FUNDAMENTALS OF IMAGING

RAD 206

Control of Scatter Radiation

Contrast changes with the use of a gridLess scatter radiation & less radiographic noise – shorter scale = “better contrast”

With Grid No Grid

GRIDS

CONTRAST & CONTRAST RESOLUTIONTwo devices are used to reduce Compton effect beam-restricting devices and radiographic gridsBeam-restricting devices effects what reaches the patient. Grids effect the remnant beam

CONTRAST & CONTRAST RESOLUTIONContrast = the comparison of areas of light, dark and shades of gray on the imageContrast Resolution = the ability to image adjacent similar tissues

BEAM-RESTRICTING DEVICESAre helpful to improve contrast resolution however the inherent problem is they are placed between the source and the patient. Even under the most favorable conditions, most if the remnant x-rays are scattered.

EFFECTS OF SCATTER RADIATION ON IMAGE CONTRAST Contrast is the degree of difference in OD between areas of an imageIf you could only capture transmitted, unscattered x-rays, the image would be very sharpThe corresponding bone-soft tissue interface, would be very abrupt, and therefore the image contrast would be high

GRIDSAre very effective device for reducing scatter radiationThe grid is a series of sections of radiopaque material (grid strips) alternating with sections of radiolucent material (interspace material)The grid is designed to transmit only x-rays that are traveling in a straight line from the source to the IR

GRIDS “CLEAN UP” SCATTER RADIATIONA high quality grid can attenuate 80 –90 percent of scatterradiation

GRID STRIPSShould be very thin and have high photon absorption properties

Lead is most common

Tungsten, platinum, gold, and uranium have been tried but Pb is still most desirable

INTERSPACE MATERIALUsed to maintain precise separation between the delicate lead strips

Aluminum or Plastic Fiber

Grid Casing = covered completely by thin aluminum to provide rigidity and to seal out moisture. Yuck!

GRID RATIO3 important dimensions on a grid: The thickness of the grid strips, the width of the interspace material, and the height of the grid

The grid ratio is the HEIGHT of the grid divided by the INTERSPACE WIDTH:

Grid ratio = h D

H = HEIGHT OF THE GRID, T = THICKNESS OF THE GRID STRIP, D = WIDTH OF THE INTERSPACE MATERIAL

GRID RATIOHigh-ratio grids are more effective in cleaning up scatter radiation than low-ratio gridsThe angle of deviation is smaller for high-ratio grids. (the photon must be traveling in a straighter line to make it through the grid)However, the higher the ratio the more radiation exposure necessary to get a sufficient number of x-rays through the grid to the IR

THE HIGHER THE RATIO THE STRAIGHTER THE PHOTON MUST TRAVEL TO REACH THE IR

Grid ratios rangefrom 5:1 to 16:1Most common8:1 to 10:1A 5:1 grid will clean up 85% 16:1 clean up 97%

GRID FREQUENCYThe number of grid strips or grid lines per inch or centimeter The higher the frequency the more strips and less interspace material and the higher the grid ratioAs grid frequency increases, patient does is increase because more scatter will be absorbed

GRID FREQUENCYSome grids reduce the thickness of the strips to reduce the exposure to the patient, this over all reduces the grid clean up

Grids have frequencies in the range of 25 to 45 lines per centimeter (60 to 110 lines per inch)

HIGHER FREQUENCY WITH THE SAME INTERSPACE DISTANCE REDUCES THE GRID EFFECTIVENESS

GRID PERFORMANCEThe principal function of a grid is to improve image contrastContrast Improvement Factor (k) = the ratio of the contrast of a radiograph made with a grid to the contrast of the radiograph made without a grid. A contrast improvement factor of 1 indicates no improvements

The higher the grid ratio & frequency the higher the k

Using grids require more patient dose. Why is this?

When a grid is used technique must be increased to maintain OD

The amount of increase is given by the Bucky factor (B) or grid factor

BUCKY FACTOR OR GRID FACTOR

BUCKY FACTOR OR GRID FACTOR

The higher the grid ratio or frequency the higher the bucky factor

The Bucky factor increases with increasing kVp

Pg 235: We will use the average values for calculations.

Selectivity or ability to “clean up”the heavier the grid the more Pb it contains

GRID CUTOFFDistance to cutoff SID Grid ratioWith decreasing SID more potential for grid cutoffIR size will alsoInfluence grid cutoff

GRID TYPES

PARALLEL GRID simplest type of gridAll the lead strips are parallelOnly clean up scatter in one direction (along the axis of the grid)

Inexpensive, Easy to make, however can cause noticeable grid cutoff with short SID’s (the greater the SID the lesser artifact seen) .

GRID CUTOFF FROM PARALLEL GRID

THE HIGHER-RATIO THE MORE CUTOFF POTENTIAL

FOCUSED GRIDDesigned to minimize grid cutoffLead strips are aligned with the divergence of the x-ray beam

Each focused grid must be identified with the appropriate SIDWrong SID = Grid cutoff

Focused grid have a little SID latitude or tolerance (e.g. 100cm Focused grid would produce artifacts if used at less than 90cm – or more than 110cm)

CROSSED GRIDAKA crosshatchHave lead strips running along the long and short axes of the gridMade by placing two parallel grid on top of each other

CROSSED GRID

Have twice the grid ratio as Parallel (linear) grids

However, CR vs grid placement is critical. The CR must align with the center of the grid and the grid and CR must be exactly parallel or grid cutoff will occur

e.g.

MOVING GRIDSAll stationary grids will give you grid lines on your radiograph. Thinner Pb strips will give you less noticeable lines. However, thinner strips have less Pb content not “cleaning up” as well

Grid Lines are made when primary x-rays are absorbed in the grid strips.

FOCUSED GRIDS ARE USUALLY USED AS MOVING GRIDSThe grid is placed in a holding mechanism that begins moving just before the x-ray exposure and continues moving after the exposure ends

2 types of movement Reciprocating & Oscillating

Reciprocating: moving back and forth about 2 cm

Oscillating: moving in a circular motion 2-3 cm

GRID MOTIONReciprocating = moves several times about 2cm back and forth during the exposure

Oscillating = moves several times about 2 – 3 cm in a circular pattern

Most grids are moving. Except for portable imaging

GRID PROBLEMSIncreased OID, especially with moving gridsThe biggest problem with grids is misalignment

GRID PROBLEMS RESULT IN:UNDEREXPOSED IMAGEOR UNDEREXPOSEDEDGES OF IMAGE

https://www.youtube.com/watch?v=mZPD_gLs5Dw

GRID PROBLEMS – OFF LEVEL

GRID PROBLEMS – OFF CENTER

A problem with focused & crossed grids

GRID PROBLEMS – OFF FOCUS (WRONG SID)

GRID PROBLEMS – UPSIDE-DOWN

A problem with focused & crossed grids

GRID SELECTIONPatient Dose

• Pg 241 – mAs changesExamDetail requiredPart thicknessDesired technique (kVp)Equipment availability

Contrast Improvement by Using an Air Gap

QUESTIONS….?