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Principles behind Magnetic Resonance Imaging (MRI)
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
The most established MRI technique for assessment of cerebral blood flow (CBF) is dynamic susceptibility contrast MRI (DSC-MRI). A Gd-based contrast agent is injected intravenously, and the signal time course is monitored by rapid T2*-weighted EPI (approximately 1 image/second for 1–2 minutes) during the first passage of the contrast agent bolus in the brain. The contrast agent concentration is quantified, as a function of time, in the tissue (pixel by pixel) as well as in a brain-feeding artery (providing the arterial input function, AIF). Perfusion-related parameters such as cerebral blood volume (CBV), CBF (Figure 32.22b), and mean transit time (MTT) can be calculated using kinetic theory for intravascular tracers, most commonly by using deconvolution of the tissue concentration time curve with the AIF to obtain CBF and MTT. The basic concept is often referred to as ‘bolus tracking’ (applicable also to, for example, computed tomography). Quantification in absolute terms has, so far, been problematic due to difficulties in obtaining accurate AIFs and uncertainties concerning the transverse relaxivities in tissue and blood.
Positron Emission Tomography (PET) in Substance-Abuse Exposed Infants: A Preliminary Report
Published in Richard J. Konkol, George D. Olsen, Prenatal Cocaine Exposure, 2020
In order to calculate the absolute rates of local cerebral glucose metabolism (LCMRgIc), it was required previously to collect arterial or arterialized (through hand warming in a small heated chamber) blood samples during the PET study and define the arterial input function to the brain.3 Recent advances allow this arterial input function to be defined based on dynamic imaging of the myocardium for 20 minutes following FDG administration and the collection of three small blood samples (1 ml each) toward the end of the procedure from a different venous source than the one used for FDG injection (unpublished data). This approach has made quantification in the PET procedure far less invasive and cumbersome than previously, and therefore easier to accomplish in young children.
Computer-Aided Diagnosis Systems for Prostate Cancer Detection
Published in Ayman El-Baz, Gyan Pareek, Jasjit S. Suri, Prostate Cancer Imaging, 2018
Guillaume Lemaître, Robert Martí, Fabrice Meriaudeau
The standardization problem can be tackled by normalizing the MRI images using the SI of some known organs present in these images. Niaf et al. and Lehaire et al. normalized T2-W-MRI images by dividing the original SI of the images by the mean SI of the bladder [126,137,138], which is depicted in Figure 10.9a. Giannini et al. also normalized the same modality but using the signal intensity of the obturator muscle [122]. Likewise, Niaf et al. standardized the T1-W-MRI images using the arterial input function (AIF) [137]. They computed the AIF by taking the mean of the SI in the most enhanced part of the common femoral arteries—refer to Figure 10.9b—as proposed in Wiart et al. [190]. Along the same line, Samarasinghe et al. normalized the SI of lesion regions in T1-W-MRI using the mean intensity of the prostate gland in the same modality [147].
Imaging-based characterization of convective tissue properties
Published in International Journal of Hyperthermia, 2020
D. Fuentes, E. Thompson, M. Jacobsen, A. Colleen Crouch, R. R. Layman, B. Riviere, E. Cressman
The widely applied Tofts model is a compartmental model that was designed for tissues with negligible blood volume [18]. Tofts models assume equilibrium of contrast media between the blood plasma and the extravascular–extracellular space (EES) as well as isodirectional permeability [19]. The notion of permeability in the Tofts’ approach is interpreted as the rate transfer constant between a blood vessel and the EES. The feeding vessels within the tissue are assumed to provide a spatially homogeneous arterial input function source term to the governing ordinary differential equation. Implicitly, this assumes that the time scale for the transport between imaging visible vessels and vessels not visible on imaging is less than the sequential time between a dynamic imaging acquisition. The extended Tofts models build upon the Tofts model and include additional parameters for intravascular signal contributions. However, permeability is still interpreted as the rate transfer constant between a blood vessel and the EES. Fundamentally, the governing equations for compartmental mass balance represent a spatially invariant mass transport between compartments and do not capture spatially variant convection phenomena.
Non-invasive imaging techniques to assess myocardial perfusion
Published in Expert Review of Medical Devices, 2020
Olivier Villemain, Jérôme Baranger, Zakaria Jalal, Christopher Lam, Jérémie Calais, Mathieu Pernot, Barbara Cifra, Mark K. Friedberg, Luc Mertens
Interpretation in clinical practice is qualitative [30], with hypoenhancement greater than 25% myocardial extent typically considered pathological. However, numerous automated quantitative CMR myocardial perfusion methods are under investigation and beginning to make headway into clinical practice. Quantitative myocardial perfusion is based on mathematical models of myocardial structure and thus contrast perfusion. Measured parameters include myocardial blood flow (MBF; ml/g/min) at stress and rest, and myocardial perfusion reserve (MPR), which is the ratio of MBF at stress to MBF at rest. Relative flow reserve can also be determined at each myocardial segment. A requirement of quantitative methods is precise measurement of the arterial input function (AIF), which represents the varying signal intensity of blood in the left ventricle due to contrast transit. This has traditionally been labor-intensive and prone to errors, though has now been improved with automated methods, including for obtaining whole-heart slice coverage using simultaneous multi-slice techniques [31–34]. In the largest quantitative perfusion CMR study to date (1049 patients), Knott KD and colleagues use stress CMR myocardial perfusion mapping via an automated artificial-intelligence-based approach to show, quantitative measures of MBF and perfusion reserve to be independent predictors for death and major adverse cardiovascular events [35]. Future trials will be needed to validate the utility of quantitative CMR myocardial perfusion imaging for clinical practice.
Late changes in blood–brain barrier permeability in a rat tMCAO model of stroke detected by gadolinium-enhanced MRI
Published in Neurological Research, 2020
Catherine A. Morgan, Michel Mesquita, Maria Ashioti, John S. Beech, Steve C. R. Williams, Elaine Irving, Diana Cash
In this study, we used a simple method to measure relative BBB permeability changes. Measurements of absolute permeability are more technically complex. A method of dynamic contrast enhanced MRI (DCE-MRI) uses compartmental analysis to derive a true measure of permeability of a medium through the vasculature into the tissue of interest, independent of cerebral blood flow (CBF) [47]. However, DCE analysis requires rapid imaging to measure the arterial input function feeding the tissue of interest and for this, in small animals with a fast heartbeat, spatial resolution and/or brain coverage is sacrificed for the temporal resolution required. Using the simpler approach here, it is assumed that the rate of CBF is similar and consistent across the subjects. We believe this assumption to be valid given the consistent location and size of the lesion produced by the tMCAO model, the fixed occlusion time and the genetic similarity between the rats. This approach allowed us to acquire multiple slices for whole-brain coverage, at a spatial resolution that allowed us to interrogate disturbances in the BBB in different structures throughout the brain. Moreover, this relatively simple method can be easily employed. Future work could benefit from employing DCE-MRI, or a related quantitative method of dynamic susceptibility contrast MRI (DSC-MRI) [48], to evaluate absolute permeability in a specific region, to provide for example, clinically relevant indication of drug delivery rates.