Variations in slice shape and absorption as artifacts in the determination of tissue parameters in NMR imaging

Minimisation of slab-selective radiofrequency excitation pulse durations constrained by an acceptable aliasing coefficient

A technical note is presented on the slab-direction aliasing of 3D imaging, introducing a simple methodology for determining the minimised duration of low flip-angle sinc radiofrequency (RF) excitation pulses, with respect to a required slab profile accuracy. The various interdependent factors affected in modifying an RF pulse duration are considered and analysed in the context of a new metric for quantifying the levels of permitted slab-aliasing. A general framework is presented for the selection of standard sinc RF excitation pulses with system-minimised durations, as well as their analysis and validation, and a demonstration of this methodology is performed for an example requirement and scanner. This methodology enables implementation of standard (vendor-generated) RF pulses with minimised duration for a required application, with high confidence in their operational reliability. Parts of such a methodology may also in theory be extended to more advanced RF pulse designs.

Determination of Flow Velocities from Magnetic Resonance Multiple Spin-Echo Images

The purpose of this study was to evaluate a method for the quantification of through-plane flow velocities by magnetic resonance imaging (MRI) from the flow characteristics of conventional multiple spin-echo (MSE) signals. Simulated inflow-outflow-dependent signals, as well as images of a flow phantom were generated and the logarithm of the flow-dependent signal value was plotted against echo time. The normalized slope of the resulting curve was calculated using a least-square fit to simulated and experimental data and was corrected for T2 relaxation effects by subtraction of a slope obtained at zero flow. After this correction, and with certain restrictions regarding the flow velocity (v), maximum number of echoes in the slope calculation and slice thickness (L), the normalized slope of the MSE signal becomes equal to the quotient v/L, and from this relation the flow velocity can be determined. The validity of the proposed method was examined for different flow velocities and for two opposite flow directions. The influence of the size of the region of interest and the number of spin echoes used in the calculation of the slope on the accuracy of the velocity determination was also studied. The sensitivity of the method to flow-induced phase changes was examined in the phantom by comparing the results obtained with different strengths of the slice-selective gradient as well as by comparing results from even-echo data with those from odd-echo data. When applied to simulated signal data, the method was found to be strictly valid only for a small velocity range, while for the flow phantom, the calculated velocities corresponded to measured velocities for values up to and over 100 mm/s. In the phantom experiment, the method was found to be insensitive to effects induced by combined changes of the slice thickness and the slice-selective gradient as well as to so-called even-echo rephasing effects. It is concluded that the examined method promises to be a rapid and easily interpretable alternative to other methods, e.g. magnetic resonance velocity-phase encoding, for the determination of flow velocities in vivo.

Rapid T1 mapping of mouse myocardium with saturation recovery Look-Locker method

Dynamic contrast-enhanced MRI using gadolinium or manganese provides unique characterization of myocardium and its pathology. In this study, an electrocardiography (ECG) triggered saturation recovery Look-Locker method was developed and validated for fast cardiac T(1) mapping in small animal models. By sampling the initial portion of the longitudinal magnetization recovery curve, high temporal resolution (∼ 3 min) can be achieved at a high spatial resolution (195 × 390 μm2) in mouse heart without the aid of parallel imaging or echo-planar imaging. Validation studies were performed both in vitro on a phantom and in vivo on C57BL/6 mice (n = 6). Our results showed a strong agreement between T(1) measured by saturation recovery Look-Locker and by the standard saturation recovery method in vitro or inversion recovery Look-Locker in vivo. The utility of saturation recovery Look-Locker in dynamic contrast-enhanced MRI studies was demonstrated in manganese-enhanced MRI experiments in mice. Our results suggest that saturation recovery Look-Locker can provide rapid and accurate cardiac T(1) mapping for studies using small animal models.

A robust methodology for in vivo T1 mapping

In this article, a robust methodology for in vivo T(1) mapping is presented. The approach combines a gold standard scanning procedure with a novel fitting procedure. Fitting complex data to a five-parameter model ensures accuracy and precision of the T(1) estimation. A reduced-dimension nonlinear least squares method is proposed. This method turns the complicated multi-parameter minimization into a straightforward one-dimensional search. As the range of possible T(1) values is known, a global grid search can be used, ensuring that a global optimal solution is found. When only magnitude data are available, the algorithm is adapted to concurrently restore polarity. The performance of the new algorithm is demonstrated in simulations and phantom experiments. The new algorithm is as accurate and precise as the conventionally used Levenberg-Marquardt algorithm but much faster. This gain in speed makes the use of the five-parameter model viable. In addition, the new algorithm does not require initialization of the search parameters. Finally, the methodology is applied in vivo to conventional brain imaging and to skin imaging. T(1) values are estimated for white matter and gray matter at 1.5 T and for dermis, hypodermis, and muscle at 1.5 T, 3 T, and 7 T.

