Methods Four patients with unresectable liver metastases from colorectal cancer were treated in 6 sessions, using an open MR system (0.2-Tesla, Magentom Open Viva, Siemens) installed in an operating room environment with MRcompatible monitoring of vital parameters and anesthesia equipment. Patients were placed within the MR-scanner in a supine position, a multipurpose-surface coil (35 cm or 45-cm diameter) mounted around the upper abdomen. Hepatospecific contrast agent (50 ml Teslascan, Nycomed, Oslo, Norway) was applied i.v. and T1and T2-weighted (fast)spin-echo images were acquired prior to MR-guided liver biopsy to localize liver metastasis. Tissue samples were obtained prior to irradiation by a MR compatible biopsy system (16 18G) to confirm malignant disease. Based on the associated susceptibility artifacts the position of the brachytherapy-applicator was identified on rapid gradient-echo images. Following MR-guided biopsy, up to four MR-compatible brachytherapy-applicators (length 20 24 cm, diameter 14 G) suitable to be attached to a high-dose-rate afterloader (MicroSelectron, Nucletron) were placed within the metastasis (Figure 1). The appropriate position of the brachytherapy-applicators was confirmed by T1and T2-weighted (fast)spin-echo images. Treatment planning was carried out on a Nucletron-PLATO-system, whereas the position of brachytherapy-applicators on MRI was correlated to their position visualized on orthogonal abdominal plane with small landmarks attached to the skin. Metastases were treated by a high-intensity Iridium-192 source (5-10 Ci; 185-370 GBq) using an afterloading technique.

The effects of bone on marrow relaxation in the trabecular volume of the most proximal 3 cm in the left tibia were studied with a RF-spoiled gradient echo MRI protocol on a 1.0 T MR unit. The MR measurements were performed on six healthy volunteers, and repeated within one month in order to assess the precision of the method. In the same subjects, the area bone mineral density (bmd, g/cm2) was measured at the left proximal femur using dual-energy X-ray absorptiometry. The calcaneus of the same side was examined with quantitative ultrasound. The marrow T2∗ relaxation deviated from a mono-exponential decay, and resembled the decay of subcutaneous fat. The shape of the relaxation curve reflected the presence of several spectral components in bone marrow, and was further influenced by the amount and structure of the surrounding trabecular bone. The bone marrow decays showed substantially reduced inter-subject variability after normalisation of the marrow data fit parameters to corresponding values for s.c. fat. This suggests the use of an internal adipose tissue reference in order to correct for diet-related variations of marrow T2∗ estimates. The mean relative precision of the MR measurements was between 5% and 10% depending on the data fit model. Moderate-to-strong correlations between DXA bmd indices in the proximal femur and MR parameters were found (rmax = −0.96; p < 0.01), while ultrasound-derived measures of bone strength measured on the calcaneus demonstrated significantly weaker correlations to the MR parameters (rmax = −0.78; p > 0.05). The method employed in this study showed reasonable precision and a moderate to good correlation compared to other bone parameters derived at the same extremity, and is a promising tool for the use on patients.

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.

A method for analysing and comparing treatment planning system (TPS) data and ferrous dosimeter gel measurements evaluated with MRI (FeMRI) was developed, including image processing to final absorbed dose images. Measurements were analysed according to this method and FeMRI data were thereby compared with the TPS-calculated dose distribution. For photons, differences between FeMRI- and TPS dose data were mainly within +/- 2%. Minor shortcomings found in both the FeMRI system and the TPS are explained and discussed. For electron beams, there was an overall good agreement. It was found that the TPS underestimates the lateral scattering dose outside the primary beam, but the reported dose difference corresponds to a small spatial deviation (less than 2 mm). It is important to consider this single beam data comparison when the method is extended to more complicated situations, for example when using several beams.

