principles ni

Type a

X-rays interact with the patient and

are scattered away from the image

receptor.

Type b

X- rays that interact with the patient

and are absorbed.

Type c

X- rays that arrive at the image

receptor without patient interaction.

Type d

X- rays that is scattered by the

patient reaching the image receptor.

type c and d

are called Image forming X-rays

remnant x-rays

x- rays that exit from the patient are called

Image-Forming X-rays

x rays that exit and interact with the image receptor are called

kVp

• As x-ray energy is increased, the absolute number of

Compton interactions decreases , but the number of

Photoelectric interactions decreases much more rapidly .

• Therefore, the relative number of x-rays that undergo

Compton scattering increases.

1%

approximately __ of x-rays incident on the patient reach the image receptor.

directly related

kVp and scatter, inverse or direct?

scatter

high kVp =

acceptable scatter and px dose

optimal kVp =

high patient dose (alara)

low kVp =

5%

• will result in a noticeable (but small) change in Image Receptor (IR) Exposure

• has the same effect on image receptor as a 30% change in mAs .

• This rule is used to make small exposure adjustments.

30% change in mAs

5% change in kVp =

reciprocity law

• Optical Density should remain the same for a given

mAs , regardless of the combination of mA and time

used.

direct

IR exposure = mas
direct or inverse?

underexposure

• Radiograph: too light or low OD 

• Rationale: too little x-rays reaches the image receptor

• Can cause quantum mottle

overexposure

✓ Radiograph: too dark or High OD 

✓ Rationale: too much x-rays reaches the image receptor

high contrast

• few shades of gray

• increase contrast

• short-scale contrast

• short (narrow) dynamic range

• narrow window width

low contrast

• many shades of gray

• decreased contrast

• long-scale contrast

• large (wide) dynamic range

• wide window width

inverse

collimation = field size
direct or inverse?

direct

field size = scatter
direct or inverse?

inverse

collimation = scatter
direct or inverse?

direct

collimation = contrast

increased collimation

• patient dose decreases

• scatter radiation decrease

• radiographic contrast increases

• exposure to image receptor decreases

increased field size

• patient dose increases

• scatter radiation increases

• radiographic contrast decreases

• exposure to image receptor increases

larger body parts

have more tissue to interact with the photons, resulting in greater scatter production .

direct

thickness = scatter
direct or inverse?

compression of anatomy

improves spatial resolution and contrast resolution and lowers the patient radiation dose.

by using a compression device

• thickness decreases

• oid decreases

• spatial resolution increases

• patient dose decreases

• contrast resolution increases

contrast

the degree of difference in OD between areas of a radiographic image

contrast resolution

the ability to image and distinguish soft tissues

reduced image contrast

results from scattered x-rays

Aperture Diaphragm

» The simplest of all beam -restricting devices .

» It is basically a lead or lead lined metal diaphragm that is attached to the x -ray tube head.

» The opening in the diaphragm usually is designed to cover just less than the size of the image receptor used.
»The main disadvantage of the diaphragm is that is does not provide a sharp demarcation of

the edge of the x -ray beam

»With a diaphragm, Off - focus is not controlled, and there is a large area of penumbra at the edge of the exposed film area.

Cones & Cylinders

» Radiographic extension cones and cylinders are considered modifications of the aperture

diaphragm

» The useful beam produced by an extension cone or cylinder is usually circular .

» The position and size of the distal end act as an aperture and determine field size.

Cone-cutting

It is impossible for the circular cross section

of the x-ray beam to be fitted to a

rectangular film.

aperture diaphragm, cones and cylinders, variable aperture collimator

types of beam restrictors

collimation

reduces the patient radiation dose and improves contrast resolution

variable-aperture collimator

is the most commonly used beam-restricting device in radiography

gustave buckey, 1913

who invented the radiographic grid and continues to be the most effective means for limiting the amount of scatter radiation that reaches the IR

grid

is a device that has very thin lead strips with radiolucent interspaces, intended to absorb scatter radiation emitted from the patient

10 cm (4 inches) or greater in thickness, and more than 60 kvp

grids are typically used only when the anatomic part is ____?

dr. Hollis potter

• from Chicago made the bucky grid practical by moving it during the radiographic exposure, a procedure that blurred the grid line out of the image

• the grid has since been known as the Potter-Bucky diaphragm, which is also the correct NEMA term for the device

high ratio grids

are more effective in reducing scatter radiation than are low-ratio grids

5;1, 16;1

higher ratio grids are used most often in high-kVp

85%

5;1 grid reduces approximately __ of the scatter radiation

97%

16;1 grid may reduce as much as __

grid frequency

the number of grid strips per centimeter

aluminum or plastic fiber

the interspace material of most grids consists of

nonhydroscopic

does not absorb moisture as plastic fiber does

aluminum

easier to form and roll into sheets of precise thickness

lead

• easy to shape and is relatively inexpensive

• high atomic number and high mass density

• high atomic number and high mass density

grid strip

it should be infinitely thin and should have high absorption properties

bucky factor (B) or grid factor

• when a grid is used, the radiographic technique must be increase to produce the same image Receptor signal

• the amount of this increase is given by the

gustave bucky

• the bucky factor is named for ______, the inventor of the grid

• it is an attempt to measure the penetration of primary and scatter radiation through the grid

air gap technique

• alternative to the use of radiographic grids

• reducing scatter radiation, thereby enhancing image contrast

10-15 cm

when the air-gap technique is used, the image receptor is moved ___ from the patient

linear/parallel grid

• easiest to manufacture, but it has some properties that are clinically undesirable