T1 measurements incorporating flip angle calibration and correction in vivo

In this work, we propose a variable FA method that combines in vivo flip angle (FA) calibration and correction with a short TR variable FA approach for a fast and accurate T(1) mapping. The precision T(1)s measured across a uniform milk phantom is estimated to be 2.65% using the conventional (slow) inversion recovery (IR) method and 28.5% for the variable FA method without FA correction, and 2.2% when FA correction is included. These results demonstrate that the sensitivity of the variable FA method to RF nonuniformities can be dramatically reduced when these nonuniformities are directly measured and corrected. The acquisition time for this approach decreases to 10 min from 85 min for the conventional IR method. In addition, we report that the averaged T(1)s measured from five normal subjects are 900 +/- 3 ms, 1337 +/- 8 ms and 2180 +/- 25 ms in white matter (WM), gray matter (GM) and cerebral spinal fluid (CSF) using the variable flip angle method with FA correction at 3 T, respectively. These results are consistent with previously reported values obtained with much longer acquisition times. The method reduces the total scan time for whole brain T(1) mapping, including FA measurement and calibration, to approximately 6 min. The novelty of this method lies in the in vivo calibration and the correction of the FAs, thereby allowing a rapid and accurate T(1) mapping at high field for many applications.

Multislice T1 relaxation time measurements in the brain using IR-EPI: Reproducibility, normal values, and histogram analysis in patients with multiple sclerosis

PURPOSE

To perform T(1) measurements using inversion recovery (IR) echoplanar imaging (EPI) to evaluate reproducibility, normal values, and T(1) histogram analysis as a measure of disease progression in multiple sclerosis (MS) patients.

MATERIALS AND METHODS

Multislice IR-EPI was performed in 10 controls and 36 MS patients. Region-of-interest (ROI) and T(1) histogram analysis were performed on T(1) maps and compared to hypointense T(1) lesions and brain atrophy in MS patients.

RESULTS

Coefficient of variation (COV) varied from 1.6% to 4.9%. Callosal normal (appearing) white matter (N(A)WM) showed the lowest and cortical gray matter the highest T(1) values. T(1) histogram analysis in controls showed a sharp WM peak centered on a T(1) value of 729 msec (range = 679-765) with extension into a shoulder of higher T(1) values. In MS patients, a shift toward higher T(1) values (mean = 788 msec, range = 700-957) with a lower relative peak amplitude was present, predominantly resulting from T(1) prolongation in NAWM. T(1) histogram parameters strongly related to hypointense T(1) lesion volume and brain atrophy in MS patients.

CONCLUSIONS

IR-EPI provides a reproducible method to obtain T(1) values in the brain. Regional variation in T(1) values is present in N(A)WM of volunteers and MS patients. Since T(1) histogram parameters reflect changes in NAWM and correlate with conventional measures of disease burden in MS patients, T(1) histogram analysis may provide a global measure of disease progression in MS.

TurboFLASH FAIR imaging with optimized inversion and imaging profiles

Optimal implementation of pulsed arterial spin labeling (PASL) methods such as flow-sensitive alternating inversion recovery (FAIR), require the minimization of interactions between the inversion and imaging slabs. For FAIR, the inversion:imaging slice thickness ratio (STR) is usually at least 3:1 in order to fully contain the extent of the imaging slice. The resulting gap exacerbates the transit time. So far, efforts to minimize the STR have concentrated on the inversion profile. However, the imaging profile remains a limiting factor especially for rapid sequences such as turbo fast low-angle shot (TurboFLASH) which uses short pulses. This study reports the implementation of a TurboFLASH sequence with optimized inversion and imaging profiles. Slice-selection is achieved with a preparation module incorporating a pair of identical adiabatic frequency offset corrected inversion (FOCI) pulses. The optimum radiofrequency (RF) and gradient scheme for this pulse combination is described, and the relaxation characteristics of the slice-selection scheme are investigated. Phantom experiments demonstrate a reduction in the STR to approximately 1.13:1. Implementation in an animal model is described, and the benefit of the improved profile in probing the sensitivity of the flow signal to tagging geometry is demonstrated. Sensitivity to transit time effects can be minimized with this sequence, and ASL methodologies can be better explored as a result of the improved conformance with the ideal of square slice profiles.

