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Equipment
Diagnostic Radiology
Introduction
Radiological imaging has a fundamental role in accurate diagnosis and successful treatment of patients. In principle, in diagnostic radiology, electromagnetic radiation is used for medical imaging. Different types of medical images are produced by different energies of X rays, different acquisition geometries and methods, referred to as imaging modalities. X rays are used in radiography, fluoroscopy, mammography and computed tomography. Radiofrequency waves are used in magnetic resonance imaging and high frequency sound waves for ultrasound imaging. The use of mathematical image reconstruction allows the visualization of sections of the body, free from tissue superposition effect, which is a principle used in computed tomography and magnetic resonance imaging.
Radiography
Projection radiography imaging dates to the discovery of X rays by Conrad Roentgen in 1895. In its simplest form, X ray imaging is the collection of attenuation shadows that are projected from an ideal X ray point source on to an image receptor. This simplified view, however, is made vastly more complex by the real X ray source and the consequences of projecting a 3-D object on to a 2-D detector. In addition, scattered radiation generated within the patient will degrade the captured image.
The principal components of a system for X ray projection radiography are: X ray generator, X tube and housing with collimation system, patient table, anti-scatter grid and image receptor (e.g. screen-film or digital). Most of X ray units would include the automatic exposure control (AEC) and optional components such as shaped filtration or compression devices. For example, radiographic X ray units use AEC devices that automatically adjust radiographic technique parameters to deliver a constant signal intensity at the image receptor in response to differences in patient thickness.
Different radiography modalities require various types of specialized equipment and techniques. The process used for chest radiography is not satisfactory for mammographic imaging, nor is it suitable for radiographs of the pelvis and abdomen. Dental radiography is included in this area of imaging and has specialized equipment for general dental imaging, cephalometric applications, and panoramic imaging. Image quality is dependent upon the X ray tube voltage, tube current and exposure time, size of the X ray tube focal spot, focal spot to image distance, and anti-scatter grids. Although patient dose is important, image quality is the essential performance element in general radiographic imaging.
Understanding of the physics of image acquisition is important to the optimization in radiography. X ray images are formed as shadows of the interior of the body and thus, the X ray receptor must be larger than the body part to be imaged. Furthermore, the detector needs to allow for good image quality in terms of object’s size and contrast. This means absorbing most of the X ray quanta and using these in an efficient( i.e., a quantum noise limited) manner, while providing adequate spatial resolution.
The capture of an X ray image may conceptually be divided into three stages. The first is the interaction of the X ray with a suitable detection medium to generate a measurable response. The second is the temporary storage of this response with a recording device. The third is the measurement of this stored response. As an example, the stages for a screen film system are: the interaction of an X ray in a phosphor material followed by generation of visible light photons, the subsequent creation of a latent image in the photographic film by these photons, and, finally, the development of a fixed photographic image. A fourth stage required for reusable systems (i.e., computed radiography systems) is the erasure of all previous images within the detection system in order to prepare for a subsequent image. The four steps for a digital direct conversion flat panel imaging system are: the absorption of an X ray followed by the release of multiple secondary electrons in a photoconductor, drifting of the electrons and holes to individual electrodes where they are stored, the readout phase when the charges are transferred to amplifiers where they are digitized line by line, and erasure which is performed simultaneously with the readout.
Medical physicist contribute to the implementation and optimization of planar radiography X ray imaging procedures. The optimization of image quality versus radiation dose is a key task for medical physicists in this area.
Fluoroscopy
Fluoroscopy refers to the use of an X ray beam and a suitable image receptor for viewing images of processes in the body in real time. Fluoroscopic imaging produces high signal to noise ratio (SNR) images at lower resolution, compared to radiography. The ability of fluoroscopy to display motion is provided by a continuous series of images produced at a rate from a few images per second up to 30 images per second.
