Recently, several implementations of arterial spin labeling (ASL) techniques have been developed for producing MRI images sensitive to local tissue perfusion. For quantitation of perfusion, both pulsed and continuous labeling methods potentially suffer from a number of systematic errors. In this study, a general kinetic model for the ASL signal is described that can be used to assess these errors. With appropriate assumptions, the general model reduces to models that have been used previously to analyze ASL data, but the general model also provides a way to analyze the errors that result if these assumptions are not accurate. The model was used for an initial assessment of systematic errors due to the effects of variable transit delays from the tagging band to the imaging voxel, the effects of capillary/tissue exchange of water on the relaxation of the tag, and the effects of incomplete water extraction. In preliminary experiments with a human subject, the model provided a good description of pulsed ASL data during a simple sensorimotor activation task.

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A general kinetic model for quantitative perfusion imaging with arterial spin labeling

Magnetic resonance imaging techniques measuring CBF have developed rapidly in the last decade, resulting in a wide range of available methods. The most successful approaches are based either on dynamic tracking of a bolus of a paramagnetic contrast agent (dynamic susceptibility contrast) or on arterial spin labeling. This review discusses their principles, possible pitfalls, and potential for absolute quantification and outlines clinical and neuroscientific applications.

https://journals.sagepub.com/doi/pdf/10.1097/00004647-199907000-00001

Measuring cerebral blood flow using magnetic resonance imaging techniques

Arterial spin labeling (ASL) is capable of noninvasively measuring blood flow by magnetically tagging the protons in arterial blood, which has been conventionally achieved using instantaneous (PASL) or continuous (CASL) RF pulses. As an intermediate method, pseudocontinuous ASL (pCASL) utilizes a train of discrete RF pulses to mimic continuous tagging that is often unavailable on imagers due to the requirement of continuous RF transmit capabilities. In the present study, we implemented two versions of pCASL (balanced and unbalanced gradient waveforms in tag and control scans) for both transmit/receive coils and array receivers. Experimental data show a 50% +/- 4% increase of signal-to-noise ratio (SNR) compared with PASL and a higher tagging efficiency than amplitude-modulated (AM) CASL (80% vs. 68%). Computer simulations predict an optimal tagging efficiency of 85% for flow velocities from 10 to 60 cm/s. It is theoretically and experimentally demonstrated that the tagging efficiency of pCASL is dependent upon the resonance offset and flip angle of the RF pulse train. We conclude that pCASL has the potential of combining the merits of PASL, including less hardware demand and higher tagging efficiency, and CASL, which includes a longer tagging bolus and thus higher SNR. These improvements provide a better balance between tagging efficiency and SNR.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.21403

A theoretical and experimental investigation of the tagging efficiency of pseudocontinuous arterial spin labeling.

Introduction Part I. An Overview of Functional Magnetic Resonance Imaging: I A. Introduction to Functional Neuroimaging: 1. Energy metabolism in the brain 2. Cerebral blood flow 3. Brain activation I B. Introduction to Functional Magnetic Resonance Imaging: 4. Nuclear magnetic resonance 5. Magnetic resonance imaging 6. Imaging functional activity Part II. Principles of Magnetic Resonance Imaging: II A. The Nature of the Magnetic Resonance Signal: 7. Basic physics of magnetism and NMR 8. Relaxation and contrast in MRI 9. Diffusion and the MR signal II B. Magnetic Resonance Imaging: 10. Mapping the MR signal 11. MRI techniques 12. Noise and artifacts in MR images Part III. Principles of Functional Magnetic Resonance Imaging: III A. Perfusion Imaging: 13. Principles of tracer kinetics 14. Contrast agent techniques 15. Arterial spin labeling techniques III B. Blood Oxygenation Level Dependent (BOLD) Imaging: 16. The nature of the BOLD effect 17. Mapping brain activation with BOLD-fMRI 18. Statistical analysis of BOLD data 19. Efficient design of BOLD experiments Appendix: the physics of NMR Index.

/pdf/introduction-to-functional-magnetic-resonance-imaging-4ohaa19hda.pdf

Introduction to Functional Magnetic Resonance Imaging: Principles and Techniques

Numerous techniques have been proposed in the last 15 years to measure various perfusion-related parameters in the brain. In particular, two approaches have proven extremely successful: injection of paramagnetic contrast agents for measuring cerebral blood volumes (CBV) and arterial spin labeling (ASL) for measuring cerebral blood flows (CBF). This review presents the methodology of the different magnetic resonance imaging (MRI) techniques in use for CBV and CBF measurements and briefly discusses their limitations and potentials.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/jmri.1073

Methodology of brain perfusion imaging

Detre et al. (Magn. Reson. Med. 23, 37-45 (1992)) and Zhang et al., (Magn. Reson. Med. 25, 362 (1992)) have recently demonstrated a technique for the measurement of regional cerebral blood flow (rCBF) based on the continuous saturation (or inversion) of the arterial blood supply to the brain in rats at 4.7 T. In the work reported here, we combined this technique with volume localized (PRESS) readouts to benefit from recording "perfusion" signals averaged over a larger volume, resulting in rapid acquisition of data with sufficient signal-to-noise ratio for application at 2.0 T. In 10 baseline flow measurements, the mean error between the NMR technique and the microsphere flow measurement was -1.5% with a standard deviation of 15.2%. For five measurements obtained with occlusion of the middle cerebral artery, the mean error was -32.4 +/- 20.2%. Perfusion measurements from a single animal under hypercapnic conditions indicated that the NMR technique could underestimate rCBF at high flow rates. An error analysis of the NMR perfusion model is also presented, along with results for typical parameters encountered at 2.0 T.

Edward G. Walsh

Papers

Radioactive microsphere validation of a volume localized continuous saturation perfusion measurement.