Yves De Deene

MRI-Based Attenuation Correction for PET/MRI Using Ultrashort Echo Time Sequences

One of the challenges in PET/MRI is the derivation of an attenuation map to correct the PET image for attenuation. Different methods have been suggested for deriving the attenuation map from an MR image. Because the low signal intensity of cortical bone on images acquired with conventional MRI sequences makes it difficult to detect this tissue type, these methods rely on some sort of anatomic precondition to predict the attenuation map, raising the question of whether these methods will be usable in the clinic when patients may exhibit anatomic abnormalities. Methods: We propose the use of the transverse relaxation rate, derived from images acquired with an ultrashort echo time sequence to classify the voxels into 1 of 3 tissue classes (bone, soft tissue, or air), without making any assumptions on patient anatomy. Each voxel is assigned a linear attenuation coefficient corresponding to its tissue class. A reference CT scan is used to determine the voxel-by-voxel accuracy of the proposed method. The overall accuracy of the MRI-based attenuation correction is evaluated using a method that takes into account the nonlocal effects of attenuation correction. Results: As a proof of concept, the head of a pig was used as a phantom for imaging. The new method yielded a correct tissue classification in 90% of the voxels. Five human brain PET/CT and MRI datasets were also processed, yielding slightly worse voxel-by-voxel performance, compared to a CT-derived attenuation map. The PET datasets were reconstructed using the segmented MRI attenuation map derived with the new method, and the resulting images were compared with segmented CT-based attenuation correction. An average error of around 5% was found in the brain. Conclusion: The feasibility of using the transverse relaxation rate map derived from ultrashort echo time MR images for the estimation of the attenuation map was shown on phantom and clinical brain data. The results indicate that the new method, compared with CT-based attenuation correction, yields clinically acceptable errors. The proposed method does not make any assumptions about patient anatomy and could therefore also be used in cases in which anatomic abnormalities are present.

Clinical delivery of intensity modulated conformal radiotherapy for relapsed or second-primary head and neck cancer using a multileaf collimator with dynamic control

BACKGROUND AND PURPOSE Concave dose distributions generated by intensity modulated radiotherapy (IMRT) were applied to re-irradiate three patients with pharyngeal cancer. PATIENTS, MATERIALS AND METHODS Conventional radiotherapy for oropharyngeal (patients 1 and 3) or nasopharyngeal (patient 2) cancers was followed by relapsing or new tumors in the nasopharynx (patients 1 and 2) and hypopharynx (patient 3). Six non-opposed coplanar intensity modulated beams were generated by combining non-modulated beamparts with intensities (weights) obtained by minimizing a biophysical objective function. Beamparts were delivered by a dynamic MLC (Elekta Oncology Systems, Crawley, UK) forced in step and shoot mode. RESULTS AND CONCLUSIONS Median PTV-doses (and ranges) for the three patients were 73 (65-78), 67 (59-72) and 63 (48-68) Gy. Maximum point doses to brain stem and spinal cord were, respectively, 67 Gy (60% of volume below 30 Gy) and 32 Gy (97% below 10 Gy) for patient 1; 60 Gy (69% below 30 Gy) and 34 Gy (92% below 10 Gy) for patient 2 and 21 Gy (96% below 10 Gy) at spinal cord for patient 3. Maximum point doses to the mandible were 69 Gy for patient 1 and 64 Gy for patient 2 with, respectively, 66 and 92% of the volume below 20 Gy. A treatment session, using the dynamic MLC, was finished within a 15-min time slot.

