Slice-selective Inversion Research Articles

Perfusion-corrected mapping of cardiac regional blood volume in rats in vivo.

Measurement of regional blood volume (RBV) in the myocardium in vivo is important for the assessment of tissue viability and function. The method in this work is based on the acquisition of a T(1) map before and after intravascular contrast agent application. It is known that this method is influenced by perfusion that causes an overestimation of RBV values. In order to solve this problem, the new method is proposed which acquires T(1) maps with slice selective inversion pulses. Due to blood flow nonexcited spins enter the detection slice, which leads to an acceleration of the relaxation time. A model that divides tissue into two compartments is adapted to slice selective inversion in order to derive a simple expression for perfusion-corrected RBV. The aim of the study is to demonstrate the feasibility and accuracy of this technique for quantification of RBV in rat myocardium in vivo. RBV maps were obtained for five rats, and the reproducibility was determined by repeating the experiment several times. A mean RBV value of 12.8 +/- 0.7% (v/v) over all animals was obtained in the myocardium. The results were compared with RBV maps obtained with perfusion-sensitive RBV imaging in the same five rats and with first-pass RBV studies. In order to demonstrate the strength of the new method the vasodilator adenosine was administered and alterations in microcirculation were imaged. Magn Reson Med 42:500-506, 1999.

Correction of errors caused by imperfect inversion pulses in MR imaging measurement of T1 relaxation times

Spin-lattice ( T 1) relaxation times were measured by an inversion-recovery magnetic resonance imaging method with a slice-selective inversion pulse (SIP), a non-selective rectangular inversion pulse (RIP), or a B 1-insensitive adiabatic inversion pulse (AIP). Data analysis either assumed perfect inversion (two-parameter fit) or allowed for imperfect inversion (three-parameter fit). Imperfect inversion pulses caused low T 1 values in phantoms with a two-parameter fit, while three-parameter T 1 estimates were accurate over the range 430–2670 ms. A difference of ∼10% between two-parameter and three-parameter T 1 values in normal human brain tissue was attributed to B 1 inhomogeneity with the slice-selective inversion pulse and rectangular inversion pulse, to the slice profile with the slice-selective inversion pulse, and to T 2 effects for the adiabatic inversion pulse. Any T 1 method that relies on accurate flip angles may have a significant systematic error in vivo. Phantom accuracy does not ensure accuracy in vivo, because phantoms may have a more homogeneous B 1 field and a longer T 2 than do biological samples.

The effect of perfusion on T1 after slice-selective spin inversion in the isolated cardioplegic rat heart: measurement of a lower bound of intracapillary-extravascular water proton exchange rate.

Many NMR measurements of cardiac microcirculation (perfusion, intramyocardial blood volume) depend on some kind of assumption of intracapillary-extravascular water exchange rate, e.g., fast exchange. The magnitude of this water exchange rate, however, is still unknown. The intention of this study was to determine a lower limit for this exchange rate by investigating the effect of perfusion on relaxation time. Studies were performed in the isolated perfused cardioplegic rat heart. After slice-selective inversion, the spin lattice relaxation rate of myocardium within the slice was studied as a function of perfusion and compared with a mathematical model which predicts relaxation rate as a function of perfusion and intracapillary-extravascular exchange rate. A linear relationship was found between relaxation rate T(-1) and perfusion P normalized by perfusate/tissue partition coefficient of water, lambda: deltaT(-1) = m x deltaP/lambda with 0.82 < or = m < or = 1.06. Insertion of experimental data in the model revealed that a lower bound of the exchange rate from intra- to extravascular space is 6.6 s(-1) (4.5 s(-1), P < 0.05), i.e., the intracapillary lifetime of a water molecule is less than 150 ms (222 ms, P < 0.05). Based on this finding, the T1 mapping after slice-selective inversion could become a valuable noncontrast NMR method to measure variations of perfusion.

OIL FLAIR: optimized interleaved fluid-attenuated inversion recovery in 2D fast spin echo.

Inversion recovery may be used to suppress signal from cerebrospinal fluid, a technique which has been named "fluid attenuated with inversion recovery" (FLAIR). This report describes interleaving a slice selective inversion pulse within a rapid spin-echo sequence to obtain the desirable contrast characteristics of FLAIR in imaging times comparable to standard rapid spin echo. Additionally, the pulse repetition time is allowed to float above a defined minimum, which can further shorten scan times and dramatically ease the optimization process. The optimized interleaved sequence is referred to as OIL FLAIR.

