Perfusion assessment with bolus differentiation: A technique applicable to hyperpolarized tracers

Nanorod-based flow estimation using a high-frame-rate photoacoustic imaging system

A quantitative flow measurement method that utilizes a sequence of photoacoustic images is described. The method is based on the use of gold nanorods as a contrast agent for photoacoustic imaging. The peak optical absorption wavelength of a gold nanorod depends on its aspect ratio, which can be altered by laser irradiation (we establish a wash-in flow estimation method of this process). The concentration of nanorods with a particular aspect ratio inside a region of interest is affected by both laser-induced shape changes and replenishment of nanorods at a rate determined by the flow velocity. In this study, the concentration is monitored using a custom-designed, high-frame-rate photoacoustic imaging system. This imaging system consists of fiber bundles for wide area laser irradiation, a laser ultrasonic transducer array, and an ultrasound front-end subsystem that allows acoustic data to be acquired simultaneously from 64 transducer elements. Currently, the frame rate of this system is limited by the pulse-repetition frequency of the laser (i.e., 15 Hz). With this system, experimental results from a chicken breast tissue show that flow velocities from 0.125 to 2 mms can be measured with an average error of 31.3%.

Hyperpolarized13C MRI of the pulmonary vasculature and parenchyma

The study of lung perfusion in normal and diseased subjects is of great interest to physiologists and physicians. In this work we demonstrate the application of a liquid-phase hyperpolarized (HP) carbon-13 ((13)C) tracer to magnetic resonance imaging (MRI) of the pulmonary vasculature and pulmonary perfusion in a porcine model. Our results show that high spatial and temporal resolution images of pulmonary perfusion can be obtained with this contrast technique. Traditionally, pulmonary perfusion measurement techniques have been challenging because of insufficient signal for quantitative functional assessments. The use of polarized (13)C in MRI overcomes this limitation and may lead to a viable clinical method for studying the pulmonary vasculature and perfusion.

Bolus delay and dispersion in perfusion MRI: implications for tissue predictor models in stroke

Perfusion maps, which are calculated from dynamic-susceptibility contrast (DSC)-MRI data by deconvolution of the arterial input function (AIF), are commonly used to predict tissue infarction in acute stroke. However, since the AIF is commonly measured in a major artery, there can be perfusion measurement errors associated with bolus delay/dispersion. Although methods to account for delay-related errors have been proposed, the effect of dispersion is more difficult to deal with and is usually left uncorrected. This study presents an assessment of the delay-dispersion relationship in a group of patients. Although a significant correlation was observed with one of the bolus delay definitions used, the estimation of the dispersion from the degree of delay was unreliable. Importantly, the dispersion observed in many patients was sufficient to result in substantial perfusion errors. The results are compared with previous numerical simulations, and their implications for the assessment and management of acute stroke are discussed.

MR coronary angiography in pigs with intraarterial injections of a hyperpolarized 13C substance

A new diagnostic application of a water-soluble contrast medium (CM) based on the hyperpolarization of a 13C substance is introduced. The degree of polarization achieved is >30%, which is about a factor of 10(5) higher than the thermal equilibrium polarization level at 1.5 T. Imaging of hyperpolarized (HP) CM during a cardiac interventional MRI procedure was studied. Catheters were positioned in the left and right coronary arteries of pigs. A coil tuned to 13C was used for nonproton imaging. The HP-13C CM ( approximately 5 ml, 0.5 M, approximately 30% polarization) was injected during projection imaging using a fully balanced steady-state free precession (SSFP) pulse sequence with and without cardiac gating. The contrast agent-filled catheter was clearly visible during the procedure. The coronary arteries were well depicted and the signal-to-noise ratios (SNRs) were in the range of 10-40. The use of HP-13C CM may provide a new diagnostic procedure for interventional MRI.

13C imaging—a new diagnostic platform

The evolution of magnetic resonance imaging (MRI) has been astounding since the early 1980s, and a broad range of applications has emerged. To date, clinical imaging of nuclei other than protons has been precluded for reasons of sensitivity. However, with the recent development of hyperpolarization techniques, the signal from a given number of nuclei can be increased as much as 100,000 times, sufficient to enable imaging of nonproton nuclei. Technically, imaging of hyperpolarized nuclei offers several unique properties, such as complete lack of background signal and possibility for local and permanent destruction of the signal by means of radio frequency (RF) pulses. These properties allow for improved as well as new techniques within several application areas. Diagnostically, the injected compounds can visualize information about flow, perfusion, excretory function, and metabolic status. In this review article, we explain the concept of hyperpolarization and the techniques to hyperpolarize 13C. An overview of results obtained within angiography, perfusion, and catheter tracking is given, together with a discussion of the particular advantages and limitations. Finally, possible future directions of hyperpolarized 13C MRI are pointed out.