Rapid relaxation times measurements by MRI: an in vivo application to contrast agent modeling for muscle fiber types characterization

This paper is a description of a simulation method to evaluate the contrast in NMR imaging and its aim is to help to optimize the use of contrast media in clinical imaging. Indeed, there is a need to define objective criteria in order to choose among several contrast media the ones that are the most effective and to define their optimal conditions of use, such as: the dose to be injected, the required time after injection to obtain the best enhancement and the optimal imaging sequence parameter values. The method is based on NMR signal simulation in the presence of contrast media and requires the fast measurement of the T1 and T2 relaxation times to obtain the dynamic relaxometry variation of tissues after contrast injection. In this work the fast imaging techniques that are to be described enable the measurement of T1 and T2 with a 30sec temporal resolution on 128*256 matrix images. The accuracy of the method was assessed in rabbit muscles after the injection of two gadolinium chelates (Gd-DTPA and Gd-DOTA) with the aim of improving the in vivo characterization of fast-twitch and slow-twitch muscle fiber types. The simulation results were in close agreement with contrast image analysis and showed, for relevant clinical doses, a small efficacy for both chelates. The interest of the proposed simulation method lies in the fact that it enables to objectively compare the efficacy of different contrast agents, to forecast the efficacy of a given contrast reagent and to define the optimal dose and the optimal imaging sequence parameters that give the best contrast. This simulation method obviates numerous prior experiments to evaluate the benefit expected from different contrast media. The method, which has been evaluated here for muscle investigations is applicable to any tissue analysis and can help to guide the best condition of use of contrast agents in MR imaging.

On the cause of increased aliasing in the slice-select direction in 3D contrast-enhanced magnetic resonance angiography

The combination of short repetition times and large flip angles typically used in 3D contrast-enhanced magnetic resonance angiography (3D CE MRA) can significantly alter the expected shape of the slab profile for unenhanced tissues, which can cause increased aliasing in the slice select direction. In this work, this increased slice select aliasing is demonstrated and explained from both theoretical and experimental points of view. The effect is due to the Ernst angle of unenhanced background tissue occurring on the falling edges of the flip angle profile that has been set for the significantly reduced T(1) of contrast-enhanced blood. The deleterious aliasing effects are magnified substantially when the chosen volume is placed close to surface coil reception with the slice select direction perpendicular to the coil axis. Magn Reson Med 44:336-338, 2000.

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: I. Analysis and compensation of eddy currents

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = I/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower, is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: II. Analysis of B1-field inhomogeneity

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = 1/T2, is related to radiation dose that has been delivered to a gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. The accuracy that is aimed for in this quantitative MR application is about 5% relative to the maximum dose. In a conventional imaging MR scanner, however, several imaging artefacts may perturb the final dose map. These deviations manifest themselves as either a deformation of the dose map or an inaccuracy of the dose pixel value. Inaccuracies in the dose maps are caused by both spatial and temporal deviations in signal intensities during scanning. This study deals with B1-field inhomogeneities as a source of dose inaccuracy. First, the influence of B1-field inhomogeneities on slice profiles is investigated using a thin-slice phantom. Secondly, a FLASH sequence is used to map the B1-field by assessing the effective flip angle in each voxel of a homogeneous phantom. In addition, both experiments and computer simulations revealed the effects of B1 field inhomogeneities on the measured R2. This work offers a method to correct R2 maps for B1 -field inhomogeneities.

T(1) fast acquisition relaxation mapping (T(1)-FARM): optimized data acquisition

A theoretical procedure for estimating the precision of the T(1) Fast Acquisition Relaxation Mapping sequence as a function of a number of acquisition parameters has been validated by both simulations and experimental results. These results have clarified the selection of sequence parameters to give optimal accuracy and precision in the R(1)* measurements. There is excellent agreement between theory, simulation, and experiment except for flip angles greater than 9 degrees, at which point slice profile imperfections significantly degrade the precision of the technique. The experimental results indicate that over a range of T(1)s that would be seen in a bolus tracking experiment (25-1200 ms), T(1) Fast Acquisition Relaxation Mapping can be used to obtain 64 x 128 R(1)* maps at a rate of 1 map/s, with a precision of 10% or better.

Accurate and rapid quantitative dynamic contrast-enhanced breast MR imaging using spoiled gradient-recalled echoes and bookend T(1) measurements

A method is presented that converts dynamic T(1)-weighted spoiled gradient-recalled echo (SGRE) image intensities into estimates of T(1) without the errors associated with imperfections in the slice profile and transmitter coil magnetic field (B(1)). The method involves T(1) measurements performed before and after a series of dynamic SGRE images. These measurements serve to calibrate and correct the SGRE signal strength equation used to estimate T(1). Simulations and phantom experiments were performed to test the method for slice-selective (two-dimensional) and slab-selective (three-dimensional) imaging, as well as for imaging performed with optimized and un-optimized B(1). For nearly all test conditions, T(1) was estimated accurately (within 10%) over a range of T(1) values expected in vivo ( approximately 1200 --> 300 msec). This method should be useful for quantifying dynamic SGRE imaging for many different applications including breast MR imaging. Magn Reson Med 42:746-753, 1999.