A dosimeter gel, based on an agarose gel infused with a ferrous sulphate solution and evaluated in a magnetic resonance scanner, was used for complete verification of calculated dose distributions. Two standard treatment procedures, treatment of cancer in the urinary bladder and treatment of breast cancer after modified radical mastectomy, were examined using pixel-by-pixel and dose volume histogram comparison. The dose distributions calculated with the dose planning system was in very good agreement with the measured ones. However, in the case of the more complicated breast cancer treatment, some discrepancies were found, mainly at the beam abutment region. This may be explained by field displacements errors and by a small limitation of the dose planning utilising small electron beams in this region. The dosimeter gel system have proven to be a useful tool for dosimetry in clinical radiation therapy applications.

The thigh and lower leg of six patients with prior polio were examined using magnetic resonance imaging (MRI), and the strength of their weak foot dorsiflexors was measured isokinetically. Spinecho images of the lower extremities were visually evaluated on a semi-quantitative four-point scale, and T1 and T2 relaxation times of the lower leg anterior compartment were analysed. There were prominent MRI signs of randomly distributed muscle degeneration. The high signal intensity changes in the affected muscles on T1-weighted images and T1 and T2 values indicated replacement of muscle fibres with fat and the accumulation of tissue water, respectively. MRI findings were compared with isokinetic strength in foot dorsiflexor muscles. Foot dorsiflexor peak torque values at 30 deg/s ranged from 6 to 29 Nm. There was no significant correlation between MRI visual scoring, T1 and T2 relaxation times and peak torque values at 30 deg/s. However, the most severe MRI changes with visual scoring and T2 relaxation times were observed in the patients with the most pronounced muscle weakness.

Verification of dose calculations in external beam treatment planning using a gel dosimetry system.

Transverse relaxation times were estimated from numerical simulations on spin systems using multi-echo spinecho 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.

Analytical calculations using the Bloch formalism were performed to assess the dependence on T-1 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 T-2 values, the echo amplitudes were erroneous, with an increasing contribution from stimulated echo components with increasing T-1. Apart from affecting T-2 estimates, stimulated echoes generated a non-monoexponential signal decay of the echo trains. The results confirmed previous simulation studies as regards the dependence on T-1 of T-2 estimates from PHAPS.

The effects of imperfect radiofrequency (RF) pulses on the echo amplitudes from the Carr-Purcell (CP), Carr-Purcell-Meiboom-Gill (CPMG), and the PHase-Alternating Phase-Shift (PHAPS; combination of CP and CPMG) multiple spin-echo schemes were studied. Properties of the PHAPS scheme for transverse relaxation time measurements was emphasized. Numerical simulations on non-relaxing spin systems were performed to assess the properties of selective (damped sinc shaped) and nonselective refocusing pulses in terms of effective spatial selectivity and generation of secondary echo signal. Analytical solutions of the Bloch equations were applied to study the generation and propagation of stimulated echo signal caused by nonideal 180-degrees phase reversals, and the results were used to analyse the numerical simulations in terms of primary and stimulated echo components. Finally, the simulated echo train patterns from the different MSE schemes were compared with MR imaging measurements. It was found that the underestimation of T2 values by the PHAPS protocol with selective refocusing pulses is mainly an effect of an ''artificial'' echo amplitude decay in the CP scheme, while the CPMG scheme produces a typical even-odd echo pattern (different from corresponding echo patterns in conventional high resolution NMR). Both effects are related to the flip angle error and phase dispersion along the slice selection direction from selective RF pulses, and are not significantly influenced by stimulated echo interference for nonrelaxing spin systems. However, the presence of stimulated echoes at the time of the primary echoes implies a dependence on T1 of the PHAPS echo amplitudes. In the CPMG protocol, different gradient schemes have been implemented to defocus stimulated echoes. However, the results indicate that there exists stimulated components that will not be affected by such gradients, and that the optimization of the RF refocusing pulses then remain the main objective.

The diffusion of ferric ions produced by irradiation in a dosimeter gel, consisting of a ferrous sulphate solution and agarose gel, has been studied. The diffusion coefficient of ferric ions in the gel was found to be 1.91*10-2 cm2 h-1+or-5%. It was shown that the dose image obtained with an MR scanner deteriorates due to diffusion. This deterioration can be predicted with the aid of the measured diffusion coefficient. It was concluded that if the MR measurements (1/T1 image) of a typical depth-dose distribution are carried out within 2 hours of irradiation the diffusion will not have a significant effect on the results.