Fluoroscopic imaging systems use much of the same technology as radiographic systems, with certain modifications and additions. Depending on the intended use, fluoroscopic systems require high-power generators and high heat capacity X ray tubes. The major difference between radiographic and fluoroscopic equipment is the image receptor. Early fluoroscopic systems used an intensifying screen, similar to that used in radiographic screen film imaging, that allowed radiologist to directly view dim images. However, the development of the X ray image intensifier was essential to the success of modern fluoroscopic imaging. Conventional and digital fluoroscopy differ primarily in the imaging system. An image intensifier-video camera system is used in conventional fluoroscopy. Flat panel X ray detectors are a technology alternative that may have neither an image intensifier nor video camera. In general, all other portions of the equipment are similar. In both cases, the equipment is used for diagnostic as well as therapeutic, i.e., fluoroscopy guided interventional procedures.
In fluoroscopic systems, the automatic exposure control (AEC) is used to control exposure parameters to deliver a constant the air kerma rate to the X ray image intensifier in response to differences in patient thickness. This prevents fluctuation in image brightness and signal-to-noise that would make diagnosis or navigation of instruments difficult.
Fluoroscopic imaging systems can be configured in several ways. The most common is the configuration used in genitourinary and gastrointestinal imaging in which the X ray tube is located under the patient table and the image intensifier and auxiliary imaging equipment are placed on a movable ‘tower’ above the patient table. Remote fluoroscopy systems, also commonly used for gastrointestinal procedures, utilize a configuration with the X ray tube located above the table and the image intensifier assembly below the table. The system is controlled remotely from a control room, which significantly reduces the exposure to the operator.
Vascular and interventional radiology procedures are usually performed in angiographic suites equipped with C-arm fluoroscopy units, which comprises a mechanically coupled X ray tube and image receptor. The X ray tube and image receptor rotate in unison about a point called the isocentre that remains at the centre of the field of view. The patient table is often cantilevered to allow continuous, unobstructed rotation of the C-arm around the patient during procedures. Vascular and interventional suites are usually equipped with more powerful generators with high heat capacity X ray tubes and additional features as spectral shaping filters.
The radiation dose rates for fluoroscopy are relatively high and the exposure times can be long, depending on the complexity of the procedure and the skill and experience of the imaging physician responsible for the procedure. The factors that must be taken into account when considering image quality in fluoroscopic imaging include contrast, noise, sharpness, temporal resolution and artefacts or image distortions. While each of these quantities is influenced and limited by the design of the fluoroscopic equipment, they are also highly dependent on equipment configuration and use. Consequently, a thorough understanding of the technology and radiation doses from fluoroscopy is essential to optimize the protocols and assure that the image quality provides adequate information while the patient dose is maintained at an acceptable level. The optimization of image quality versus radiation dose is a key task for medical physicists in diagnostic and interventional radiology.
Mammography
Mammography is a radiographic procedure designed for imaging of the breast. For many women, mammography is a highly effective means of detecting early-stage breast cancer. It is one of the most demanding examinations in medical imaging requiring fine detail, high contrast, low patient motion, low noise images, and appropriate viewing conditions. For example, the mammography system must have sufficient spatial resolution at high spatial frequencies to delineate the edges of fine structures in the breast, as fine as 50 μm. The breast tissues intrinsically lack subject contrast, requiring the use of low energy X ray photons, able to emphasize the compositional differences in the breast tissues. The breast is sensitive to ionizing radiation, which requires use of the lowest absorbed dose compatible with high diagnostic image quality.
Mammography is performed using a dedicated mammography X ray unit, consisting of an X ray tube and an image receptor mounted on opposite sides of a mechanical assembly. Because the breast must be imaged from different angles, the assembly is able to rotate around the horizontal axis. Radiation leaving the X ray tube passes through a metallic spectral filter and compression plate, which compresses the breast on to the breast support platform. The X rays transmitted through the breast and breast support are incident on a anti-scatter grid, and subsequently on the image receptor, where they interact and deposit most of their energy locally. In screen film and cassette based digital mammography systems, a fraction of the X rays passes through the receptor without interaction and these X rays impinge upon the sensor of the automatic exposure control (AEC) mechanism of the mammography unit. In other digital mammography systems, the AEC mechanism is typically integral with the digital image receptor.