Effects of slow flow on slice profile and NMR signal in fast imaging sequences

A computer program has been developed to evaluate the selective-slice profiles obtained in the steady state for fast gradient-echo imaging. Both spoiled and refocused gradient-echo pulse sequences have been considered. By numerically solving the Bloch equations modified for the effects of flow, for a three-dimensional volume of spins, for realistic RF excitations and linear gradient combinations, the program permits the combined effects of flow and imaging variables on the magnetization slice profile to be assessed quantitatively. We have found that the gradient pattern in gradient-echo pulse sequences is a significant factor for determining the steady-state slice profiles and the strength of the NMR signal from the flowing spins.

Flip angle effects in STEAM and PRESS-optimized versus sinc RF pulses

Flip angle dependence of the localized single-voxel 1H NMR spectroscopic sequences STEAM and PRESS using numerically optimized Shinnar-Le Roux (SLR) and conventional sinc RF pulses has been evaluated. Phantom experiments were used to evaluate voxel profiles from MR images of the selected voxels. Information on the total excited volume was recorded from the integrated area under the water peak in the localized spectrum at different flip angles (theta = 0 degrees-180 degrees). The voxel profiles for both the STEAM and PRESS sequences using the SLR RF pulses were found to be identical, unlike the case for the sinc RF pulses. The SLR RF pulses in the PRESS sequence were found to be more sensitive to flip angle variations. Localized, water-suppressed 1H NMR spectra recorded from the frontal gray matter in healthy volunteers (n = 3) showed less lipid contamination using the SLR RF pulses compared with the sinc RF pulses.

Improvements in absorbed dose measurements for external radiation therapy using ferrous dosimeter gel and MR imaging (FeMRI)

A ferrous gel, based on ferrous (Fe) sulphate and agarose, was used with a clinical magnetic resonance imaging (MRI) scanner to obtain relative dose distribution data from therapeutic photon and electron beams. The FeMRI gel was scanned using a new MRI acquisition protocol optimized for T1 measurements. Thorough comparisons with silicon semiconductor detector and ionization chamber measurements, as well as with Monte Carlo calculations, were performed in order to quantify the improvements obtained using FeMRI for dose estimations. Most of the relative doses measured with FeMRI were within 2% of the doses measured with other methods. The larger discrepancies (2-4%) found at shallow depths are discussed. The uncertainty in relative dose measurements using FeMRI was significantly improved compared with previously reported results (5-10%, one standard deviation, 1 SD), and is today between 1.6% and 3.3% (depending on dose level, 2 SD). This corresponds to an improvement in the minimum detectable dose (3 SD above background) from approximately 2 Gy to better than 0.6 Gy. The results obtained in this study emphasize the importance of obtaining basic FeMRI dose data before the method is extended to complicated treatment regimes.

Magnetic resonance renography: optimisation of pulse sequence parameters and Gd-DTPA dose, and comparison with radionuclide renography

The aim of this study was to assess the feasibility of magnetic resonance renography (MRR) using gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) in comparison with conventional radionuclide renography (RR) using technetium-99m-DTPA (99mTc-DTPA). MRR has many advantages over RR, including lack of ionising radiation, increased spatial resolution, and visible background anatomy. By optimising the pulse sequence, we developed an MRR protocol in which signal intensity is linear with Gd-DTPA concentration over a clinically relevant range. Twenty-nine patients and a volunteer were studied using this protocol. Magnetic resonance renography was performed using three different doses of Gd-DTPA: 0.1 mmol kg-1 (n = 13), 0.05 mmol kg-1 (n = 7), and 0.025 mmol kg-1 (n = 9). Each patient was also assessed using radionuclide renography. The resulting renograms were assessed in terms of time to peak signal intensity, signal decrease after peak, and kidney function ratios calculated from both the areas underneath and the slopes of the uptake curves. We have shown that the MR renograms obtained using low dose Gd-DTPA correlate best with the radionuclide renograms. Remaining discrepancies may be explained by variations in the injection procedures (hence in arterial input functions) and the limited coverage of the three MRR slices compared to the whole body projection of RR. Furthermore, at high local concentrations, signal becomes independent of T1 and is dominated by T2.

19F chemical shift imaging technique to measure intracellular pO2 in vivo using perflubron

RATIONALE AND OBJECTIVES

There is a linear relation between the T1 relaxation rate of fluorine-19 (19F) of perfluorochemicals (PFCs) and the partial pressure of the oxygen (pO2) dissolved in the PFC. A line scan technique was used to overcome the significant chemical shift and low signal-to-noise ratio (SNR) of in vivo 19F magnetic resonance imaging. This study was designed to determine whether the line scan technique could detect the effect of oxygen on 19F T1. In addition, its ability to detect changes in intracellular pO2 when the inspired gas was raised from 20% to 100% O2 also was investigated.