Whole abdominopelvic radiotherapy (WAPRT) using intensity-modulated arc therapy (IMAT): First clinical experience

PURPOSE Whole abdominopelvic radiation therapy (WAPRT) is a treatment option in the palliation of patients with relapsed ovarian cancer. With conventional techniques, kidneys and liver are the dose- and homogeneity-limiting organs. We developed a planning strategy for intensity-modulated arc therapy (IMAT) and report on the treatment plans of the first 5 treated patients. METHODS AND MATERIALS Five consecutive patients with histologically proven relapsed ovarian cancer were sent to our department for WAPRT. The target volumes and organs at risk (OAR) were delineated on 0.5-cm-thick CT slices. The clinical target volume (CTV) was defined as the total peritoneal cavity. CTV and kidneys were expanded with 0.5 cm. In a preset range of 8 degrees interspaced gantry angles, machine states were generated with an anatomy-based segmentation tool. Machine states of the same class were stratified in arcs. The optimization of IMAT was done in several steps, using a biophysical objective function. These steps included weight optimization of machine states, leaf position optimization adapted to meet the maximal leaf speed constraint, and planner-interactive optimization of the start and stop angles. The final control points (machine states plus associated cumulative monitor unit counts) were calculated using a collapsed cone convolution/superposition algorithm. For comparison, two conventional plans (CONV) were made, one with two fields (CONV2), and one with four fields (CONV4). In these CONV plans, dose to the kidneys was limited by cerrobend blocks. The IMAT and the CONV plans were normalized to a median dose of 33 Gy to the planning target volume (PTV). Monomer/polymer gel dosimetry was used to assess the dosimetric accuracy of the IMAT planning and delivery method. RESULTS The median volume of the PTV was 8306 cc. The mean treatment delivery time over 4 patients was 13.8 min. A mean of 444 monitor units was needed for a fraction dose of 150 cGy. The fraction of the PTV volume receiving more than 90% of the prescribed dose (V(90)) was 9% higher for the IMAT plan than for the CONV4 plan (89.9% vs. 82.5%). Outside a build-up region of 0.8 cm and 1 cm away from both kidneys, the inhomogeneity in the PTV was 15.1% for the IMAT plans and 24.9% for the CONV4 plans (for CONV2 plans, this was 34.9%). The median dose to the kidneys in the IMAT plans was lower for all patients. The 95th percentile dose for the kidneys was significantly higher for the IMAT plans than for the CONV4 and CONV2 plans (28.2 Gy vs. 22.2 Gy and 22.6 Gy for left kidney, respectively). No relevant differences were found for liver. The gel-measured dose was within clinical planning constraints. CONCLUSION IMAT was shown to be deliverable in an acceptable time slot and to produce dose distributions that are more homogeneous than those obtained with a CONV plan, with at least equal sparing of the OARs.

The design of anisotropic diffusion phantoms for the validation of diffusion weighted magnetic resonance imaging

Diffusion weighted magnetic resonance imaging offers a non-invasive tool to explore the three-dimensional structure of brain white matter in clinical practice. Anisotropic diffusion hardware phantoms are useful for the quantitative validation of this technique. This study provides guidelines on how to manufacture anisotropic fibre phantoms in a reproducible way and which fibre material to choose to obtain a good quality of the diffusion weighted images. Several fibre materials are compared regarding their effect on the diffusion MR measurements of the water molecules inside the phantoms. The diffusion anisotropy influencing material properties are the fibre density and diameter, while the fibre surface relaxivity and magnetic susceptibility determine the signal-to-noise ratio. The effect on the T(2)-relaxation time of water in the phantoms has been modelled and the diffusion behaviour inside the fibre phantoms has been quantitatively evaluated using Monte Carlo random walk simulations.

Age-related differences in metabolites in the posterior cingulate cortex and hippocampus of normal ageing brain: A 1H-MRS study