In mammography, the spectral shape is controlled by adjustment of the tube voltage, the target material and the type and thickness of the metallic filter placed between the X ray tube and the breast. The strategies for optimization of the X ray spectrum for screen film mammography and digital mammography are quite different. In screen film mammography, the contrast of the displayed image is constrained by the fixed gradient of the film, while in digital mammography, the quality of the displayed image is constrained by the image signal to noise ratio (SNR).
As noted above, image quality is extremely important in mammography, and it is essential to assure that the entire imaging chain is functioning optimally. An effective quality assurance programme is required to achieve this goal. The optimization of image quality versus radiation dose and quality assurance are the key tasks for medical physicists in mammography.
Computed tomography
Computed tomography (CT) is a radiographic process that produces a photon attenuation map of the patient based on the variable attenuation of a beam of X rays as it passes through a patient. The process of CT image acquisition involves the measurement of X ray transmission profiles through a patient for a large number of views. A profile from each view is achieved primarily by using a detector arc generally consisting of several hundreds of detector elements, referred to as a detector row. By rotation of the X ray tube and detector row around the patient, a large number of views can be obtained. The acquired transmission profiles are used to reconstruct the CT image, composed of a matrix of picture elements (pixels). The gantry contains all the system components that are required to record transmission profiles of the patient. The X ray tube with high voltage generator and tube cooling system, the collimator, the beam shaping filters, the detector arc and the data acquisition system are all mounted in the gantry.
CT produces cross-sectional images of high radiographic contrast, particularly important for diagnosis involving soft tissue and vastly superior to that contrast gained from projection radiography. On the other hand, the dose to the patient may be significantly higher in comparison with other imaging modalities. The simplest image acquisition is the scan projection radiograph (SPR), also known as a scoutview, scanogram or topogram. This is taken to plan the CT acquisition with the X ray tube and detector moving in one plane relative to the patient. The axial slice scan involves acquiring a collection of attenuation profiles around a patient who is stationary on the scan table, which ensures that all the profiles are in the one plane and allows rapid reconstruction computation. Helical, or spiral, scanning is achieved when the couch is moved at the same time as the scan profiles are acquired. In spiral scanning the reconstruction is more complicated, due to the fact that the profiles are no longer in the same plane and may be interpolated to a pseudo-planar state before reconstruction. The maximum benefit from helical CT acquisition is achieved with multidetector CT (MDCT) acquisition. Currently, most scanners are helical MDCT scanners, but the technologies of dual source and cone beam CT (CBCT) have been implemented on a wide scale.
In modern scanners, relevant exposure parameters such as the X ray tube voltage, tube current and exposure time are automatically controlled based on individual patient size., The exposure parameters are also adjusted during the scan of a patient as the beam passes through different angles at any particular cross-sectional position. This is often described as a form of automatic exposure control (AEC).
After preclinical research and development during the early 1970s, CT developed rapidly as an indispensable imaging modality in diagnostic radiology, resulting in increasing clinical application. As per United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) 2020/2021 Report, CT makes the largest contribution (about 62%) to the collective effective dose to the population from the man-made sources of radiation, but accounts for only about 10 % of all procedures. Furthermore, the total number of CT examinations has increased by about 80 %, and its contribution to the collective effective dose has increased from 37 % to 62 % period 2009–2018. These facts along with increasing complexity of scanners, require careful monitoring by the medical physicist in conjunction with the radiologist and radiographer to ensure that appropriate examination conditions exist and that procedures are optimized for diagnostic quality and patient dose.