METHODS

The T1 relaxation rate of samples of perflubron emulsion diluted from 3.5% to 70% w/v and equilibrated with N2-O2 gas mixtures (pO2 range = 10-450 mm Hg) was measured using the line scan technique. The gas and emulsion pO2 were measured with a blood gas analyzer. The liver T1 relaxation rate was measured in three rabbits given 5 ml/kg perflubron emulsion 4 and 8 days earlier as they breathed room air and then 100% O2. We used a prototype cylindrical coil double-tuned to hydrogen-1 (1H) and 19F and selected a line through the liver. The scanning parameters yielded a voxel size of 20 x 20 x 15.6 mm. Liver and blood samples were obtained postsacrifice for perflubron concentration measurement.

RESULTS

A linear relation between the 19F T1 relaxation rate (1/T1) of the 3.5% w/v emulsion and dissolved pO2 was established with a slope of 0.0033 (sec-1/mm Hg) and a correlation coefficient of .991. As the PFC concentration increased by 1,900%, the slope increased by 21.2%. The 1/T1 for the liver was 0.182 +/- 0.004 sec-1 at baseline. It increased to 0.247 +/- 0.022 sec-1 when rabbits breathed 100% O2 (p = .023), which corresponded to an increase in intracellular pO2 of 19.7 mm Hg. The liver-to-blood PFC concentration ratio was 500:1.

CONCLUSION

In vitro measurements with the line scan technique replicated the established linear dependence of 1/T1 on pO2. In vivo measurements indicated a 20-mm Hg increase in intracellular pO2 of liver phagocytes when the inspired gas was changed from 20% to 100% O2.

Leukoencephalopathy with swelling and a discrepantly mild clinical course in eight children

An identical syndrome of cerebral leukoencephalopathy and megalencephaly with infantile onset was discovered in 8 children, including 2 siblings. Neurological findings were initially normal or near normal, despite megalencephaly and magnetic resonance imaging (MRI) evidence of severe white matter affection. Slowly progressive ataxia and spasticity developed, while intellectual functioning was preserved for years after onset of the disorder. MRI characteristics included diffuse abnormality in signal intensity and swelling of the cerebral hemispheral white matter with cyst-like spaces in the frontoparietal and anterior-temporal subcortical areas. MR spectra were relatively mildly abnormal. Screening for inborn errors, especially those that cause either megalencephaly or white matter disease or both was negative. A distinguishing feature of the present disorder is the apparently severe abnormality of the cerebral white matter as demonstrated by MRI, which contrasts with the remarkably slow course of functional deterioration.

Methods for reconstructing phase sensitive slice profiles in magnetic resonance imaging

The experimental determination of slice profiles excited by applying radiofrequency pulses in the presence of a gradient generally results in magnitude profiles. The conditions necessary to obtain a phase-sensitive picture of the profile of a slice are discussed. A distinction is made between the "excitation profile" (distribution of the transverse magnetization immediately after the RF pulse) and the "slice profile" (distribution after refocusing by gradient reversal and/or imperfect gradient switching). Methods are presented that allow one to obtain either the excitation profile or the slice profile. It is shown that phase encoding along the direction of the slice selection gradient provides a convenient protocol for obtaining the distribution of both the real and imaginary parts of the slice profile. The phase sensitive excitation profile can be obtained by frequency encoding. These methods were used to evaluate the performance of various shaped pulses.

Optimization of flip angle for T1 dependent contrast in MRI

The flip angle which maximizes contrast between materials with different T1 can be calculated from the root of a cubic expression. A simple closed form expression can be used if contrast is defined in a differential sense and results in only slight contrast loss even with large T1 differences.

Magnetization transfer effects in multislice MR imaging

A theoretical model is presented which describes the effects of magnetization transfer in multislice MR imaging of a tissue-mimicking phantom composed of cross-linked agar gel. The model is successful in explaining differences between single and multislice image signal intensities observed for the agar gel but not seen in a simple aqueous solution. Magnetization transfer leads to a reduction in the image signal intensity of a slice of interest due to off-resonance RF irradiation arising from 90 degrees and 180 degrees pulses intended for neighboring slices. The contribution of magnetization transfer to multislice MR imaging depends on the amount of off-resonance RF irradiation during the imaging sequence repetition interval. For the tissue-mimicking agar gel, conventional spin-echo multislice imaging gave rise to a negligible image signal intensity reduction (< or = 2%); however, fast spin-echo (FSE) imaging, which employs up to 16 times as many RF pulses per slice, exhibited as much as a 13% reduction in image signal intensity (13 slices). The reduction in multislice image signal intensity due to magnetization transfer is sample specific and is shown to be more dramatic for in vivo human leg muscle (10% for conventional spin echo, 40% for FSE) where magnetization transfer rates are greater than in the cross-linked agar gel.

