Radiologic Imaging

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1

Scatter reduces radiographic quality by...

Adding unwanted density to the film without adding any patient information ; it is the radioographers job to minimize the amount of Scatter hitting the Image receptor.

2

Beam restriction devices and grids can be used to....

Limit Scatter hitting the IR

3

Beam restriction devices...

Decrease the filed size and the amount of tissue being irritated, reducing the Scatter produced.

Grids absorb Scatter radiation produced by the patient before it hits the IR.

Does not get rid of all of it.

Low kVp produces less scatter

4

žThe two factors that affect the amount of scatter produced are

kVp and the volume of tissue irradiated.

žUsing a high kVp increases the chance for Compton interactions

žThe volume of the part depends on the thickness and the field size; a larger volume creates more scattered photons

5

As kv increases scatter _________

As part thickness decreases scatter ________

decreases, (but increases over PE) photoelectric

decreases

Low kv is white

high kv is grey

6

žBeam restriction is used to limit scatter radiation produced and to limit patient exposure

žThe terms beam restriction and collimation are used interchangeably

žIncreasing collimation means decreasing field size; decreasing collimation means increasing field size

7

žAs beam restriction increases, field size decreases, patient dose decreases, scatter decreases and image quality increases.

žIncreasing collimation decreases the amount of photons hitting the IR, reducing quantity

žmAs may need to be increased with increasing collimation (decreasing field size)

žSignificant collimation may require a 30% to 50% increases in mAs

8

Aperture Diaphragms are Beam restricting devices

  • žA flat piece of Lead that has a hole in it
  • žSlide onto the tube just below the window

ž

9

Cones and cylinders

  • žAn aperture that has an extended flange attached to it
  • žSlide onto the tube just below the window
  • žProduce a circular projected field

E.G. Skull work/nasal

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Collimator

  • žTwo to three sets of lead shutters located immediately below the tube window
  • žAnother set of shutters are located 3-7” below the tube with longitudinal and lateral blades with their own control
  • žA projected field size will be rectangular or square and should NEVER exceed the size of the IR
11

PBL (Positive Beam Limitation Device)

  • žAn automatic collimator or a positive beam-limiting device automatically limits the size and shape of the beam to the size of the IR
  • žPBL does not allow the size of the beam to exceed the size of the IR, yet there is an override button
12

Grids

  • žInvented by Gustave Bucky in 1913
  • žDevice used to reduce the amount of scatter radiation reaching the IR
  • žA grid is placed between the patient and the IR
  • žA grid is only used when the part is 10 cm or thicker, and more than 60 kVp is needed for the exam
13

Grid Construction

  • žGrids contain strips that have a precise height, thickness and space between them
  • žGrids have lead strips, aluminum interspaces with aluminum fronts and backs
  • žGrid construction can be described by grid frequency and grid ratio.
14

ž Grid frequency expresses the number of lead strips per unit length, either cms or inches

ž Grid frequencies range between 25-45 lines/cm (60 to 110 lines/inch)

5% to the image receptor

(2.5 cm =I inch)

15

žGrid ratio is the ratio of the height of the lead strips to the distance between them

žGrid ratio = h/D Height over space in between

žGrid ratios range from 4:1 to 16:1

ž The higher the grid ratio, the more scatter is cleaned up, improving quality

No Grid = 1

5.1 = 2

6.1 = 3

8.1 = 4

12.1 = 5

16.1 = 6

žWhat is the grid ratio when the lead strips are 3.2 mm high and separated by 0.2mm?

= 16 (3.2 divided by 0.2)

16

Grid Pattern

  • žTwo types: linear or cross hatched
  • žA linear is more common because the tube can be angled with a linear grid

ž

  • A cross hatched absorbs more scatter, but causes limitations
17

Grid Focus

  • žTwo types of grid focus: parallel same as (non-focused) and focused
  • žParallel grids have strips that run parallel to each other; primarily used in mobile and fluoroscopy
  • žA focused grid has lines that are angled to match the diverging beam
  • žFocused grid allow more transmitted photons, not considered a good x ray
18

Convergent point

If imaginary lines were drawn from the divergent beam to the grid, these lines with meet at a point

19

Convergent Line

If the points were connected along the length of the grid

20

Focal Distance and Focal range

  • žThe focal distance (grid radius) is the distance between the grid and the convergent point or line
  • žThe focal distance determines the focal range of a focused grid
  • žThe focal range is the recommended range of SID that can be used with a focused grid
21

žA common focal range is 36-42” with a focal distance of 40” and focal range of 60-74” with a focal distance of 72

72 Wall Bucky

40 Table Bucky

22

Types of Grids (3 Types)

  • žStationary grids are either wafer, grid caps or grid cassettes
  • žReciprocating or oscillating grids move slightly to blur the grid lines
  • žMoving grids are termed Potter- bucky diaphragm
  • žThe grid moves slightly during the entire exposure
23

GCF- Grid ConversionFactot (Bucky Factor)

Notes:

Absorbs transmission, not a good thing

Are they good or bad for patients exposure? Bad

increasing Mas increases quantity, more x ray exposure

žGrid conversion chart:

žGrid ratios GCF

  • no grid 1
  • 5:1 2
  • 6:1 3
  • 8:1 4
  • 12:1 5
  • 16:1 6
24

Grid conversion formula

mAs 1 = grid conversion factor 1

mAs2 = grid conversion factor 2

žEx. A quality radiograph used 70@4 without a grid. What new mAs is needed when using a 8:1 grid to maintain quantity?

žGrid cutoff- a decrease in the number of transmitted photons that reach the IR because of grid misalignment, resulting in reduced density

ž

žThe higher the grid ratio, the more potential cutoff

25

Grid errors

žUpside-down focused- occurs when a focused grid is placed upside down

žThe grid lines go the opposite the angle of divergence

žAppears radiographically

as a loss of density along

the edges of the image

26

In a Portable situation or grid Pan

Loss of Quantity

Loss of Density on the whole film

  • žOff-level- occurs when the beam is angled across the lead strips
  • žmost common
  • žfrom tube or grid being angled
  • žAppears radiographically as a loss of density across the entire film
  • žCan happen with a parallel or focused grid
27

žOff-center (lateral decentering)

Density on the entire film

  • occurs when the CR is nor placed in the center of the grid
  • žAppears radiographically as an overall loss of density
28

Off Focus

  • žOff-focus- occurs when using a SID outside the recommended focal range
  • žAppears radiographically as a loss of density at the periphery of the film
29

A Typical Grid

Mid ratio of 8.1 - 12.1 improves the film quality without exposing the patient to more radiation

  • žIs linear instead of crossed
  • žIs focused instead of parallel
  • žIs mid-ratio (8:1 to 12:1)
  • žHas a focal range that includes a SID of 40 or 72 inches
30

Air Gap Technique

  • žCan be used to reduce scatter on the film if a grid is not available
  • žOID is increased to reduce the number of scattered photons hitting the IR, requiring and increase in mAs
  • žA large OID will reduce sharpness, so SID is increased to compensate

OID - Object to Image distance - patient

SID - Source to Image distance - x ray tube