Interventional procedures
In interventional procedures, X ray imaging is used to guide the operator during the positioning of catheters, coils, stents, etc., placed with the intention of obtaining diagnostic information or a therapeutic effect from the procedure in radiology and cardiology. Fluoroscopically guided interventional procedures are widely used as an alternative to conventional surgery, resulting in reduced patient morbidity and mortality. These procedures are usually performed in interventional suites equipped with C-arm fluoroscopy units. A C-arm fluoroscopy X ray units comprises a mechanically coupled X ray tube and image receptor. The X ray tube and image receptor rotate in unison about a point called the isocentre that remains at the centre of the FOV when the C-arm is rotated. The table is often cantilevered to allow continuous, unobstructed rotation of the C-arm around the patient during procedures, allowing for positioning at a variety of angles around the patient. Interventional suites are equipped with more powerful generators with high heat capacity and water or oil cooled X ray tubes. Also, variable spectral shaping filters are often included to maximize iodine contrast while maintaining the patient dose at an acceptable level. Image receptors used in interventional procedures can be either image intensifier based or digital, e.g., flat panel detectors. The image receptors used for cardiac imaging are smaller than those used for vascular and interventional radiology, owing to the small size of the heart.
Interventional suites can be either single plane or biplane systems. Biplane systems use two C-arms that can be independently positioned around the patient for simultaneous digital acquisitions during a single contrast injection.
The interventional procedures are, by their nature, very variable and dynamic. Radiation doses to patients from fluoroscopically guided interventional procedures may be high enough to cause skin injuries and increased probability of developing cancer in future years. There is also a risk to staff members of deterministic effects such as cataract formation. Changes in acquisition mode, i.e., fluoroscopy, image acquisition, exposure factors, filtration, projection, collimation and body part irradiated may all take place during such examinations. The patient dose will depend on the size of the patient, the image acquisition protocol and the complexity of the case. Therefore, monitoring of relevant dosimetric quantities is essential. Kerma area product (KAP) is the dosimetric quantity of choice for the estimation of radiological risk. The use of incident air kerma and entrance surface air kerma is needed for examinations where there is a risk of skin injury. The total KAP for the examination, air kerma at reference point and the total fluoroscopy time are displayed on the X ray console. Medical physicists provide consultation on the doses received by patients or personnel and on the associated risks. They have responsibilities in the optimization of the physical and technical aspects of the imaging equipment and process in fluoroscopy guided interventional procedures.
Dental radiography
The tooth is a low attenuation static object that, when radiographed directly, places very limited demands on X ray generation. The image receptor is placed inside the mouth and irradiated externally. Dental radiography is, typically, limited to a several types of examinations including: 1) intraoral radiographs; 2) panoramic radiographs; 3) cephalometric radiographs; and 4) cone-beam CT imaging.
Intraoral radiography results in the most dental radiographs and the highest radiation dose to the patient. Panoramic radiographs, a tomographic image of all of the teeth and jaw, result in a relatively low dose, with much lower volume of images being produced. Cephalometric images are primarily lateral radiographs of the head used for orthodontic measurement purposes. Cone-beam CT (CBCT) is used usually for treatment planning before placing dental implants.
Dental radiography is a relatively simple procedure. The intraoral X ray tube is a small robust device with a stationary target operating with a tube current of only a few milliamperes (e.g. 7 mA). The X ray generator is typically very simple, often with a fixed tube voltage (e.g. 70 kV) and tube current allowing output changes only by variations in exposure time. The collimation is used to restricts the beam to the region of the mouth being radiographed. If film is used for image reception, the film processing requires diligent attention. Most recently, films have been replaced by digital detectors. For example, digital image capture can be achieved from an intensifying screen that is linked to a charge coupled device camera through a tapered fibre optic coupling. The electronic signal can be transferred to an acquisition computer and processing, storage and viewing.
In panoramic radiography, X ray unit uses the principle of tomography and, more importantly, the principle of panoramic photography. The panorama of the teeth is acquired by a narrow vertical fan beam of X rays as the tube rotates around the back of the head. Simultaneously, the image receptor is moved behind the aperture to capture the image. Panoramic systems also use fixed techniques factors, including fixed exposure time.
