Paul R. Moran

Near-resonance spin-lock contrast

Spin-lock and spin-tip excitations are the two magnetization components created by the preparatory RF pulse of an MRI contrast enhancement sequence. Only spin-lock is inherently adiabatic, preserving spin alignment so that tissue-specific relaxation can generate desired saturation contrasts. Spin-tip is the rotating-frame oscillating excitation, and generally causes nonadiabatic loss of all detectable magnetization. Relative levels of spin-lock and spin-tip are important to understand as a function of the preparatory B1 delta amplitude, resonance frequency offset, delta, and the pulse waveform. These MR responses can be accurately analyzed theoretically and numerically by using Torreys tipped coordinates to formulate Blochs equations. At near-resonance offsets, (delta/gamma B1) less than 2.0, spin-lock contrast (SLC) depends strongly on T2, due to the nature of spin-lock T1 rho relaxation in the RF pulse interval. The relaxation rates 1/T1 rho and 1/T2 rho apply for active B1 delta, but remain linear combinations of ordinary (1/T1) and 1/T2) for motionally narrowed MR. The SLC increases rapidly as delta decreases below 2000 Hz; carefully chosen B1 delta rise times avoid spin-tip losses down to 150 Hz or less. The SL saturation enhances or multiplies any other indirect saturation effects that may be also present, such as magnetization transfer. A strong near-resonance SLC multiplier is advantageous for clinically practical MRI sequences that use short B1 delta pulses and fast SE multislice scan modes. Simulations based upon spin-lock/spin-tip theory and measured (T1,T2) yield excellent agreement with real MRI results for clinically practical fast multislice scans.

Spatially resolved flow velocity measurements and projection angiography by adiabatic passage

This paper describes the basic principles of gradient modulated adiabatic passage using a CW radiofrequency excitation. The possible applications of this technique include a direct assessment of in-plane and oblique directional flow velocities, and visualization of flow velocity profiles. Flow angiography based on the time-of-flight technique is also discussed with experimental results.

MR flow imaging in projection through a stationary surround.

A magnetic resonance imaging technique is discussed which, by cyclic inversion of the longitudinal magnetization, produces boli of moving material with alternating sign of the magnetization. At periodic spacings along the flow direction, the signal strength from magnetization of positive sign is equal to that of negative sign. This results in a minimum in the intensity distribution. A banded intensity structure results reflecting the distribution of flow velocities across the imaged vessel. The inversion of the longitudinal magnetization causes an inherent suppression of the signal from stationary material allowing the collection of flow images in projection through a stationary surround without the need for image subtraction.

A general approach to T1, T2, and spin-density discrimination sensitivities in NMR imaging sequences

Previous specific empirical studies and computer simulations show convincing evidence for superior sensitivity in imaging T1 differences (T1-discrimination) by the simple saturation-recovery NMR sequence above all other common sequences. This occurs under optimum conditions where system repetition time is shorter than commonly employed in most currently recommended NMR sequences. This paper presents a general theory of discrimination sensitivity; its application to the human subject shows that the specific empirical results must, in general, be true in all cases. This occurs because all functional forms entering the NMR relaxation modulations of imaged-magnetization (in the current sequences) are the same exponential-function. Moreover, density-discrimination and T2-discrimination are optimized by short repetition times just as for T1-discrimination; the lower limit is a T2-controlled data-window constraint not treated in previous descriptions. These results have significant practical consequences for NMR scanner design.

Velocity sensitivity of slice-selective excitation

The purpose of this study was to investigate how flow affects slice-selective excitation, particularly for radiofrequency (rf) pulses optimized for slice-selective excitation of stationary material. Simulation methods were used to calculate the slice profiles for material flowing at different velocities, using optimal flow compensation when appropriate. Four rf pulses of very different shapes were used in the simulation study: a 90 degrees linear-phase Shinnar-LeRoux pulse; a 90 degrees self-refocusing pulse; a minimum-phase Shinnar-LeRoux inversion pulse; and a SPINCALC inversion pulse. Slice profiles from simulations with a laminar flow model were compared with experimental studies for two different rf pulses using a clinical magnetic resonance imaging (MRI) system. We found that, for a given rf pulse, the effect of flow on slice-selective excitation depends on the product of the selection gradient amplitude, the component of velocity in the slice selection direction, and the square of the rf pulse duration. The shapes of the slice profiles from the Shinnar-LeRoux pulses were relatively insensitive to velocity. However, the slice profiles from the self-refocusing pulse and the SPINCALC pulse were significantly degraded by velocity. Experimental slice profiles showed excellent agreement with simulation. In conclusion, our study demonstrates that slice-selective excitation can be significantly degraded by flow depending on the velocity, the gradient amplitude, and characteristics of the rf excitation pulse used. The results can aid in the design of rf pulses for slice-selective excitation of flowing material.

Flow field mapping by multi-zone adiabatic passage excitation☆

This paper describes a robust method for flow field mapping by multi-zone adiabatic fast passage (AFP). It provides a quick and simple way to simultaneously acquire flow profiles at several locations and arbitrary orientations inside the field-of-view. The flow profile is the time-averaged evolution of the labeled flowing material. Results obtained using a carotid bifurcation and jet phantoms are similar to the previous experimental studies employing Laser Doppler Anemometry (LDA), and other flow visualization techniques. In addition, the preliminary results obtained with a human volunteer support the feasibility of the technique for in vivo flow quantification.