Projectional radiography

Imaging principles

Projectional radiography relies on the characteristics of x-ray radiation (quantity and quality of the beam) and knowledge of how it interacts with human tissue to create diagnostic images. X-rays are a form of ionizing radiation, meaning it has sufficient energy to potentially remove electrons from an atom, thus giving it a charge and making it an ion. Ionizing radiation has sufficient energy to penetrate human tissue.

X-ray attenuation

When an exposure is made, x-ray radiation exits the tube as what is known as the primary beam. When the primary beam passes through the body, some of the radiation is absorbed in a process known as attenuation. Anatomy that is denser has a higher rate of attenuation than anatomy that is less dense, so bone will absorb more x-rays than soft tissue. What remains of the primary beam after attenuation is known as the remnant beam. The remnant beam is responsible for exposing the image receptor. Areas on the image receptor that receive the most radiation (portions of the remnant beam experiencing the least attenuation) will be more heavily exposed, and therefore will be processed as being darker. Conversely, areas on the image receptor that receive the least radiation (portions of the remnant beam experience the most attenuation) will be less exposed and will be processed as being lighter. This is why bone, which is very dense, process as being ‘white’ on radio graphs, and the lungs, which contain mostly air and is the least dense, shows up as ‘black’.

Density

Radiographic density is the measure of overall darkening of the image. Density is a logarithmic unit that describes the ratio between light hitting the film and light being transmitted through the film. A higher radiographic density represents more opaque areas of the film, and lower density more transparent areas of the film.

With digital imaging, however, density may be referred to as brightness. The brightness of the radiograph in digital imaging is determined by computer software and the monitor on which the image is being viewed.

Contrast

Contrast is defined as the difference in radiographic density between adjacent portions of the image. The range between black and white on the final radiograph. High contrast, or short-scale contrast, means there is little gray on the radiograph, and there are fewer gray shades between black and white. Low contrast, or long-scale contrast, means there is much gray on the radiograph, and there are many gray shades between black and white.

Closely related to radiographic contrast is the concept of exposure latitude. Exposure latitude is the range of exposures over which the recording medium (image receptor) will respond with a diagnostically useful density; in other words, this is the "flexibility" or "leeway" that a radiographer has when setting his/her exposure factors. Images having a short-scale of contrast will have narrow exposure latitude. Images having long-scale contrast will have a wide exposure latitude; that is, the radiographer will be able to utilize a broader range of technical factors to produce a diagnostic-quality image.

Contrast is determined by the kilovoltage (kV; energy/quality/penetrability) of the x-ray beam and the tissue composition of the body part being radiographed. Selection of look-up tables (LUT) in digital imaging also affects contrast.

Generally speaking, high contrast is necessary for body parts in which bony anatomy is of clinical interest (extremities, bony thorax, etc). When soft tissue is of interest (ex. abdomen or chest), lower contrast is preferable in order to accurately demonstrate all of the soft tissue tones in these areas.

Variations by target organs

Projection radiography uses X-rays in different amounts and strengths depending on what body part is being imaged:

Hard tissues such as bone require a relatively high energy photon source, and typically a tungsten anode is used with a high voltage (50-150 kVp) on a 3-phase or high-frequency machine to generate bremsstrahlung or braking radiation. Bony tissue and metals are denser than the surrounding tissue, and thus by absorbing more of the X-ray photons they prevent the film from getting exposed as much. Wherever dense tissue absorbs or stops the X-rays, the resulting X-ray film is unexposed, and appears translucent blue, whereas the black parts of the film represent lower-density tissues such as fat, skin, and internal organs, which could not stop the X-rays. This is usually used to see bony fractures, foreign objects (such as ingested coins), and used for finding bony pathology such as osteoarthritis, infection (osteomyelitis), cancer (osteosarcoma), as well as growth studies (leg length, achondroplasia, scoliosis, etc.).

Soft tissues are seen with the same machine as for hard tissues, but a "softer" or less-penetrating X-ray beam is used. Tissues commonly imaged include the lungs and heart shadow in a chest X-ray, the air pattern of the bowel in abdominal X-rays, the soft tissues of the neck, the orbits by a skull X-ray before an MRI to check for radiopaque foreign bodies (especially metal), and of course the soft tissue shadows in X-rays of bony injuries are looked at by the radiologist for signs of hidden trauma (for example, the famous "fat pad" sign on a fractured elbow).

Dental radiography uses a small radiation dose with high penetration to view teeth, which are relatively dense. A dentist may examine a painful tooth and gum using X-ray equipment. The machines used are typically single-phase pulsating DC, the oldest and simplest sort. Dental technicians or the dentist may run these machines; radiographers are not required by law to be present.

Mammography is an X-ray examination of breasts and other soft tissues. This has been used mostly on women to screen for breast cancer, but is also used to view male breasts, and used in conjunction with a radiologist or a surgeon to localise suspicious tissues before a biopsy or a lumpectomy. Breast implants designed to enlarge the breasts reduce the viewing ability of mammography, and require more time for imaging as more views need to be taken. This is because the material used in the implant is very dense compared to breast tissue, and looks white (clear) on the film. The radiation used for mammography tends to be softer (has a lower photon energy) than that used for the harder tissues. Often a tube with a molybdenum anode is used with about 30 000 volts (30 kV), giving a range of X-ray energies of about 15-30 keV. Many of these photons are "characteristic radiation" of a specific energy determined by the atomic structure of the target material (Mo-K radiation).

Projectional radiography terminology

NOTE: The word 'view' is often used erroneously to describe a radiographic projection.