Dependence on T1 of the echo amplitudes from multiple spin-echo sequences with equidistant echoes: simulation studies

Transverse relaxation times were estimated from numerical simulations on spin systems using multi-echo spin-echo MRI protocols. The influence of T1 on the echo amplitudes via stimulated echo components was studied. The resulting effects on T2 estimates from the Carr-Purcell (CP), Carr-Purcell-Meiboom-Gill (CPMG), and Phase-Alternating-Phase-Shift (PHAPS; combination of CP and CPMG), multiple echo schemes were examined. Protocols with either spatially selective or nonselective refocusing pulses were studied. An intravoxel static field inhomogeneity of 0.1, 1, and 10 ppm was stimulated. The dependence on T1 of the T2 estimates was notable for T1 values below approximately 800 msec for all protocols. The PHAPS scheme provided rather accurate, but underestimated, T2 values when selective refocusing was used. With nonselective refocusing, PHAPS T2 values were overestimated and demonstrated a pronounced dependence on magnetic field inhomogeneity. In general, long T2 values were erroneous with the PHAPS protocol. The results indicate that a CPMG protocol structure provides a more robust method for T2 estimations than the PHAPS protocol.

MRI measurements of the dependence on T1 of the echo amplitudes using a multiple spin-echo scheme

Analytical calculations using the Bloch formalism were performed to assess the dependence on T1 of the echo amplitudes for the Phase-Alternating Phase-Shift (PHAPS) multiple spin-echo protocol. Measurements in a 0.5 T MR imaging unit were performed to ratify the analytical results. Especially for low T2 values, the echo amplitudes were erroneous, with an increasing contribution from stimulated echo components with increasing T1. Apart from affecting T2 estimates, stimulated echoes generated a non-monoexponential signal decay of the echo trains. The results confirmed previous simulation studies as regards the dependence on T1 of T2 estimates from PHAPS.

Radiofrequency map of an NMR coil by imaging

We propose a new imaging method to obtain a map of the radiofrequency (RF) field amplitude over a sample. The sequence contains three RF pulses (alpha, 2 alpha, and alpha) and produces two images by a classical spin echo and a stimulated echo. A third image is computed and gives the distribution of the flip angle alpha, and so the RF amplitude, over the sample. The accuracy of the flip angle determination is verified on an homogeneous sample and results show a good correlation between experimental and theoretical flip angles in the range of 50 degrees to 130 degrees. Experiments with a surface coil and a resonator show the method is available in an inhomogeneous RF field. Images obtained on the calf of a volunteer confirms the independence of the computed RF distribution from proton density, T1, or T2 contrast.

Interstitial 1.06 Nd:YAG laser thermotherapy for brain tumors under real-time monitoring of MRI: experimental study and phase I clinical trial

This paper presents the experimental and clinical results of interstitial 1.06 Nd:YAG laser thermotherapy (ILTT) for brain tumors under real-time monitoring by magnetic resonance imaging. The authors chose a laser heat source for interstitial thermotherapy of brain tumors for several important reasons: (1) Laser heat delivery is less complicated and more controlled; (2) laser effects on tissue can be tested, monitored, and controlled by MRI. A 1.064 nm Nd:YAG laser and a specially designed laser optic fiber (ILTT) were used in C.W. mode this study. The laser was used at 4 W at a C.W. mode pulse and total exposure duration was 10 minutes (total energy was 2400 joules). Temperature distribution was determined with a microprocessor-based thermometer and by the levels of the signal intensity under MRI. The relationship between the temperature and MRI signal intensity allowed exploration of the possibility of using MRI as a noninvasive temperature monitoring method. Two patients with glioblastoma and one patient with a brain metastasis were treated with this modality. The results and indications are presented and discussed.

Practical T2 quantitation for clinical applications

Many studies have investigated the use of magnetic resonance relaxation times for tissue characterization. A number have been performed in vivo with clinical whole-body imagers. Unfortunately, the results have yet to establish the role of quantitative tissue relaxation time measurements in clinical diagnosis. One of the major problems is that the techniques used in many of these studies are error prone, making the results inconclusive. In the present study, the problems associated with clinical T2 measurements were systematically evaluated in an attempt to obtain reliable in vivo quantitation. The authors demonstrate that spoiler gradients are the most effective technique for artifact suppression but that they render ineffective radio-frequency phase schemes such as the Meiboom-Gill modification to the Carr-Purcell sequence to compensate experimental imperfections. The present study results in a more reliable multi-echo sequence for T2 measurement. Preliminary clinical results in brain and cervix demonstrate the performance of the new technique.