A new method has been investigated for the mapping of dose distributions in three dimensions delivered by the Leksell gamma knife. The irradiation unit is used to selectively treat small volumes in the brain with single high doses of ionising radiation -a treatment procedure known as radiosurgery. The dosimetry method investigated utilises a dosimeter gel consisting of ferrous sulphate solution and agarose which is, prior to irradiation, loaded into a cavity in a spherical phantom. Chemical changes induced in the gel by the radiation are measured by means of an MR-scanner. This imaging method permits rapid evaluation of the dose distribution in an irradiated volume. It thus offers a potential verification of individual radiation intracranial target treatment regimes as well as quality assurance measurements, assuming that the precision and accuracy of the dose mapping are adequate. The dose and its distribution registred by the gel dosimeter, in this initial experiment, are in good agreement with corresponding computed data obtained with the KULA treatment planning system of the gamma knife. The gel has thus the potential of being an attractive alternative dose mapping method to those used at present in radiosurgery, i.e. radiographic film and small ionisation chambers. The precision of the dosimeter gel is, however, not yet sufficient high to be used as a basic dosimetry system for the gamma knife.

The measurement of absorbed dose distributions using dosemeter gel and magnetic resonance imaging (MRI) in a standard geometry has been investigated. Absorbed depth-dose curves and profiles measured with this new technique show good agreement with corresponding measurements using diodes. This was proven in a 60Co beam as well as an electron beam. The dosemeter gel is made of agarose and ferrous sulphate solution. The dose response is linear (r = 0.9996) in the investigated dose interval, 0-40 Gy. The sensitivity is a factor of about six higher compared to ordinary ferrous sulphate solution, known as 'Fricke'. This is a true 3D dose measurement technique which will have a number of applications in radiation therapy, since it is possible to mould the gel to arbitrary geometries, mix different radiation qualities and integrate the absorbed dose from different kinds of fields.

A sequence of decaying data, uniformly spaced in time, may be rapidly analysed as a sum of two monoexponential decays by multiplying the amplitudes by four Poisson distributions and summing. The resulting four sums form a system of equations which is easily solved for the parameters of the decays. On simulated noisy data, the method showed precision comparable with the established eigenfunction expansion method (DISCRETE). The method is so fast that it may be applied pixel by pixel to MR images. A faster but less exact variant of the method was also investigated. The method may also be extended to sums of more than two exponentials, but at a considerable cost in speed.

The influence of energy and angular spread and electron and photon contamination on the water/air stopping-power ratios for 'realistic' electron beams of 10 MeV has been investigated using the Monte Carlo method. Differences smaller than 0.5% have been found in the sW,air value at the depth of maximum absorbed dose compared with sW,air(E0,z) determined according to most dosimetry protocols. The use of broad independent energy and angular distributions to sample the initial state of electrons for Monte Carlo simulations has been analysed. Uncertainties in sSAW,air values, evaluated with a Monte Carlo iterative technique, are approximately 0.5% at dmax. The combination of uncertainties in sSAW,air due to the calculation procedure and to the 'sW,air(E0,z) method' yields an estimated total uncertainty of approximately 1% for the sW,air at dmax in clinical electron beams with energies around 10 MeV, which is smaller than values quoted recently.

The accuracy in measurements of mono- and biexponential transverse relaxation processes with an MRI unit (0.5 T) was studied with a binary phantom. Comparison with spectrometer measurements (0.5 T) demonstrated that the imager underestimated the T2 values for monoexponential processes. Numerical resolution of biexponential processes also yielded underestimated relaxation times, but the resolution of a slow, constant component from faster components was relatively precise and consistent, provided the T2 ratio was above 2.5 in the T2 range 200-800 ms for a spin-echo sequence with 32 echoes. The effects of signal-averaging, strength of slice-selective gradient, single- versus multi-slice mode and repetition time were of little importance. In coronal slices a spurious biexponentiality occurred occasionally from monoexponential sources. The influence of stochastic noise was of minor importance compared to the effect of systematic noise.