Projectional radiography


Projectional radiography, also known as conventional radiography, is a form of radiography and medical imaging that produces two-dimensional images by x-ray radiation. The image acquisition is generally performed by radiographers, and the images are often examined by radiologists. Both the procedure and any resultant images are often simply called "X-ray". Plain radiography generally refers to projectional radiography. Plain radiography can also refer to radiography without a radiocontrast agent or radiography that generates single static images, as contrasted to fluoroscopy, which are technically also projectional.

Equipment

X-ray generator

Projectional radiographs generally use X-rays created by X-ray generators, which generate X-rays from X-ray tubes.

Grid

A Bucky-Potter grid may be placed between the patient and the detector to reduce the quantity of scattered x-rays that reach the detector. This improves the contrast resolution of the image, but also increases radiation exposure for the patient.

Detector

Detectors can be divided into two major categories: imaging detectors and dose measurement devices.

Shielding

is the main material used by radiography personnel for shielding against scattered X-rays.

Image properties

Projectional radiography relies on the characteristics of x-ray radiation 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.

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 will be more heavily exposed, and therefore will be processed as being darker. Conversely, areas on the image receptor that receive the least radiation 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 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 of the x-ray beam and the tissue composition of the body part being radiographed. Selection of look-up tables in digital imaging also affects contrast.
Generally speaking, high contrast is necessary for body parts in which bony anatomy is of clinical interest. When soft tissue is of interest, lower contrast is preferable in order to accurately demonstrate all of the soft tissue tones in these areas.

Geometric magnification

Geometric magnification results from the detector being farther away from the X-ray source than the object. In this regard, the source-detector distance or SDD is a measurement of the distance between the generator and the detector. Alternative names are source/focus to detector/image-receptor/film distance.
The estimated radiographic magnification factor is the ratio of the source-detector distance over the source-object distance. The size of the object is given as:
where Sizeprojection is the size of the projection that the object forms on the detector. On lumbar and chest radiographs, it is anticipated that ERMF is between 1.05 and 1.40. Because of the uncertainty of the true size of objects seen on projectional radiography, their sizes are often compared to other structures within the body, such as dimensions of the vertebrae, or empirically by clinical experience.
The source-detector distance is roughly related to the source-object distance and the object-detector distance by the equation SOD + ODD = SDD.

Geometric unsharpness

Geometric unsharpness is caused by the X-ray generator not creating X-rays from a single point but rather from an area, as can be measured as the focal spot size. Geometric unsharpness increases proportionally to the focal spot size, as well as the estimated radiographic magnification factor.

Geometric distortion

Organs will have different relative distances to the detector depending on which direction the X-rays come from. For example, chest radiographs are preferably taken with X-rays coming from behind. However, in case the patient cannot stand, the radiograph often needs to be taken with the patient lying in a supine position with the X-rays coming from above, and geometric magnification will then cause for example the heart to appear larger than it actually is because it is further away from the detector.

Scatter

In addition to using a Bucky-Potter grid, increasing the ODD alone can improve image contrast by decreasing the amount of scattered radiation that reaches the receptor. However, this needs to be weighted against increased geometric unsharpness if the SDD is not also proportionally increased.

Imaging variations by target tissue

Projection radiography uses X-rays in different amounts and strengths depending on what body part is being imaged:
NOTE: The simplified word 'view' is often used to describe a radiographic projection.
Plain radiography generally refers to projectional radiography. Plain radiography can also refer to radiography without a radiocontrast agent or radiography that generates single static images, as contrasted to fluoroscopy.

Breasts

Projectional radiography of the breasts is called mammography. 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 on the film. The radiation used for mammography tends to be softer than that used for the harder tissues. Often a tube with a molybdenum anode is used with about 30 000 volts, 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.

Chest

Chest radiographs are used to diagnose many conditions involving the chest wall, including its bones, and also structures contained within the thoracic cavity including the lungs, heart, and great vessels. Conditions commonly identified by chest radiography include pneumonia, pneumothorax, interstitial lung disease, heart failure, bone fracture and hiatal hernia. Typically an erect postero-anterior projection is the preferred projection. Chest radiographs are also used to screen for job-related lung disease in industries such as mining where workers are exposed to dust.
For some conditions of the chest, radiography is good for screening but poor for diagnosis. When a condition is suspected based on chest radiography, additional imaging of the chest can be obtained to definitively diagnose the condition or to provide evidence in favor of the diagnosis suggested by initial chest radiography. Unless a fractured rib is suspected of being displaced, and therefore likely to cause damage to the lungs and other tissue structures, x-ray of the chest is not necessary as it will not alter patient management.

Abdomen

In children, abdominal radiography is indicated in the acute setting in suspected bowel obstruction, gastrointestinal perforation, foreign body in the alimentary tract, suspected abdominal mass and intussusception. Yet, CT scan is the best alternative for diagnosing intra-abdominal injury in children. For acute abdominal pain in adults, an abdominal x-ray has a low sensitivity and accuracy in general. Computed tomography provides an overall better surgical strategy planning, and possibly less unnecessary laparotomies. Abdominal x-ray is therefore not recommended for adults presenting in the emergency department with acute abdominal pain.
The standard abdominal X-ray protocol is usually a single anteroposterior projection in supine position. A Kidneys, Ureters, and Bladder projection is an anteroposterior abdominal projection that covers the levels of the urinary system, but does not necessarily include the diaphragm.

Axial skeleton

Head

In case of trauma, the standard UK protocol is to have a CT scan of the skull instead of projectional radiography. A skeletal survey including the skull can be indicated in for example multiple myeloma.

Other axial skeleton

These include:
;AP-projection 40° posterior oblique after Grashey
The body has to be rotated about 30 to 45 degrees towards the shoulder to be imaged, and the standing or sitting patient lets the arm hang. This method reveals the joint gap and the vertical alignment towards the socket.
;Transaxillary projection
The arm should be abducted 80 to 100 degrees. This method reveals:
;Y-projection
The lateral contour of the shoulder should be positioned in front of the film in a way that the longitudinal axis of the scapula continues parallel to the path of the rays. This method reveals:
This projection has a low tolerance for errors and accordingly needs proper execution. The Y-projection can be traced back to Wijnblath’s 1933 published cavitas-en-face projection.
In the UK, the standard projections of the shoulder are AP and Lateral Scapula or Axillary Projection.

Extremities

A projectional radiograph of an extremity confers an effective dose of approximately 0.001 mSv, comparable to a background radiation equivalent time of 3 hours.
The standard projection protocols in the UK are:
Certain suspected conditions require specific projections. For example, skeletal signs of rickets are seen predominantly at sites of rapid growth, including the proximal humerus, distal radius, distal femur and both the proximal and the distal tibia. Therefore, a skeletal survey for rickets can be accomplished with anteroposterior radiographs of the knees, wrists, and ankles.

General disease mimics

Disease mimics are visual artifacts, normal anatomic structures or harmless variants that may simulate diseases or abnormalities. In projectional radiography, general disease mimics include jewelry, clothes and skin folds.