OBJECTIVE To study age-related metabolic changes in N-acetylaspartate (NAA), total creatine (tCr), choline (Cho) and myo-inositol (Ins). MATERIALS AND METHODS Proton magnetic resonance spectroscopy (1H-MRS) was performed in the posterior cingulate cortex (PCC) and the left hippocampus (HC) of 90 healthy subjects (42 women and 48 men aged 18-76 years, mean±SD, 48.4±16.8 years). Both metabolite ratios and absolute metabolite concentrations were evaluated. Analysis of covariance (ANCOVA) and linear regression were used for statistical analysis. RESULTS Metabolite ratios Ins/tCr and Ins/H2O were found significantly increased with age in the PCC (P<0.05 and P≤0.001, respectively), and in the HC (P<0.01 for both). An increased tCr/H2O was only observed in the PCC (P<0.01). Following absolute quantification based on the internal water signal, significantly increased concentrations of Ins and tCr in the PCC confirmed the relative findings (P<0.01 for both). CONCLUSION Age-related increases of tCr and Ins are found in the PCC, whereas this holds only true for Ins in the HC, indicating possible gliosis in the ageing brain. No age-dependent NAA decreases were observed in the PCC nor the HC. The 1H-MRS results in these specific brain regions can be important to differentiate normal ageing from age-related pathologies such as mild cognitive impairment (MCI) and Alzheimers disease.

31P-MRS demonstrates a reduction in high-energy phosphates in the occipital lobe of migraine without aura patients

Background: Differences in brain energy metabolism have been found between migraine patients and controls in previous phosphorus magnetic resonance spectroscopy (31P-MRS) studies, most of them emphasizing migraine with aura (MwA). The aim of this study was to verify potential changes in resting-state brain energy metabolism in patients with migraine without aura (MwoA) compared to control subjects by 31P-MRS at 3 tesla. Methods: Quantification was performed using the phantom replacement technique. MRS measurements were performed interictally and in the medial occipital lobe of 19 MwoA patients and 26 age-matched controls. Results: A significantly decreased phosphocreatine concentration ([PCr]) was found as in previous studies. While adenosine triphosphate concentration ([ATP]) was considered to be constant in previously published work, this study found a significant decrease in the measured [ATP] in MwoA patients. The inorganic phosphate ([Pi]) and magnesium ([Mg2+]) concentrations were not significantly different between MwoA patients and controls. Conclusions: The altered metabolic concentrations indicate that the energy metabolism in MwoA patients is impaired, certainly in a subgroup of patients. The actual decrease in [ATP] adds further strength to the theory of the presence of a mitochondrial component in the pathophysiology of migraine.

Application of monomer/polymer gel dosimetry to study the effects of tissue inhomogeneities on intensity-modulated radiation therapy (IMRT) dose distributions

BACKGROUND AND PURPOSE When planning an intensity-modulated radiation therapy (IMRT) treatment in a heterogeneous region (e.g. the thorax), the dose computation algorithm of a treatment planning system may need to account for these inhomogeneities in order to obtain a reliable prediction of the dose distribution. An accurate dose verification technique such as monomer/polymer gel dosimetry is suggested to verify the outcome of the planning system. MATERIALS AND METHODS The effects of low-density structures: (a) on narrow high-energy (18 MV) photon beams; and (b) on a class-solution IMRT treatment delivered to a thorax phantom have been examined using gel dosimetry. The used phantom contained air cavities that could be filled with water to simulate a homogeneous or heterogeneous configuration. The IMRT treatment for centrally located lung tumors was delivered on both cases, and gel derived dose maps were compared with computations by both the GRATIS and Helax-TMS planning system. RESULTS Dose rebuildup due to electronic disequilibrium in a narrow photon beam is demonstrated. The gel measurements showed good agreement with diamond detector measurements. Agreement between measured IMRT dose maps and dose computations was demonstrated by several quantitative techniques. An underdosage of the planning target volume (PTV) was revealed. The homogeneity of the phantom had only a minor influence on the dose distribution in the PTV. An expansion of low-level isodoses in the lung volume was predicted by collapsed cone computations in the heterogeneous case. CONCLUSIONS For the class-solution described, the dose in centrally located mediastinal tumors can be computed with sufficient accuracy, even when neglecting the lower lung density. Polymer gel dosimetry proved to be a valuable technique to verify dose calculation algorithms for IMRT in 3D in heterogeneous configurations.

