Potential diagnostic role of the MRI-derived internal magnetic field gradient in calcaneus cancellous bone for evaluating postmenopausal osteoporosis at 3T.

INTRODUCTION: Bone mineral density (BMD) result has a low predictive value on patients' risk for future fractures. Thus, new approaches for examining patients at risk for developing osteoporosis would be desirable. Magnetic resonance (MR) investigations in cancellous bone have been shown to yield useful quantitative information on both trabecular-bone microstructure and bone marrow composition. This work was undertaken to address the hypothesis that the effective internal magnetic field gradient (IMFG), a new MR parameter, discriminates between healthy, osteopenic and osteoporotic postmenopausal women, classified on the basis of bone mineral density (BMD) criteria. The work builds on preliminary results indicating that IMFG, measured in trabecular-bone pores and quantified by spin-echo decay and water diffusion MR near the bone-bone marrow interface depends on both the bone marrow water rate of diffusion and the magnetic susceptibility difference (ΔX) between water and bone. MATERIALS AND METHODS: MR relaxometry, MR spectroscopy and diffusion-weighted MR imaging of the heel was performed in fifty-five women (mean age, 62.9±6.6years) at 3T. Moreover, in order to study the reproducibility of IMFG measurement, five young women (mean age 31.0±3.2years; age range, 28-36years) were scanned and rescanned. The study protocol was approved by the local Ethics Committee. Quantitative Computer Tomography (QCT) of the L1-L3 vertebral segments was performed to classify the postmenopausal women into three groups according to QCT BMD: healthy (n=8); osteopenic (n=25); and osteoporotic (n=22). In all subjects, BMD T-scores, marrow fat content (Mfc), T2*, apparent diffusion coefficient (ADC) and IMFG (estimated from the additional spin-echo decay due to diffusion of water in local magnetic field gradients), were assessed in the whole calcaneus as well as in three calcaneal subregions: subtalar, tuber calcaneus, and cavum calcaneus. Between-group comparisons to assess group differences and Pearson correlation analysis were performed. Short and long-term coefficients of variation (CVS and CVL, respectively) were evaluated in young subjects. RESULTS: Reproducibility of the IMFG measurement was satisfactory. No significant difference was found in the IMFG measurement performed in both calcaneus and subtalar calcaneal region between the two separate sessions comprised of five young women. Mfc did not significantly differ between groups. The IMFG in the subtalar region was significantly different between all three groups (P<0.01), being greatest in healthy women, intermediate in those with osteopenia, and lowest in osteoporotic subjects. Conversely neither T2* nor ADC is able to discriminate healthy subjects from those with osteopenia and osteoporosis. Increased inter-trabecular space, as it typically occurs in patients with osteoporosis, modifies water diffusion, conferring higher ADC values, thereby lowering the IMFG. CONCLUSION: The IMFG measured in the calcaneal subtalar region shows a high ability in identifying healthy subjects. The new quantitative MR method based on measurement of the IMFG may provide a new means for assessing patients with osteoporosis.

Is the in vivo dosimetry with the OneDosePlusTM system able to detect intra-fraction motion? A retrospective analysis of in vivo data from breast and prostate patients.

BACKGROUND: The OneDosePlusTM system, based on MOSFET solid-state radiation detectors and a handheld dosimetry reader, has been used to evaluate intra-fraction movements of patients with breast and prostate cancer. METHODS: An Action Threshold (AT), defined as the maximum acceptable discrepancy between measured dose and dose calculated with the Treatment Planning System (TPS) (for each field) has been determined from phantom data. To investigate the sensitivity of the system to direction of the patient movements, fixed displacements have been simulated in phantom. The AT has been used as an indicator to establish if patients move during a treatment session, after having verified the set-up with 2D and/or 3D images. Phantom tests have been performed matching different linear accelerators and two TPSs (TPS1 and TPS2). RESULTS: The ATs have been found to be very similar (5.0% for TPS1 and 4.5% for TPS2). From statistical data analysis, the system has been found not sensitive enough to reveal displacements smaller than 1 cm (within two standard deviations). The ATs applied to in vivo treatments showed that among the twenty five patients treated for breast cancer, only four of them moved during each measurement session. Splitting data into medial and lateral field, two patients have been found to move during all these sessions; the others, instead, moved only in the second part of the treatment. Patients with prostate cancer have behaved better than patients with breast cancer. Only two out of twenty five moved in each measurement session. CONCLUSIONS: The method described in the paper, easily implemented in the clinical practice, combines all the advantages of in vivo procedures using the OneDosePlusTM system with the possibility of detecting intra-fraction patient movements.