained using a one-tissue compartment model and image-derived input function.
Results
Phantom study: PET and CMR MBF measurements demonstrated high repeatability with intraclass coefficients 0.98 and 0.99, respectively. There was high correlation between PET and CMR MBF (r = 0.98, p < 0.001) and good agreement (bias − 0.85 mL/g/min; 95% limits of agreement 0.29 to − 1.98). Volunteer study: Mean global stress MBF for CMR and PET were 2.58 ± 0.11 and 2.60 ± 0.47 mL/g/min respectively. On a per territory basis, there was moderate correlation (r = 0.63, p = 0.03) and agreement (bias − 0.34 mL/g/min; 95% limits of agreement 0.49 to − 1.18).
Conclusion
Simultaneous MBF quantification using hybrid PET-MR imaging is feasible with high test repeatability and good to moderate agreement between PET and CMR. Future studies in coronary artery disease patients may allow cross-validation of techniques
Multicentre evaluation of MRI variability in the quantification of infarct size in experimental focal cerebral ischaemia
Ischaemic stroke is a leading cause of death and disability in the developed world.
Despite that considerable advances in experimental research enabled understanding
of the pathophysiology of the disease and identified hundreds of potential
neuroprotective drugs for treatment, no such drug has shown efficacy in humans. The
failure in the translation from bench to bedside has been partially attributed to the
poor quality and rigour of animal studies. Recently, it has been suggested that
multicentre animal studies imitating the design of randomised clinical trials could
improve the translation of experimental research. Magnetic resonance imaging (MRI)
could be pivotal in such studies due to its non-invasive nature and its high sensitivity
to ischaemic lesions, but its accuracy and concordance across centres has not yet been
evaluated.
This thesis focussed on the use of MRI for the assessment of late infarct size, the
primary outcome used in stroke models. Initially, a systematic review revealed that a
plethora of imaging protocols and data analysis methods are used for this purpose.
Using meta-analysis techniques, it was determined that T2-weighted imaging (T2WI)
was best correlated with gold standard histology for the measurement of infarctbased
treatment effects. Then, geometric accuracy in six different preclinical MRI
scanners was assessed using structural phantoms and automated data analysis tools
developed in-house. It was found that geometric accuracy varies between scanners,
particularly when centre-specific T2WI protocols are used instead of a standardised
protocol, though longitudinal stability over six months is high. Finally, a simulation
study suggested that the measured geometric errors and the different protocols are
sufficient to render infarct volumes and related group comparisons across centres
incomparable. The variability increases when both factors are taken into account and
when infarct volume is expressed as a relative estimate. Data in this study were
analysed using a custom-made semi-automated tool that was faster and more reliable
in repeated analyses than manual analysis.
Findings of this thesis support the implementation of standardised methods for the
assessment and optimisation of geometric accuracy in MRI scanners, as well as image
acquisition and analysis of in vivo data for the measurement of infarct size in
multicentre animal studies. Tools and techniques developed as part of the thesis show
great promise in the analysis of phantom and in vivo data and could be a step towards
this endeavour
A calibrated physical flow standard for medical perfusion imaging
In the medical sector, various imaging methodologies or modalities (e.g. MRI, PET, CT) are used to assess the health of various parts of the bodies of patients. One such investigation is the blood flow or perfusion of the heart muscle, expressed as the (blood) flow rate normalized by the mass of the volume of interest. Currently there is no physical flow standard for the validation of quantitative perfusion measurements. This need has been addressed in the EMPIR 15HLT05 PerfusImaging project. A phantom simulating the heart muscle has been developed with the capability that it can reproducibly generate a flow profile with individual flow rates known with a relative uncertainty of about 10% (k = 2) and total flow rate known with an uncertainty of 1% (k = 2). An overview of the phantom and its validation is given. Next, a new analysis method is presented to analyse the sequence of images which are acquired when using a standard dynamic imaging protocol. It is concluded that the new, alternative approach gives results comparable to the standard analysis method.Multi Phase System