Computed tomography imaging has been used for some time in dentistry, including the use of custom designed units for dental applications. However, the use has become more widespread recently with the advent of cone beam technology. There are many CBCT models available, using a variety of acquisition schemes. All models have a flat panel detector for acquisition, typically using digital radiography technology. A CBCT can acquire a full field of view (FOV) that covers the whole head, although acquisitions that are restricted to the mandible are possible. The use of these lower cost CT units opens new potential in some areas of dental diagnosis, although their significantly higher dose compared with panoramic radiography should be noted.
Like in other X ray imaging modalities, the medical physicist has responsibilities for optimization of radiation protection and safety in dental radiography, for image quality and patient dose assessment, and physical aspects of the programme of quality assurance, including medical radiological equipment acceptance and commissioning.
Dual-energy absorptionmetry (DXA)
Dual-energy x-ray absorptiometry (DXA) is an X ray imaging technique primarily used to derive the mass of one material in the presence of another through knowledge of their unique X ray attenuation at different energies.
Dedicated commercial DXA systems first became available in the late 1980s. DXA techniques were originally based on single-photon absorptiometry, in which transmission of a scanning pencil beam, from a radionuclide source, through the patient was measured. The first-generation of modern DXA scanners used a pencil X ray beam; later designs employ fan beams, cone beams and C-arm technology.
The principle of operation for DXA involves two images that are made from the attenuation of a low and a high X ray energy beam, using special imaging equipment comprising special beam filtering and near perfect spatial registration of the two attenuation maps. With this technique the amount of bone, or calcium, usually in the vertebral bodies or hip, but also in other bones is assessed. The DXA unit must be calibrated with a phantom suitable for a particular examination (e.g., spine) and for a particular DXA system type. Universal phantoms that can be used between different types of systems have been developed. However, the calibration of DXA units is an important practical subject that is essential for the viability of DXA usage. The T score is the primary diagnostic value used for osteoporosis and is inversely related to fracture risk. The Z score is used to diagnose low bone mass in young adults and children.
DXA service should be characterized by a good quality control program and standardization of practice, which requires adequate medical physics support.
Magnetic resonance imaging
Since its discovery, magnetic resonance imaging (MRI) has quickly become one of the most important medical imaging devices available to physicians today. Unlike other imaging modalities, such as X ray and computed tomography, MRI does not involve ionizing radiation. MRI also offers superior soft tissue contrast that is not possible with other imaging modalities. Furthermore, the MRI acquisition parameters can be precisely controlled to adjust the image contrast among different tissues. For all these reasons, MRI has become an invaluable tool for the assessment of many types of disease.
MRI involves imaging the nucleus of hydrogen atom, naturally abundant in water and fat, based on their ability to absorb radio frequency (RF) energy when placed in an external magnetic field. The resultant evolving spin polarization can induce a RF signal in a RF coil and thereby be detected. Pulses of radio waves excite the nuclear spin energy transition, and magnetic field gradients localize the polarization in space. By varying the parameters of the pulse sequence, different contrasts may be generated between tissues based on the relaxation properties of the hydrogen atoms in different tissues. The major components of an MRI scanner are the main magnet, which polarizes the sample, the shim coils for correcting inhomogeneities in the main magnetic field, the gradient system which is used to localize the magnetic resonance signal and the radiofrequency system, which excites the sample and detects the resulting magnetic resonance signal. For this reason, most MRI scans essentially map the location of water and fat in the body.
MRI typically produces cross-sectional images but also provides other information such as flow and perfusion, and spectroscopic information.
The primary concern for the medical physicist in MRI is assuring optimum image quality and minimizing the risk to patients and staff due to the high static magnetic field.
Ultrasound
Ultrasound is the most commonly used diagnostic imaging modality, accounting for approximately 25% of all imaging examinations performed worldwide. It is a non-ionizing radiation modality, performed using the relatively low cost and portable ultrasound scanner and able to produce real time images of moving structures, blood flow and soft tissue structures. For these reasons, the ultrasound is used for a wide range of applications, e.g., in cardiac and vascular imaging, imaging of the abdominal organs, imaging of the developing fetus, cancer imaging, musculoskeletal imaging and many other areas.
The term ultrasound refers specifically to acoustic waves at frequencies greater than the maximum frequency audible to humans. Diagnostic imaging is generally performed using ultrasound in the frequency range of 2–15 MHz, which is selected as a trade-off between spatial resolution and penetration depth.
The information contained in an ultrasonic image is influenced by the physical processes underlying propagation, reflection, and attenuation of ultrasound waves in tissue. The ultrasound imaging system has three main components: a transducer, the processing unit with control unit and the display. In the conventional method of ultrasonography, images are acquired in reflection, or pulse echo, mode. The transducer, composed of an array of small piezoelectric elements, transmits a focused pulse along a specified line of sight known as a scan line. Echoes returning from the tissue are received by the same array, focused via the delay-and-sum beam forming process and demodulated to obtain the magnitude, or envelope, of the echo signal. The scanner measures the arrival time of the echoes relative to the time the pulse was transmitted and maps the arrival time to the distance from the array, using an assumed speed of sound. The earliest ultrasound systems would display the result of a single pulse acquisition in 1-D A-mode (amplitude mode) format by plotting echo magnitude as a function of distance. A 2-D or 3-D B-mode (brightness mode) image is acquired by performing a large number of pulse echo acquisitions. The term B-mode imaging reflects the fact that the echo magnitude from each point in the FOV is mapped to the grey level, or brightness, of the corresponding pixel in the image. Modern ultrasound units are capable of producing two-, three-, and even four-dimensional imaging using sophisticated transducer arrays. Modern ultrasound imaging methods include contrast enhanced imaging, tissue harmonic imaging, coded excitation imaging, three and four dimensional imaging and Doppler imaging.
As in other modalities, a medical physicist provides professional support in ultrasound imaging. For example, artefacts are much more prevalent in ultrasound imaging than in other areas. The medical physicist must be able to investigate and identify causes of artefacts and to undertake routine quality control tests of ultrasound imaging systems, including systems used for recording of the images.
Automatic exposure control systems
Automatic exposure control (AEC) systems sense the amount of radiation reaching the image receptor and adjust the dose or dose rate to the patient size to assure good quality images. AEC systems work on different principles, primarily based on the design goals of the manufacturer.
Radiographic systems use automatic exposure control (AEC) devices that automatically adjust radiographic technique factors (most often the X ray tube current time product) to deliver a constant signal intensity at the image receptor in response to differences in patient thickness, X ray tube energy, focus to detector distance and other technical factors. Similarly, in fluoroscopic systems, the AEC controls the incident air kerma rate at image receptors, to prevent fluctuation in image brightness and image contrast. AEC systems may also be known by other names including automatic dose control, automatic dose rate control and automatic brightness control.
AEC systems for CT derive information on the X ray transmission through the patient from the scan projection radiography and, based on this information, the optimal tube current as a function of longitudinal position of the X ray tube relative to the patient is calculated. This is called z axis tube current modulation.
Likewise, the AEC systems are essential parts of mammography X ray units. The AEC systems in digital mammography units differs from that in analogue mammography X ray units, which are design to control the optical density of the film by adjusting mainly X ray tube current and exposure time. The wider dynamic range of digital detectors allows for extra freedom and extends to the choice of technique factors such as tube voltage and target and filter material. Most digital mammography systems use a measurement of the compressed breast thickness (produced by a sensor in the compression mechanism) to select the technique factors (e.g., X ray tube voltage, target, filter) to be employed in the exposure. Furthermore, some digital mammography units use a trial exposure to determine the transmission through the breast. In a further refinement of this approach, some digital mammography systems utilize sophisticated AEC that identify the area of greatest attenuation within a defined area of the detector during this trial exposure. This is then used to select an appropriate X ray tube voltage and filtration, and sufficient exposure to achieve a predetermined pixel value, contrast or detector dose.
An essential part of a good quality control program will be to test performance of AEC systems, e.g., image quality and patient dose over a typical range of patient thicknesses. The medical physics has a leading role in this process.