AP - Antero-Posterior

PA - Postero-Anterior

DP - Dorsal-Plantar

Lateral - Projection taken with the central ray perpendicular to the midsagittal plane

Oblique - Projection taken with the central ray at an angle to any of the body planes. Described by the angle of obliquity and the portion of the body the X-ray beam exits; right or left and posterior or anterior. For example a 45 degree Right Anterior Oblique of the Cervical Spine.

Flexion - Joint is radiographed while in flexion

Extension - Joint is radiographed while in extension

Stress Views - Typically taken of joints held in a 'stressed' position. Test of stability.

HBL, HRL, HCR or CTL - Horizontal Beam Lateral, Horizontal Ray Lateral, Horizontal Central Ray, or Cross Table Lateral. Used to obtain a lateral projection usually when patients are unable to move.

Prone - Patient lies on their front

Supine - Patient lies on the back

Decubitus - Patient lying down. Further described by the downside body surface: dorsal (backside down), ventral (frontside down), or lateral (left or right side down).

OM - occipito-mental, an imaginary positioning line extending from the menti (chin) to the occiput (particularly the external occiputal protuberance)

Cranial or Cephalad - Tube angulation towards the head

Caudal - Tube angulation towards the feet

Equipment Used in Projectional Radiography

Ceiling or Floor Mounted X-ray tube

Height adjustable table

Bucky or Grid

User Interface

Image Receptor - Film / Screen Cassette or CR Plate / DR Detectors

Processor or Image Reader

Chest Stand

Chest - Erect PA Only. Lateral on request by a Radiologist

Sternum - PA chest and lateral sternum

Abdomen - Supine AP Only. Decubitus on special request

Kidney, Ureter, Bladder (KUB) - AP Only.

Cervical Spine - AP and Lateral. Peg projection with trauma only. Obliques and Flexion and Extension on special request

Thoracic Spine - AP and Lateral

Lumbar Spine - AP and Lateral +/- L5/S1 view. Obliques and Flexion and Extension requests are rare

Pelvis - AP only. SIJ projections (prone) on special request

Hip - AP and Lateral

Skull - None for trauma, patient goes to CT. Only on request for skeletal suvery in cases for example like multiple myeloma

Sinus - OM with open mouth

Facial Bones - OM and OM 30°

Shoulder - AP and Lateral Scapula or Axillary Projection. Other Special projections available on request

Clavicle - AP and AP Cranial

Humerus - AP and Lateral

Elbow - AP and Lateral. Radial head projections available on request

Radius and Ulna - AP and Lateral

Wrist - DP and Lateral

Scaphoid - DP with Ulna deviation, Lateral, Oblique and DP with 30° angulation

Hand - DP and Oblique

Fingers - DP and Lateral

Thumb - AP and Lateral

Femur - AP and Lateral

Knee - AP and Lateral. Intra Condular projections on request

Patella - Skyline Projection

Tibia and Fibula - AP and Lateral

Ankle - AP/Mortice and Lateral

Calcaneum - Axial and Lateral

Foot / Toes - AP and Oblique

Routine projections used in the US

Chest - (CXR) Includes a PA and Lateral with the patient standing or sitting up. Special projections include an AP in cases where the image needs to be obtained stat and with a portable device, particularly when a patient cannot be safely positioned upright. Lateral Decubitus may be used for visualization of air-fluid levels if an upright image cannot be obtained. AP Axial Lordotic projects the clavicles above the lung fields, allowing better visualization of the apices (which is extremely useful when looking for evidence of primary tuberculosis)

Abdomen - Usually a single AP supine (KUB—kidney, bladder, and ureter) projection. Special projections include a PA prone, Lateral Decubitus, upright AP, and Lateral Cross-Table (with the patient supine) A minimal acute obstructive series (for the purpose of ruling out small bowel obstruction) would include two views: typically, a supine view and an upright view (which would be sufficient to detect air-fluid levels), although a lateral decubitus could be substituted for the upright.

Cervical Spine - Five or six projections are common; a Lateral, two 45 degree obliques, an AP axial (Cephalad), an AP "Open Mouth" for C1-C2, and Cervicothoracic Lateral (Swimmer's) to better visualize C7-T1 if necessary. Special projections include a Lateral with Flexion and Extension of the cervical spine, an Axial for C1-C2 (Fuchs or Judd method), and an AP Axial (Caudad) for articular pillars.

Thoracic Spine - An AP and Lateral are basic projections. Obliques 20 degrees from lateral may be ordered to better visualize the zygapophysial joint

Lumbar Spine - Basic projections include an AP, two Obliques, a Lateral, and a Lateral L5-S1 spot to better visualize the L5-S1 interspace. Special projections are AP Right and Left bending, and Laterals with Flexion and Extension.

Sacrum and Coccyx - If both bones are to be examined separate cephalad and caudad AP axial projections are obtained for the sacrum and coccyx respectively as well as a single Lateral of both bones.

Sternum - The two basic projections are a 15 to 20 degree Right Anterior Oblique and a Lateral.

Sternoclavicular Joints - Are usually ordered as a single PA and a Right and Left 15 degree Right Anterior Obliques.

Ribs - Common rib projections are based on the location of the area of interest. These are obtained with shorter wavelengths/higher frequencies/higher levels of radiation than a standard CXR.

Anterior area of interest - a PA chest X-ray, a PA projection of the ribs, and a 45 degree Anterior Oblique with the non-interest side closest to the image receptor.

Posterior area of interest - a PA chest X-ray, an AP projection of the ribs, and a 45 degree Posterior Oblique with the side of interest closest to the image receptor.