Evaluation of radiochromic gel dosimetry and polymer gel dosimetry in a clinical dose verification

A quantitative comparison of two full three-dimensional (3D) gel dosimetry techniques was assessed in a clinical setting: radiochromic gel dosimetry with an in-house developed optical laser CT scanner and polymer gel dosimetry with magnetic resonance imaging (MRI). To benchmark both gel dosimeters, they were exposed to a 6 MV photon beam and the depth dose was compared against a diamond detector measurement that served as golden standard. Both gel dosimeters were found accurate within 4% accuracy. In the 3D dose matrix of the radiochromic gel, hotspot dose deviations up to 8% were observed which are attributed to the fabrication procedure. The polymer gel readout was shown to be sensitive to B0 field and B1 field non-uniformities as well as temperature variations during scanning. The performance of the two gel dosimeters was also evaluated for a brain tumour IMRT treatment. Both gel measured dose distributions were compared against treatment planning system predicted dose maps which were validated independently with ion chamber measurements and portal dosimetry. In the radiochromic gel measurement, two sources of deviations could be identified. Firstly, the dose in a cluster of voxels near the edge of the phantom deviated from the planned dose. Secondly, the presence of dose hotspots in the order of 10% related to inhomogeneities in the gel limit the clinical acceptance of this dosimetry technique. Based on the results of the micelle gel dosimeter prototype presented here, chemical optimization will be subject of future work. Polymer gel dosimetry is capable of measuring the absolute dose in the whole 3D volume within 5% accuracy. A temperature stabilization technique is incorporated to increase the accuracy during short measurements, however keeping the temperature stable during long measurement times in both calibration phantoms and the volumetric phantom is more challenging. The sensitivity of MRI readout to minimal temperature fluctuations is demonstrated which proves the need for adequate compensation strategies.

On the validity of 3D polymer gel dosimetry: III. MRI-related error sources

In MRI (PAGAT) polymer gel dosimetry, there exists some controversy on the validity of 3D dose verifications of clinical treatments. The relative contribution of important sources of uncertainty in MR scanning to the overall accuracy and precision of 3D MRI polymer gel dosimetry is quantified in this study. The performance in terms of signal-to-noise and imaging artefacts was evaluated on three different MR scanners (two 1.5 T and a 3 T scanner). These include: (1) B₀-field inhomogeneity, (2) B₁-field inhomogeneity, (3) dielectric effects (losses and standing waves) and (4) temperature inhomogeneity during scanning. B₀-field inhomogeneities that amount to maximum 5 ppm result in dose deviations of up to 4.3% and deformations of up to 5 pixels. Compensation methods are proposed. B₁-field inhomogeneities were found to induce R₂ variations in large anthropomorphic phantoms both at 1.5 and 3 T. At 1.5 T these effects are mainly caused by the coil geometry resulting in dose deviations of up to 25%. After the correction of the R₂ maps using a heuristic flip angle-R₂ relation, these dose deviations are reduced to 2.4%. At 3 T, the dielectric properties of the gel phantoms are shown to strongly influence B₁-field homogeneity, hence R₂ homogeneity, especially of large anthropomorphic phantoms. The low electrical conductivity of polymer gel dosimeters induces standing wave patterns resulting in dose deviations up to 50%. Increasing the conductivity of the gel by adding NaCl reduces the dose deviation to 25% after which the post-processing is successful in reducing the remaining inhomogeneities caused by the coil geometry to within 2.4%. The measurements are supported by computational modelling of the B₁-field. Finally, temperature fluctuations of 1 °C frequently encountered in clinical MRI scanners result in dose deviations up to 15%. It is illustrated that with adequate temperature stabilization, the dose uncertainty is reduced to within 2.58%.

On the accuracy and precision of gel dosimetry

Gel dosimeters are radiation sensitive chemical dosimeters that aim at displaying the absorbed dose in three dimensions (3D) and integrated over time. Starting from the fundamental definitions of accuracy and precision this paper examines the unique meaurement challenges inherent to gel dosimetery.