ASN Report 2017

267 ASN report on the state of nuclear safety and radiation protection in France in 2017 Chapter 09  - Medical uses of ionising radiation a series of slice images, is developing in Europe. The evaluation of this technique, currently in progress in several European countries, should enable its advantages comparedwith the traditional planar technique to be determined. At present, this technique is not recognised for use in organised breast cancer screening. Computed Tomography Computed Tomography (CT) scanners use a beam of X-rays emitted by a tube which moves in a spiral around the body of the patient (spiral or helical CT scanner). Based on a computerised image acquisition and processing system, these scanners produce a three-dimensional reconstruction of the organs with very much better image quality than that of conventional radiology devices. The number of rows of detectors (multi-detector-row CT scanner, also known as a multi-slice or volumetric CT scanner) has been increased in recent machines, enabling thinner slices to be produced. An examination can comprise several helical image acquisitions of a specific anatomical region (with or without injection of a contrasting agent) or of different anatomical regions. This technique can, like Magnetic Resonance Imaging (MRI), be associated with functional imaging provided by nuclear medicine in order to obtain fusion images combining functional information with structural information. The technologies developed over the last few years have made examinations easier and faster to perform, and led to an increase in exploration possibilities (example of dynamic volume acquisitions) and in the indications. 1 The placing of mobile computed tomography systems on the market for intraoperative use is to be underlined, as is the increase in fluoroscopy-guided interventional CT procedures. On the other hand, these technological developments have led to an increase in the number of examinations, resulting in an increase in the doses delivered to patients and thus reinforcing the need for strict application of the principles of justification and optimisation (see chapter 1). Technical progress has nevertheless brought a new mode of image reconstruction in the form of iterative reconstruction. Computed tomography can thus provide consistent image quality at reduced doses. As at 31st December 2017, the French pool of radiological devices included slightly more than 1,100 computed tomography facilities covered by an ASN license. 1 . The term indication means a clinical sign, an illness or a situation affecting a patient which justifies the value of a medical treatment or a medical examination. FUNDAMENTALS Medical imaging: several imaging techniques can be used for a given organ The physician’s diagnostic approach based on the history of the illness and the clinical examination of the patient can be supplemented by specific examinations (medical imaging, biological analyses, etc.). Four broad medical imaging techniques are available today. They use X-rays (radiology), gamma rays (nuclear medicine), ultrasounds (ultrasonography) and magnetic fields (MRI - Magnetic Resonance Imaging). These techniques enable the morphology of an organ to be analysed or its function to be studied; the intrinsic qualities and the medical interpretation of the resulting images are fundamentally dependent on the physical principle used: ཛྷ ཛྷ Radiology reveals differences in density in a tissue (due to the presence of a tumour, for example) or between different organs. Radiology, mammography and X-ray computed tomography are radiological examinations. The scanner enables an organ to be reconstructed in 3D and slices of an organ to be created (slice imaging or tomography). ཛྷ ཛྷ Nuclear medicine analyses the distribution of a radiopharmaceutical (a drug consisting of a vector marked by a radioactive isotope or an isolated radionuclide) administered to a patient. This is functional imaging which enables the physiopathological processes to be studied and provides important information on the normal or pathological functioning of a tissue or organ. The radiopharmaceutical is chosen according to the target and the studied organ. ཛྷ ཛྷ Ultrasonography uses the properties of ultrasounds to echo (reflect) off interfaces, whether these interfaces are the anatomical boundaries of organs or heterogeneous areas within an organ or tissue. The recorded echoes allow the reconstruction of an image of the explored area. By combining this with the Doppler effect it is also possible to measure the rate of blood flow in the vessels. ཛྷ ཛྷ MRI uses the magnetic properties of hydrogen nuclei placed in a strong and stable magnetic field. The proton (H + ) is the main constituent of the molecule of water, an element that is present to a greater or lesser extent in all the tissues of the human body. After “excitation” by radiofrequency waves, the signals from the protons in the water of the human body are picked up by dedicated antennas and analysed by computer in order to produce a slice image. Radiology and nuclear medicine that use ionising radiation are regulated by ASN, the French nuclear safety Authority. Ultrasonography and MRI do not use ionising radiation. The Guide to good medical imaging examination practices , produced by the French Society of Radiology (SFR) and the French Society of Nuclear Medicine and Molecular Imaging (SFMN), helps physicians to choose the most appropriate examination according to the symptomatology, the suggested diagnoses and the patient’s medical history. It takes into account the proof of the level of diagnostic performance of the examinations in each of the situations (analysis of international publications), whether the examination involves radiation or not, and if so, the corresponding doses. No technique is universal; a technique that gives good results for one organ or function of that organ may be less effective for another organ, and vice versa .