Quantitative magnetic resonance imaging of perfusion using magnetic labeling of water proton spins within the detection slice

A technique for noninvasive quantitative magnetic resonance imaging of perfusion is presented. It relies on using endogenous water as a freely diffusible tracer. Tissue water proton spins are magnetically labeled by slice-selective inversion, and longitudinal relaxation within the slice is detected using a fast gradient echo magnetic resonance imaging technique. Due to blood flow, nonexcited spins are washed into the slice resulting in an acceleration of the longitudinal relaxation process. Incorporating this phenomenon into the Bloch equation yields an expression that allows quantification of perfusion on the basis of a slice-selective and a nonselective inversion recovery experiment. Based on this technique, quantitative parameter maps of the regional cerebral blood flow (rCBF) were obtained from eight rats. Evaluation of regions of interest within the cerebral hemispheres yielded an average rCBF value of 104 +/- 21 ml/min/100 g, which increased to 219 +/- 30 ml/min/100 g during hypercapnia. The measured rCBF values are in good agreement with previously reported literature values.

Erkennbarkeit fokaler Leberläsionen: Vergleich von MRT bei 1,5 T und dynamischer Spiral-CT

To compare dynamic spiral CT with MR imaging in the detectability of focal liver lesions. Thirty-one patients with 62 focal liver lesions (27 benign) were evaluated retrospectively. Dynamic spiral CT scans were compared with T1- and T2- weighted spin-echo (SE) sequences and in part with multi slice 2-D-FLASH and single-shot slice-selective inversion recovery Turbo-FLASH sequences at 1.5 T. Dynamic spiral CT detected 89% of all lesions and was superior to each sequence alone (56-70%), and also to MRI overall (79%) regardless of lesion size, localization, and histology. However, statistical significance (p < 0.05) was only found for the differences between CT and the T1-weighted SE sequence. Compared to conventional SE and GE MR imaging sequences, dynamic spiral CT scanning seems to be more effective in the detection of focal liver lesions.

Quantification of relative cerebral blood flow change by flow-sensitive alternating inversion recovery (FAIR) technique: application to functional mapping.

Relative cerebral blood flow changes can be measured by a novel simple blood flow measurement technique with endogenous water protons as a tracer based on flow-sensitive alternating inversion recovery (FAIR). Two inversion recovery (IR) images are acquired by interleaving slice-selective inversion and nonselective inversion. During the inversion delay time after slice-selective inversion, fully magnetized blood spins move into the imaging slice and exchange with tissue water. The signal enhancement (FAIR image) measured by the signal difference between two images is directly related to blood flow. For functional MR imaging studies, two IR images are alternatively and repeatedly acquired during control and task periods. Relative signal changes in the FAIR images during the task periods represent the relative regional cerebral blood flow changes. The FAIR technique has been successfully applied to functional brain mapping studies in humans during finger opposition movements. The technique is capable of generating microvascular-based functional maps.

The use of slice-selective inversion to improve the dynamic range for measurement of pseudodiffusion coefficient

The use of slice-selective inversion to improve the dynamic range for measurement of pseudodiffusion coefficient

Signal-to-Noise and Contrast in Fast Spin Echo (FSE) and Inversion Recovery FSE Imaging

Fast spin echo (FSE) imaging has recently experienced a renewed enthusiasm in the clinical setting for its ability to provide high contrast T2-weighted images in short imaging times. This article evaluates the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) properties of the FSE sequence, inversion recovery (IR) FSE sequence, and conventional SE imaging. The results indicate that FSE imaging displays similar contrast properties to SE imaging, but that the SNR and CNR are improved secondary to the longer TRs and longer effective TEs that may be used. The SNR per unit time of the FSE sequence, and hence its efficiency, is at least a factor of 8 better than the SE sequence when 16 echoes are acquired for each excitation. The addition of a slice selective inversion pulse in IR-FSE allows rapid generation of IR images with image contrast similar to that of conventional IR sequences. When used with a multicoil array for abdominal, pelvic, and spine imaging, the IR-FSE sequence produces images that are virtually free of motion artifact from the subcutaneous fat immediately adjacent to the coils. Both FSE and IR-FSE, when compared with SE imaging, provide superior image contrast and SNR in reduced imaging time.