On the accuracy of noninvasive thermometry using molecular diffusion magnetic resonance imaging

Temperature measurement using magnetic resonance imaging (MRI) of water self-diffusion is investigated. Diffusion images and derived temperatures are obtained in polyacrylamide gel phantom. The temperatures measured from MRI are compared with those from temperature probes to verify their accuracy. In general, the difference between temperatures determined from MRI diffusion images over 0.3 cm3 regions of interest and from temperature probes were 0.2 degrees C. It is concluded that current MRI technology allows noninvasive temperature tomography that is comparable with invasive thermometry with respect to temperature accuracy, has spatial and time resolutions that would be useful in hyperthermic oncology.

Observations on the choice of reconstruction matrix in magnetic resonance imaging

This paper points out a number of ways in which the choice of imaging matrix can affect the performance of a machine in Magnetic resonance imaging. It observes that, contrary to most other techniques, there are advantages in some instances both in sensitivity and detectability through the use of lower resolution images which may not be obtainable in higher resolution ones--although it is equally true that there are other circumstances where the reverse is true. Considerations of effects arising from field inhomogeneity, patient motion, susceptibility mapping, and fluid flow are used to illustrate the argument.

Temporal evolution of ischemic damage in rat brain measured by proton nuclear magnetic resonance imaging

We studied the effect of focal cerebral ischemia on the "state" of brain water using proton nuclear magnetic resonance imaging. Focal cerebral ischemia was induced in five halothane-anesthetized rats via tandem occlusion of the left common carotid artery and the left middle cerebral artery. The proton transverse relaxation time, the proton density, and the water diffusion coefficient were measured at various times from the same region of brain tissue from 1.5 to 168 hours after occlusion. Early measurements indicated significant changes in the transverse relaxation time (p = 0.004) and water diffusion coefficient (p = 0.002) of ischemic brain tissue compared with a homologous region from the contralateral hemisphere. However, the transverse relaxation time, proton density, and water diffusion coefficient in ischemic brain tissue showed different temporal evolutions over the study period. Diffusion coefficient weighting was superior to relaxation time and proton density weighting for the visualization of early cerebral ischemia. Our data suggest that nuclear magnetic resonance imaging is sensitive in detecting changes in proton-associated parameters during early cerebral ischemia and confirm significant changes (p less than or equal to 0.01) in the temporal evolution of transverse relaxation times, proton densities, and diffusion coefficients following middle cerebral artery occlusion.

Target-point combination of MR images

A method is described for combining multiple magnetic resonance images of the same anatomic slice to produce a single image which incorporates the favorable contrast features of each of the original images. The target-point method is a general method that includes linear combination as a subset and is designed to deal with the clinical need to maximize the contrast-to-noise ratio between several pairs of tissue simultaneously. Although it is intrinsically a nonlinear method, noise propagates approximately uniformly into the combined image. In examples of brain images the target-point method produces images with higher mutual contrast than the first principal component weighted sun image.

Measurement of the blood-brain barrier permeability and leakage space using dynamic MR imaging. 1. Fundamental concepts

Leakage of Gd-DTPA through a defective blood-brain barrier is measured quantitatively using dynamic MRI scanning, in which repeated scans are made after a bolus injection. Image registration artifacts are minimized; a dose of 0.1 mM/kg and an IR sequence enable enhancement to be measured quantitatively. The triexponential enhancement curve is fitted to a theoretical model based on compartmental analysis. The transfer constant, or permeability surface area product per unit volume of tissue (k), and leakage space per unit volume of tissue (v1) are measured. Estimates for a quickly enhancing multiple sclerosis lesion are k = 0.050 min-1, v1 = 21%; for a slow one k = 0.013 min-1, v1 = 49%. This implies permeability in the range 4-17 x 10(-6) cm s-1, in broad agreement with other physiological methods. The method is noninvasive and can be used to make serial measurements in patients and in experimental animal models. The time course of pathological aspects of diseases with blood-brain barrier breakdown, such as multiple sclerosis, tumors, and infections (e.g., HIV) can be studied, along with their response to therapy. The measurements are of physiological variables and are therefore independent of imaging equipment and field.

