How to work together between nuclear medicine and radiotherapy departments?

Among the available imaging techniques, functional imaging provided by nuclear medicine departments represents a tool of choice for the oncoradiotherapist for targeting tumour activity, with positron emission tomography as the main modality. Before, during or after radiotherapy, functional imaging helps guide the oncoradiotherapist in making decisions and in the strategic choice of pathology management. Setting up a working group to ensure perfect coordination at all levels is the first step. Key points for a common and coordinated management between the two departments are the definition of an organizational logistic, training of personnel at every levels, standardization of nomenclatures, the choice of adapted and common equipment, implementation of regulatory controls, and research/clinical routine continuum. The availability of functional examinations dedicated to radiotherapy in clinical routine is possible and requires a convergence of teams and a pooling of tools and techniques.

Interest of positron-emission tomography and magnetic resonance imaging for radiotherapy planning and control.

Computed tomography (CT) in the treatment position is currently indispensable for planning radiation therapy. Other imaging modalities, such as magnetic resonance imaging (MRI) and positron emission-tomography (PET), can be used to improve the definition of the tumour and/or healthy tissue but also to provide functional data of the target volume. Accurate image registration is essential for treatment planning, so MRI and PET scans should be registered at the planning CT scan. Hybrid PET/MRI scans with a hard plane can be used but pose the problem of the absence of CT scans. Finally, techniques for moving the patient on a rigid air-cushioned table allow PET/CT/MRI scans to be performed in the treatment position while limiting the patient's movements exist. At the same time, the advent of MRI-linear accelerator systems allows to redefine image-guided radiotherapy and to propose treatments with daily recalculation of the dose. The place of PET during treatment remains more confidential and currently only in research and prototype status. The same development of imaging during radiotherapy is underway in proton therapy.

[Inter- and intrafraction imaging during stereotactic body radiation therapy: Which solutions for which tumours?] / Imagerie inter- et intrafractions en radiothérapie stéréotaxique extracrânienne : quelles solutions pour quelles tumeurs ?

Due to high dose gradients, stereotactic body radiation therapy requires high precision in the location of the tumour. Uncertainties in the positioning can introduce serious damage on organs at risk and consequently can reduce tumour local control. A better tumour location can be achieved by controlling its position with an efficient inter and intrafraction imaging procedure. The various imaging techniques available on treatment systems are presented and performances are discussed. Finally, propositions are given in terms of imaging system according to the location treated by stereotactic body radiation therapy.

["Definition of target volume: When et how can the radiation oncologist use PET?"] / « Définition des volumes cibles : quand et comment l'oncologue radiothérapeute peut-il utiliser la TEP ? ¼.

PET/CT has become a standard examination in oncology but is probably still underused for radiotherapy planning. However, except for the clinical research data that shows the interest of this examination in considering personalized and adaptive radiotherapy, it is also important in defining target volumes. However, before using it in clinical practice, a few prerequisites are required to know the acquisition and segmentation methods. Ideally, PET/CT should become a standard examination for radiotherapy departments in the same way as planning CT and tomorrow as MRI.

Human immunodeficiency virus atropy induces modification of subcutaneous adipose tissue architecture: in vivo visualization by high-resolution magnetic resonance imaging.

BACKGROUND: Human immunodeficiency virus (HIV) infection generally induces lipodystrophy. For targeted treatment a better understanding of its development is necessary. The utility of high-resolution magnetic resonance imaging (MRI) is explored. OBJECTIVES: The present study presents a way to visualize the adipose tissue architecture in vivo and to inspect modifications associated with the atrophy. METHODS: High-resolution MRI scans with surface coils were performed on the calf and at the lumbar region of three groups of patients: HIV patients with lipoatrophy, HIV patients without lipoatrophy and healthy volunteers. All patients underwent a clinical examination. In addition, dual energy X-ray absorptiometry (DEXA) measurements were taken. On the MRI scans adipose tissue thickness and adipose nodule size were measured. Results High-resolution MRI enabled identification of a clear disorganization of adipose tissue in patients with lipoatrophy. In addition, these patients presented a very small adipose tissue thickness on the calf and a very small nodule size. RESULTS: led to the hypothesis that adipose tissue disorganization appears before changes in DEXA measurements or clinically visible modifications. CONCLUSIONS: High-resolution MRI enabled visualization in vivo of precise changes in tissue organization due to HIV lipoatrophy. This imaging technique should be very informative for better monitoring of the atrophy.

Quantitative magnetic resonance imaging of subcutaneous adipose tissue.

