Chapter 13

large focal spot size

- large = 1.0 mm, 1.3 mm -For general imaging, the large focal spot is used. This ensures that sufficient mAs can be used to image thick or dense body parts. The large focal spot also provides for a shorter exposure time, which minimizes motion blur. -more x-rays can be produced with the large focal spot because anode heat capacity is higher.

Inherent filtration

-All x-ray beams are affected by the inherent filtration properties of the glass or metal envelope of the x-ray tube. For general-purpose tubes, the value of inherent filtration is approximately 0.5 mm Al equivalent. -Variable-aperature light-localizing collimator = 1.0 mm 0.5 + 1.0 = 1.5 mm AL equivalent

Compensating filters

-Compensating filters are shapes of aluminum mounted onto a transparent panel that slides in grooves beneath the collimator. -These filters balance the intensity of the x-ray beam so as to deliver a more uniform exposure to the image receptor. -As added filtration is increased, the result is increased x-ray beam quality and penetrability. The result on the image is the same as that for increased kVp, that is, more scatter radiation and reduced image contrast. -shapes of AL mounted onto a transparent panel that slides in grooves beneath the collimator -these filters balance the intensity of the x-ray beam so as to deliver a more uniform exposure to the image receptor -spine = wedge shaped -chest = trough shaped

Direct square law

-Distance has no effect on radiation quality. -The following relationship, called the square law, is derived from the inverse square law. It allows a radiologic technologist to calculate the required change in mAs after a change in SID to maintain constant OD. -this law can be flipped and still receive the same result -Distance (SID) affects OD. -The use of a longer SID results in less magnification, less focal spot blur, and improved spatial resolution. However, more mAs must be used because of the effects of the direct square law. -overall density decreases/increases by a factor of 4

Composition

-Measurement of the thickness of the anatomical part does not release the radiologic technologist from exercising some additional judgment when selecting a proper radiographic technique. -The radiologic technologist must estimate the mass density of the anatomical part and the range of mass densities involved. -when only soft tissue is being imaged, low kVp and high mAs are used. With an extremity,which consists of soft tissue and bone, low kVp is used because the body part is thin. -The chest has high subject contrast; the abdomen has low subject contrast.

Focal Spot Size

-On the operating console, identified as small and large, 0.5 mm/1.0 mm, 0.6 mm/1.2 mm, or 1.0 mm/2.0 m -These are called microfocus tubes and are designed for imaging very small microcalcifications at short SIDs. -Changing the focal spot for a given kVp/mAs setting does not change the x-ray quantity or quality.

Body Habitus

-The patient's size, shape, and physical condition greatly influence the required radiographic technique. -Sthenic—meaning "strong, active"—patients are average patients. -Hyposthenic patients are thin but healthy appearing; these patients require less radiographic technique. -Hypersthenic patients are big in frame and usually overweight. -Asthenic patients are small, frail, sometimes emaciated, and often elderly. -Radiographic technique charts are based on sthenic patients.

Added filtration

-The radiologic technologist has no control over these sources of filtration but may control stages of added filtration. -Some x-ray imaging systems have selectable added filtration -Usually, the imaging system is placed into service with the lowest allowable added filtration. -When added filtration is changed, be sure to return it to its normal position before beginning the next examination. -the manufacture adds 1 mm AL filter inserted between the x-ray tube housing and the collimator -this meets the required total filtration of 2.5 mm AL equivalent (inherent) 1.5 + (Added) 1.0 = 2.5 mm AL equivalent

Thickness

-The thicker the patient, the more x-radiation is required to penetrate the patient to expose the image receptor. -Patient thickness should not be guessed. -Depending on the type of radiographic technique practiced, the mAs setting or the kVp will be altered as a function of the thickness of the part.

small focal spot size

-small = 0.5 mm, 0.6 mm -Mammography x-ray tubes have 0.1 mm/0.3 mm focal spots. -small focal spot, electron interaction occurs over a smaller area of the anode, and the resulting heat limits the capacity of x-ray production. -A small focal spot is reserved for fine-detail radiography, in which the quantity of x-rays is relatively low. -Small focal spots are always used for magnification radiography. These are normally used during extremity radiography and in examination of other thin body parts in which higher x-ray quantity is not necessary.

Filtration

-three types of x-ray filtration 1. inherent 2. added 3. compensating =required 2.5 mm AL equivalent

Patient Factors

Radiographic techniques may be described by identifying three groups of factors. The first group includes patient factors, such as anatomical thickness and body composition. The second group consists of image-quality factors, such as OD, contrast, detail, and distortion. Also of importance is how these image-quality factors are influenced by the patient. The final group includes the exposure technique factors, such as kVp, milliamperage, exposure time, and SID, as well as grids, screens, focal-spot size, and filtration. These factors determine the basic characteristics of radiation exposure of the image receptor and patient radiation dose, and they provide the radiologic technologist with a specific and orderly means of producing, evaluating, and comparing radiographs. An understanding of each of these factors is essential for the production of high-quality images.