Towards quantitative measurements of relaxation times and other parameters in the brain

The nature and physical significance of the relaxation times T1 and T2 and of proton density are described. Methods of measuring T1 and T2 are discussed with emphasis on the establishment of precision and the maintenance of accuracy. Reported standards of success are briefly reviewed. We expect sensitivities of the order of 1% to be achievable in serial studies. Although early hopes of disease diagnosis by tissue characterisation were not realised, strict scientific method and careful calibration have made it practicable to apply relaxation time measurement to research into disease process. Serial measurements in patients and correlation with similar studies in animal models, biopsy results and autopsy material taken together have provided new knowledge about cerebral oedema, water compartmentation, alcoholism and the natural history of multiple sclerosis. There are prospects of using measurement to monitor treatment in other diseases with diffuse brain abnormalities invisible on the usual images. Secondarily derived parameters and notably the quantification of blood-brain barrier defect after injection of Gadolinium-DTPA also offer prospects of valuable data.

Problems with achieving saturation using methods based on bursts of rf pulses with spoilers in magnetic resonance imaging

Attempts at saturation by nonselective rf pulses, followed by gradient spoiler pulses, are sometimes used as the basis of a method of measuring T1 in magnetic resonance imaging, because a method of this type is perceived as being less affected by slice shape artifact than partial saturation methods. This note suggests that, unless care is taken, this assumption can be quite erroneous.

Fast Field Echo imaging: an overview and contrast calculations

Current fast imaging techniques are based on gradient echo sequences with reduced flip angle excitation pulses and very short repetition times TR. Practical T2 values may be of the order of TR or longer. In this situation, a different image contrast can be obtained, depending on details of the sequence. Four essentially different versions of the basic Fast Field Echo (FFE) sequence can be distinguished and are described systematically in this article. For these sequences, image contrast formulas are presented. Practical imaging should tolerate small field inhomogeneities. This requirement can be satisfied by only three of the four versions. Numerical simulations are used to study the influence of a modified phase alternation scheme on image contrasts of two of the remaining sequences. The results of the calculations are verified by phantom studies on a 1.5-T whole-body imager. Implications for contrast in clinical images are discussed in relation to head images obtained on the same machine.

A comparison of fast spin echo and gradient field echo sequences

Optimal angle, fast repeat time, gradient field echo imaging techniques such as FISP (Fast Imaging with Steady Precession) and FLASH (Fast Low Angle Shot) often fail to discriminate disease from healthy tissue for two main reasons. First, T1 and T2 of the affected tissue may increase such that the ratio of T1 to T2 remains nearly unchanged, hence there is no contrast change with FISP. Second, T2 weighted gradient field echo images suffer severely from T2* signal and resolution loss leading to a reduction in C/N. Although FLASH imaging with two separate angles can, in principle, extract the longer T1 tumors, contrast is often not good. To overcome the inhomogeneity and contrast problems, we have implemented a FAst optimal angle spin-echo sequence with a short TE(FATE). For the first echo, FATE has the same contrast properties as FLASH with a slight decrease in signal intensity. The advantage is that the intensity of the signal does not suffer from T2* signal decay, hence improved contrast and disease detection via T2 weighted FATE images is possible. Contrast-to-noise in lesion detection is also considered for CE FAST (Contrast Enhanced Fast), a T2-weighted version of FISP, and HYBRID.

Optimization of MR protocols: a statistical decision analysis approach

A new method of optimizing MRI data acquisition protocols is presented. Tissues are modeled with probability density functions (PDFs) of tissue parameter values (such as T1, T2). The imaging data acquisition process is modeled as a mapping from a tissue parameter space to a signal strength space. Tissue parameter PDFs are mapped to signal strength PDFs for each tissue in a clinical problem. The efficacy of an MRI protocol is evaluated using the methods of statistical decision analysis applied to the signal strength PDFs, including the propagation of noise. This procedure evaluates the ability to discriminate different tissues based on the signal strengths produced with the protocol. The model can incorporate an arbitrary number of tissues, parameters, and pulse sequences in the protocol. The multivariate nature of MRI and the observed broad distribution of tissue parameter values makes this model more appropriate for optimizing data acquisition protocols than methods which maximize the signal-difference-to-noise ratio between discrete values of the tissue parameters. It is shown that these two methods may calculate different optimal protocols. The method can be used to optimize data acquisition for quantitative computer-based tissue classification, as well as imaging. Data acquisition and image processing philosophies are discussed in light of the method.

Slice-shape artifact changes with precession angle in rapid MR imaging

One possible method of measuring T1 in rapidly acquired magnetic resonance images using variants of the FLASH technique is by the manipulation of the magnetization precession angle. It is suggested here that this method is as prone to slice-shape artifacts as many other approaches.

MRI: the development of T2 contrast with rapid field echo sequences

The soft tissue contrast obtained in fast imaging using rapidly repeated partial saturation sequences is usually based on differences in T1. We describe here an extension of the method which results in the development of clinically useful T2-based contrast.