BACKGROUND/PURPOSE: Quantitative transverse relaxation time (T(2)) magnetic resonance (MR) imaging has been used with the aim to characterize subcutaneous adipose tissue. Protons in adipose tissue have a fast exchange behavior giving bi-component transverse relaxation processes with short and long relaxation time values depending on the tissue properties. METHODS: MR images were acquired on a 1 T Siemens MR scan using a multi-spin-echo sequence. A high sensitive surface coil, enabling low noise MR images with voxel size of 10 mm(3), was used for performing accurate quantitative T(2) imaging. These acquisition parameters were determined by a preliminary study performed on an oil phantom known to be a valuable model for mimicking in vivo adipose tissue. In vivo study of the thigh adipose tissue was carried out on 30 volunteers. 20 women with various clinically diagnosed cellulite grades and 10 males, among them five showed overweight. Tissue characterization was finally performed through the analysis of the T(2) distributions. RESULTS: Phantom study showed that improvements in the precision in T(2) measurement are obtained at the expense of the spatial resolution. Uncertainties in T(2) measurements are three times lower by considering a region of interest of 3 x 3 pixels compared with a pixel by pixel analysis. The in vivo study showed that women groups present higher mean short T(2S) component values than men. Histogram of T(2) distribution showed that the maximum amplitude is observed at a lower value for the overweight men group. In addition, larger values around the septae were visualized on the long relaxation time images. CONCLUSIONS: This study shows that precise T(2) map of adipose tissue can be computed. The balance between precision and spatial resolution is examined. Preliminary results relative to tissue organization and to difference between clinical groups proves the potential of the quantitative MRI.

In vivo visualization of hyaluronic acid injection by high spatial resolution T2 parametric magnetic resonance images.

BACKGROUND/PURPOSE: In recent years, increasing use of injectable resorbable fillings has been reported for facial wrinkle treatment. However, the physiological processes involved such as the localization and subsequent diffusion of the injected product in skin tissues are poorly documented. This may be noninvasively achieved using quantitative magnetic resonance imaging (MRI), which is duly presented in this pilot study. METHODS: Hyaluronic acid (HA) was injected intradermally in the forearm of a young male volunteer. High-resolution MRI scans using a surface antenna were performed just after injection, and after 2, 4 and 9 months. Morphological images were compared with transverse relaxation time (T(2)) images computed from a pixel-by-pixel analysis. RESULTS: On high-resolution morphological MR images the HA injection is barely visible, but with quantitative MRI the zone of injection is clearly seen. This is due to HA having a distinctly different transverse relaxation time, T(2) approximately 600 ms, compared with dermal and hypodermal tissues, 35 and 80 ms, respectively. CONCLUSION: These preliminary results demonstrate the ability of the T(2) images for in vivo visualization of the filler agent and also for characterization of tissue modifications. In addition, the diffusion and progressive degradation of the filler agent can be monitored by T(2) measurements over time.

High spatial resolution quantitative MR images: an experimental study of dedicated surface coils.

Measuring spin-spin relaxation times (T2) by quantitative MR imaging represents a potentially efficient tool to evaluate the physicochemical properties of various media. However, noise in MR images is responsible for uncertainties in the determination of T2 relaxation times, which limits the accuracy of parametric tissue analysis. The required signal-to-noise ratio (SNR) depends on the T2 relaxation behaviour specific to each tissue. Thus, we have previously shown that keeping the uncertainty in T2 measurements within a limit of 10% implies that SNR values be greater than 100 and 300 for mono- and biexponential T2 relaxation behaviours, respectively. Noise reduction can be obtained either by increasing the voxel size (i.e., at the expense of spatial resolution) or by using high sensitivity dedicated surface coils (which allows us to increase SNR without deteriorating spatial resolution in an excessive manner). However, surface coil sensitivity is heterogeneous, i.e., it--and hence SNR--decreases with increasing depth, and the more so as the coil radius is smaller. The use of surface coils is therefore limited to the analysis of superficial structure such as the hypodermic tissue analysed here. The aim of this work was to determine the maximum limits of spatial resolution and depth compatible with reliable in vivo T2 quantitative MR images using dedicated surface coils available on various clinical MR scanners. The average thickness of adipose tissue is around 15 mm, and the results obtained have shown that obtaining reliable biexponential relaxation analysis requires a minimum achievable voxel size of 13 mm3 for a conventional volume birdcage coil and only of 1.7 mm3 for the smallest available surface coil (23 mm in diameter). Further improvement in spatial resolution allowing us to detect low details in MR images without deteriorating parametric T2 images can be obtained by image filtering. By using the non-linear selective blurring filter described in a previous work, the voxel size was reduced to 0.8 mm3, allowing us to detect microstructures such as fibrous septae while preserving precision in T2 measurements. This paper provides practical information allowing us to perform reliable T2 quantitative MR micro images. High resolution imaging with dedicated surface coils, which is only well-suited to near surface organs, might lead to highly valuable results in this context, especially to analyse the hypodermis involved in the lipodystrophy seen in patients with human immuno-deficiency virus (HIV).

A post-processing method for multiexponential spin-spin relaxation analysis of MRI signals.

Quantitative MR imaging is a potential tool for tissue characterization; in particular, proton density and proton relaxation times can be derived from MR signal analysis. However, MR image noise affects the accuracy of measurements and the number of tissue parameters that can be reliably estimated. Filtering can be used to limit image noise; however this reduces spatial resolution. In this work we studied, using both simulations and experiments, a filter called a 'selective blurring filter'. Compared to other classical filters, this filter achieves the best compromise between spatial resolution and noise reduction. The filter was specifically used to reliably determine the bi-component transverse relaxation of protons in adipose tissue. Long and short relaxation times and the relative proton fraction of each component were obtained with a degree of uncertainty of less than 10% and an accuracy of 95%.