• LESS COMMONLY EMPLOYED
  - because the strips do not try to coincide with the divergence of the x-ray beam
  - some grid cutoff will occur along the lateral edges, especially when the grid is employed at short SID

grid cutoff

• the undesirable absorption of primary x-rays by the gird

• most common with parallel grids

• most pronounced when
  1. short SID
  2. large-area image receptor

parallel grid

best employed at long SID

• because the beam will be straighter and perpendicular

cross-hatch/ cross grid

• two sets of lead strips superimposed and running and 90* to one another

• this design is used only in stationary grids

• generally, a stationary grid is used for high kV work and no x-ray tube angulation

• beam must be aligned with the center of grid! there should be no angulation

• more efficient than parallel grids in cleaning up scatter radiation

• has a higher contrast improvement factor than a parallel grids of twice the gird ratio

• ex: 6:1 cross grid will clean up more scatter radiation than a 12:1 parallel grid

disadvantages of cross grids

1. positioning the grid critical
   - central ray of the x-ray beam must coincide with the center of the grid

2. tilt-table techniques
   - possible only if the x-ray tube and table are properly aligned

3. exposure technique
   - requires a substantial increase with resulting higher patient radiation dose

grid cutoff

the main disadvantage of parallel and crossed grids

focused grid

• lead strips are inclined inward

• focused on a pre-determined point above the grid (focal range)

• designed to minimize grid cutoff

• coincide with the divergence of the x-ray beam

• match the angle of divergence of the primary beam

non-focused grid

• a parallel line grid

• relationship of lead strips is uniform to one another

focused grid disadvantages

• more difficult to manufacture than parallel grids

• they are characterized by all the properties of parallel grids except when properly positioned, they exhibit no grid cutoff

• should be used carefully for there are geometric limitations (SID)

• every focused grid is marked with its intended focal distance and the side of the grid that should face the x-ray tube

grid lines

• an obvious and annoying shortcoming of the grids

• the presence of grid lines can be demonstrated simply by radiographing a grid

• grid lines are the images made when primary x-rays are absorbed within the grid strips. EVEN THOUGH THE GRID STRIPS ARE VERY SMALL, their image is still observable

• high-frew grids present less obvious grid lines compared to low-freq grids

• visibility of grid lines = width of grid strips

moving grids

• placed in a holding mechanism that begins moving just before x-ray exposure and continues moving after the exposure ends.

• 2 basic types of moving grid mechanisms:
  - reciprocating grid
  - oscillating grid

reciprocating grid

is a moving grid that is motor driven back and forth several times during x-ray exposure. the total distance of drive is approximately 2cm

oscillating grid

is positioned within a frame with a 2- to 3-cm tolerance on all sides between the frame and the grid. delicate, springlike devices located in the four corners hold the grid centered within the frame. a powerful electromagnet pulls the grid to one side and releases it at the beginning of the exposure. Thereafter, the grid oscillates in a circular fashion around the grid frame, coming to rest after 20 to 30 seconds.

3 possible motions of moving grids

• single stroke (one way)

• reciprocating (forward & backward)

• catapult

off level

• usually produced with an improperly positioned x-ray tube, not an improperly positioned grid

• occurs when the grid tilts during horizontal beam radiography or during mobile radiography

off center-off axis

• problem usually seen in focused grids

• any lateral shift results in grid cutoff across the entire radiograph, producing lower OD

• this error in positioning is called LATERAL DECENTERING

off- focused grid

• problems in using focused grid is when using SIDs unspecified for that grid or outide its focal range.

• the farther the grid from its focal range, the more severe the grid cutoff

• cutoff is more severe at edges

upside-down grid

• shows a severe cutoff on either side of the central ray

mammography

low-ratio grids are commonly used during?

grid ratios up to 8:1

• grid represents a good compromise between desired levels of scatter radiation reduction and patient radiation dose .

• are satisfactory at tube potential below 90 kVp
  - are used when kVp exceeds 90 kVp

grid selection

» Grids absorb scatter

» Scatter adds exposure to the image receptor

» The more efficient a grid is at absorbing scatter, the less image receptor exposure

» Therefore, compensations must be made to increase this Exposure!

» This compensation is generally accomplished by INCREASING mAs !

» Increasing mAs results to increase in patient dose .

» The better the grid cleans up scatter, the greater will be

the dose to the patient in achieving proper IR exposure!

grid conversion factor (gcf)

the amount of mas needed can be calculated using the:

post-reduction

those taken after manipulation and casting have occured—are commonly performed to confirm alignment of the bone.

fiberglass/plaster mix

increase mAs 50% or 8% kVp

soft tissue technique

decrease kVp 20%, leave mAs on the same on

35% increase in technique

for pooling of blood in head, thorax, and abdomen on postmortem (dead person)

expiration chest technique

post mortem- an

increase 35% in mAs

post-mortem chest x-ray for expiration chests immediately after death

increase 50% in mAs

• is required on post-mortem chest x-ray for expiration chests (if ½ HR or more has elapsed)

• commonly used for board question

conversion factors for cylindrical cones

• extension cylinder collapsed- increase mAs by 40% or 5 kVp

• extension cylinder extended increase 60% mAs or 10 kVp