31P Spectroscopic localization using pinwheel NMR excitation pulses

Spectroscopic imaging with a one-dimensional phase-encoding gradient and surface-coil reception relies on the restricted range of sensitivity of the surface coil to provide localization in the dimensions transverse to the coil axis and consequently suffers from relatively poor localization in these dimensions. A two-dimensional (2D) cylindrically selective excitation pulse with a large spectral bandwidth is presented here to remedy this problem. The gradient waveforms are derived from multiple spirals in k space which form an overall pinwheel pattern, resulting in a pulse which is much shorter than the equivalent single-spiral trajectory. Nonuniform traversal of the spirals further reduces the pulse width under conditions of gradient slew-rate limitations, yielding overall gains in bandwidth of up to about 30 compared with the equivalent single-spiral trajectory traversed at constant angular rate. The accompanying rf waveform is obtained by weighted 2D Fourier transformation of the desired sensitivity profile. A new weighting factor is introduced into the rf waveform to compensate for nonuniform sampling of k space by the pinwheel near the origin. This factor is independent of the weighting used to account for the rate of traversal of the trajectory and is applicable to 2D pulse design in general. Pulse sequences employing pinwheel excitation in conjunction with either phase-encoding or slice-selective inversion are used to produce multiple-voxel and single-voxel localization in a human heart and a phantom. Pinwheel pulses may be used to advantage on moieties with long spin-lattice relaxation times and short transverse relaxation times and are therefore ideal for applications in phosphorus (31P) NMR.

Magnetic resonance imaging with adiabatic pulses using a single surface coil for RF transmission and signal detection.

In order to overcome the problems that arise from nonuniform B1 fields, there has been interest in developing pulses that are insensitive to large variations in RF power. Pulses derived from adiabatic passage principles that can execute spin inversion, excitation, and 90 degrees and 180 degrees plane rotations in the presence of B1 inhomogeneities have recently been described. When driven with optimized modulation functions, these pulses can execute uniform excitation, refocusing, and slice-selective inversion over a 10-fold or greater variation in B1 magnitude. This insensitivity to B1 strength enables the execution of T1- and/or T2-weighted spin-echo imaging experiments using coils, such as the surface coil, with extremely inhomogeneous B1 profiles. We have successfully acquired images with these pulses at 200 MHz using a single surface coil as the transmitter and receiver. Images of the slice definition, the region over which the excitation and refocusing pulses operate with a surface coil, and brain images obtained with slice planes perpendicular to the plane of the surface coil are presented. Results demonstrate that these pulses can be transmitted with a surface coil to yield high-quality T1- and/or T2-weighted images without B1 artifacts.

Volume-selected proton spectroscopy in the human brain

We have obtained fully localized, high-resolution spectra from selected volumes of the human brain as small as 1 cm3 rapidly and reproducibly, using a combined depth pulse/two-dimensional ISIS method (2, 2) and a single transmit/receive surface coil. In the standard ISIS experiment (2), three orthogonal slice-selective inversion pulses are used to select a cubic volume of interest. Here we use only two-slice-selective inversion pulses which produce slices perpendicular to the plane of a surface coil. These slices in our studies were in planes perpendicular to y and z, while the transmit/ receive surface coil is at constant x. The result is that a column of sensitive volume is produced which extends perpendicularly from the center of the coil. To remove high-flux signals from the regions in this column which lie close to the coil, a series of depth pulses were included prior to the excitation pulse. This localizes signals in the third dimension, parallel to the plane of the coil. The potential advantages of using a depth-pulse approach for localization in the x direction rather than a third ISIS type slice selection are, first, that chemical-shift effects on slice location in the x direction are symmetrical about the carrier, and, second, by suppressing the high-flux signals with a series of depth pulses, better cancellation of the surface signals may be achieved. We are currently comparing results from the depth-pulse/2D ISIS method with a 3D ISIS or depth-pulse/3D ISIS method in order to optimize localization. By combining the depth-pulse/2D ISIS technique with a 1 -t1 binomial spin-echo sequence (the depth pulses also being l-t-l binomial pulses), the following overall pulse sequence was obtained: