Chapter 1
Basic principles of radiologic modalities

Mamdoh AlObaidy, Kiran Reddy Busireddy, and Richard C. Semelka

Department of Radiology, University of North Carolina, Chapel Hill, USA

Introduction

In this chapter, we will describe the features and basic imaging principles of the various modalities employed in radiology. Since many specialties perform these types of studies, "radiology" is often also referred to generically as "imaging." A basic feature of all imaging is that pictures are generated, and the quality of the pictures oftentimes depends on how pathologies stand out compared to normal tissues.

Each of the different modalities uses their own terms to describe pathology, which relate back to how the images themselves are created. In this chapter, brief technical descriptions of each modality will be discussed with special emphasis on image production, image description, factors that influence image quality, and associated imaging artifacts with each modality.

X-ray-based imaging modalities

Plain radiography, mammography, fluoroscopy, and computed tomography (CT) all use X-rays as the source of generating images. All these modalities employ an X-ray tube to generate the images. The controllable factors are tube voltage, measured in kVp; tube current, measured in mA; and total exposure time, measured in seconds.

The X-ray tubes produce X-rays by accelerating electrons to high energies from a filament (cathode) to a tungsten target (anode) by heating the filaments to a very high temperature, which then emits electrons. The flow of electrons from the filament to the target constitutes the tube current (mA). X-rays are produced when energetic electrons strike the target material; electron kinetic energy is transformed into heat and X-rays, which are then filtrated at the X-ray tube window to achieve higher beam quality. The term mAs refers to the product of tube current and time duration.

These X-rays are then directed to the imaged subject (the patient). The number of X-rays produced by the X-ray beam is related to the X-ray beam intensity, measured in terms of air kerma (mGy). X-ray beam intensity (mA) is proportional to the X-ray tube current. X-ray beam intensity is also proportional to the exposure time, which is the total time during which a beam current flows across the X-ray tube. Doubling the tube current, the number of X-rays or the exposure time will double the X-ray beam intensity, but will not affect the average energy of the beam. KVp affects the penetrating power of X-rays and hence tissue contrast.

Image production can be achieved using analog or digital systems. Analog radiography uses films to capture, display, and store radiographic images. Digital systems can be classified as cassette and noncassette systems.

Plain radiography (X-rays)

Image production

X-ray tube voltage varies according to imaged body part. Exposure times range between tens and hundreds of milliseconds.

The typical settings to obtain an erect posteroanterior chest radiograph are a kVp of 100 and mAs of 4. The typical settings to obtain an erect anteroposterior abdominal radiograph are a kVp of 80 and mAs of 40. The typical kVp and mAs settings for imaging the appendicular skeleton are 52-60 and 2.5-8, respectively. Note that there are slight variations between the kVp and mAs for these different regions. This reflects that more current is needed to penetrate regions with more tissue (abdomen compared to chest), and optimal contrast is different to study the disease processes of these different regions as well (abdomen compared to skeleton).

Image descriptors

The most common projections in plain radiography are frontal (anteroposterior or posteroanterior), lateral, oblique, or cross-table, based on the direction of X-ray beam in relation to the patient. Special positions and projections are used in musculoskeletal (MSK) imaging.

Frontal projection images are interpreted as if the patient is sitting in front of the reader; where the left side of the image corresponds to the right side of the patient.

The brightness of a structure on plain radiography is related to its atomic number; structures containing material with higher atomic number absorb more photons before they reach the detector or film. In plain radiography, bright areas are described as radiopaque or radiopacity, and dark areas are described as radiolucent. Metals, bones, some stones, contrast materials, and various pathologies appear as radiopaque. Air/gas appears as radiolucent.

Image performance

X-ray-based imaging modalities including plain radiography, mammography, fluoroscopy, and CT share the same parameters that can influence image quality. Combinations of tube voltage, tube current, and exposure time, and focal spot size govern the final image quality.

Optimization of these parameters to achieve a diagnostic quality image with minimum radiation is the principal goal. Plain radiographic studies generally offer the highest spatial resolution, with the subcategory of mammography having the very highest, followed by CT, magnetic resonance imaging (MRI), and then nuclear medicine.

Mammography

Mammography is an X-ray-based imaging modality that uses low-energy X-rays to image the breasts as a diagnostic and screening tool.

Image production

X-ray tubes in mammography units used molybdenum as a target and a much smaller focal spots. The tube voltage in mammography ranges from 25 to 34 kV. The heel effect, described as higher X-ray intensity on the cathode side, is utilized in mammography to increase the intensity, that is, penetration, of radiation near the chest wall where tissue thickness is relatively greater.

Compression is used in mammography to reduce the breast parenchymal thickness, which achieves immobilization and reduction in radiation dose, thereby decreasing blurring and increasing sharpness.

Digital tomosynthesis mammography is a newer form of mammography that offers high resolution and is performed using limited-angle tomography (multiple projections at different angles) at mammographic dose levels. The acquired data set is reconstructed using iterative algorithms.

Stereotaxic localization is achieved by acquiring two images, each 15° from the normal projection. This technique provides good localization of masses and is used to perform core needle biopsies.

Image descriptors

The two routinely used mammography views are craniocaudal (CC) and mediolateral oblique (MLO). Other additional views include true lateral, exaggerated, axillary, and cleavage views. Compression views can also be acquired in cases of where the presence of a tumor is uncertain and to resolve any possible parenchymal overlap.

Images are usually reviewed in pairs to help assess for any asymmetry. Mammographic findings are usually described using the terminology of Breast Imaging Reporting and Data System (BI-RADS) lexicon, which includes the description of breast parenchyma, masses, calcifications, and distortion, followed by the assignment of a BI-RADS score, which is used for patient management and to determine follow-up intervals.

Fluoroscopy

Fluoroscopy is an X-ray-based imaging technique commonly used to obtain real-time images of the internal structures of a patient through the use of a fluoroscope.

Image production

Fluoroscopy units are composed of X-ray generator, X-ray tube, collimator, filters, patient table, grid, image intensifier, optical coupling, television system, and image recording. Fluoroscopy units operate using low tube currents (1-6 mA) and tube voltages (70-125 kV). When the X-ray beam is switched off, last image hold (LIH) software permits the visualization of the last image. Newer fluoroscopy systems use pulsed fluoroscopy to reduce dose by acquiring frames that are less than real time (quarter to half the number of frames per second).

Fluoroscopy systems use a television camera to view the image output of the image intensifiers by converting light images into electric (video) signals that can be recorded or viewed on a monitor. Fluoroscopy allows real-time observation and imaging of dynamic activities. It has many applications in radiology, including gastrointestinal (GI), genitourinary, cardiovascular, neuromuscular, and MSK procedures. It can be used for diagnostic and interventional procedures, whether in the fluoroscopy, cardiology, endoscopy, and interventional suites as well as in the operating room.

Cineradiography refers to real-time visualization of motion with fluoroscopy, and frame rate varies from very fast (30 frames/s) in vascular studies during injection of contrast injection to slower to observe motility of the GI tract.

Digital subtraction angiography (DSA) is a fluoroscopic technique used for imaging the vascular system following intravascular contrast injection. In this technique, subtracting the acquired noncontrast mask image, from subsequent frames following contrast administration, allows the removal of static nonenhancing vascular structures that augments visualization of even the smallest contrast differences. This permits using a much lower intravenous (IV) contrast...