Horst Aichinger

Recent developments in breast imaging

A review of breast imaging has already appeared in 1982 in this journal. Consequently, the present article concentrates on a discussion of only those developments of a more recent nature. Although the emphasis is placed on the physical aspects of the different imaging methods concerned, the essential factors relating to the clinical background and the associated radiation risk are also outlined. The completeness of detail depends on the present clinical importance of the method under discussion. X-ray mammography, which is still the most important breast imaging technique and has proved to be an effective method for breast cancer screening, is therefore treated in greater detail. Since the early 1980s, ultrasound B-mode scanning has evolved to an indispensable adjunct to x-ray mammography. For Doppler sonography, diaphanography, contrast-enhanced MRI, CT and DSA, the visualization of a tumour depends essentially on the enhanced vascularity of the lesion. Whether this will prove to be a reliable indicator for malignancy remains to be shown in controlled clinical studies. Common to all imaging systems is the increasing use of digital methods for signal processing, which also offers the possibility of computer-aided diagnosis by texture analysis and pattern recognition.

Determination of average glandular dose with modern mammography units for two large groups of patients

Until recently, for mammography Mo anode-Mo filter x-ray tube assemblies were almost exclusively used. Modern mammography units provide the possibility to employ a variety of anode-filter combinations with the aim of adapting the x-ray spectrum to compressed breast thickness and composition. The present contribution provides information on the radiation exposure of two large groups of patients (one of 1678 and one of 945 women) who were mammographed with modern x-ray equipment, and on the dosimetry necessary for the evaluation. For dosimetric purposes spectral information is essential. X-ray spectra have been determined for various anode-filter combinations from measurements with a Ge detector. Based on these spectra, conversion factors from air kerma free in air to average glandular dose (g factors) have been calculated for different anode-filter combinations, compressed breast thickness ranging from 2 to 9 cm and breast compositions varying from 0 to 100% glandular tissue. Determinations of various quantities, including entrance surface air kerma (ESAK), tube output, tube loading (TL), fraction of glandular tissue (FGL) and compressed breast thickness, were made during actual mammography. Average glandular dose (AGD) was determined using g factors corrected for tissue composition as well as g values for standard breast composition, i.e. 50% adipose tissue and 50% glandular tissue by mass. It is shown that, on average, the influence of the actual breast composition causes variations of the order of about 15%. For group 1 and group 2, the mean values of average glandular dose (using g factors corrected for tissue composition) were 1.59 and 2.07 mGy respectively. The number of exposures per woman was on average 3.4 and 3.6 respectively. The mean value of compressed breast thickness was 55.9 and 50.8 mm respectively. The mean age of group 1 was 53.6 years (for group 2 the age was not recorded). The fraction by mass of glandular tissue FGL decrease with increasing compressed breast thickness and age of patient (from 75% at 25 mm to 20% at 80 mm, and from 65% at 20 years to 30% at 75 years). For a medium-sized breast, i.e. a compressed breast thickness of 55 mm, FGL is about 35%, indicating that the standard mix (FGL = 50%) might need some modification, particularly because of additional evidence from another investigation with similar results on FGL.

Radiation Exposure and Image Quality in X-Ray Diagnostic Radiology

Radiation exposure and image quality in X-ray diagnostic radiology , Radiation exposure and image quality in X-ray diagnostic radiology , کتابخانه دیجیتالی دانشگاه علوم پزشکی و خدمات درمانی شهید بهشتی

Image Quality and Dose

The radiological image is composed of the spatial variations of a physical quantity, e.g. the X-ray fluence at the input of the imaging chain (radiation image). When using a film-screen system then this spatial variation is represented by the resulting distribution of the optical density on the film (radiograph), when using a digital imaging system it is – after corrections of the raw data with reference to IEC 62494 (see IEC 62494, 2008) and postprocessing – shown by the resulting grey-scale values e.g. on the monitor of the viewing station (X-ray image). The radiological image represents the projection of the spatial distribution of the patient tissue components within the field of view. Visualisation of important details requires separation of the ‘structures of interest’ against the ‘background’ (e.g. in mammography, micro-calcifications in the breast glandular tissue). Loosely speaking, the difference between structures of interest and background is referred to as the signal.

Patient Dose Estimation

Dose output Y100 of X-ray tube assemblies with W/Re-anode at a target angle of 10° and various additional filtration; focus distance of 100 cm (the figure is equivalent to Fig. 10.3)

Production and Measurement of X-Rays

X-rays are produced when a beam of fast electrons strikes a target. The electrons lose, on this occasion, most of their energy in collisions with atomic electrons in the target, causing ionisation and excitation of atoms. In addition they can be sharply deflected by the electric field of the atomic nuclei, thereby losing energy by emitting X-ray photons.

Characteristics of the Imaging Radiation Field

Typical geometrical and physical characteristics of anti-scatter grid Pb 12/40 Typical geometrical and physical characteristics of anti-scatter grid Pb 13/75 Typical geometrical and physical characteristics of anti-scatter grid Pb 15/80

Radiation Field and Dosimetric Quantities

The physical characteristics of the radiation source and the exposure parameters, which together determine the radiation quality, are the anode material of the X-ray tube and the filtration of the primary radiation beam, the X-ray peak tube voltage selected, its temporal course (e.g. especially at short exposure times or, in pulsed exposure techniques, its rise and drop) and the inherent waveform of the tube voltage (2-, 6-, 12-, multi-pulse or DC). The radiation quality (i.e. the photon energy spectrum) influences both patient dose and image quality. An increase in the X-ray tube voltage for a certain anode-filter combination at a definite image receptor dose (see Sect. 5.2) will result in an increased penetration of the X-ray beam and consequently in a reduction of the absorbed dose and the contrast observed in the image.

Optimisation of Image Quality and Dose

The knowledge of the relationship that links image quality and radiation dose is a prerequisite to any optimisation of medical diagnostic radiology, because — according to the ALARA concept — the dose received by the patient during a radiological examination should be kept “as low as reasonably achievable” (see preface). The image quality and dose required for a successful and reliable diagnosis depends on physical parameters such as contrast, resolution and noise, the constitution of the patient, the viewing conditions (Brandt et al. 1983) and also on the characteristics of the observer that assesses the image. In Chap. II.7 (see Fig. II.7.2) the importance of the coordination of these influencing quantities has been pointed out. Furthermore, one should take into consideration that reducing the system noise by increasing the dose will not always improve task performance. This observation indicates that the imaging process might be optimised by accepting a higher system noise (e.g. in paediatrics). The following shows how exposure parameters can be adapted to the medical indication.

Penetration of X-Rays

The attenuation properties of the various kinds of tissue in the patient’s body with respect to X-ray photons in the energy range of about 10 keV to 150 keV is determined principally by the photoelectric effect and Compton scattering (see Chap. II.2). Therefore, in X-ray imaging, photons emitted from the focal spot of the X-ray tube enter the patient, where they may be absorbed, transmitted without interaction (primary photons) or scattered (secondary photons). The radiation image is formed from the emergent primary photons while impaired by the secondary photons (see Chaps. II.5 and III.2). By the interaction of all these photons with a suitable image receptor, the radiographic image is built up. So the X-ray image consists of a two-dimensional projection of the attenuating properties of the tissues in the three-dimensional volume of the patient’s body along the path of the X-ray photons superimposed by scattered radiation.