Control of Control of Scatter RadiationScatter Radiation

Bushong Ch. 14Bushong Ch. 14

ObjectivesObjectives Begin discussing factors that influence Begin discussing factors that influence

image detail or visibility of detailimage detail or visibility of detail Spatial & Contrast resolutionSpatial & Contrast resolution Radiographic NoiseRadiographic Noise Scatter RadiationScatter Radiation Ways to reduce scatter & improve image Ways to reduce scatter & improve image

qualityquality Primary beam restriction & GridsPrimary beam restriction & Grids Technique adjustments when using Technique adjustments when using

gridsgrids

What are some factors that What are some factors that increase scatter radiation?increase scatter radiation?

3 factors contribute to an 3 factors contribute to an increase in scatter increase in scatter

Increased kVpIncreased kVp

Increased x-ray field sizeIncreased x-ray field size

Increased patient thicknessIncreased patient thickness

2 principal characteristics of 2 principal characteristics of any image are Spatial & any image are Spatial &

Contrast ResolutionContrast Resolution Spatial resolution Spatial resolution

Resolution is the ability to image two separate Resolution is the ability to image two separate objects and visually distinguish one from the objects and visually distinguish one from the otherother

Spatial resolution is the ability to image small Spatial resolution is the ability to image small objects that have high subject contrast (eg. objects that have high subject contrast (eg. bone-soft tissue interface, calcified lung bone-soft tissue interface, calcified lung nodules)nodules)

Determined by focal-spot size and other factors Determined by focal-spot size and other factors that contribute to blurthat contribute to blur

Diagnostic x-ray has excellent spatial resolution. Diagnostic x-ray has excellent spatial resolution. It is measured in line pairs per mm. It is measured in line pairs per mm. (CT measured (CT measured in cm)in cm)

SMPTE Test Pattern

Contrast Resolution Contrast Resolution

Determined by scatter radiation Determined by scatter radiation and other sources of radiographic and other sources of radiographic noisenoise

Radiographic noise (image fog) = Radiographic noise (image fog) = A uniform signal produced by A uniform signal produced by

scattered x-rays scattered x-rays Digital imaging the grainy or uneven Digital imaging the grainy or uneven

appearance of an image caused by an appearance of an image caused by an insufficient number of primary x-raysinsufficient number of primary x-rays

MRI CT

Image-forming x-raysImage-forming x-rays

Two kinds of photons are responsible Two kinds of photons are responsible for the OD and contrast on an image: for the OD and contrast on an image: Photons that pass through without Photons that pass through without interacting and those that are interacting and those that are scattered though Compton.scattered though Compton.

X-rays that exit the patient are X-rays that exit the patient are remnant and those that interact with remnant and those that interact with the IR are image-forming. the IR are image-forming.

Ideally, only those x-rays that do Ideally, only those x-rays that do not interact with the patient not interact with the patient

should reach the IR….should reach the IR…. However, scatter radiation is a However, scatter radiation is a

factor that must be managedfactor that must be managed Proper collimation has the PRIMARY Proper collimation has the PRIMARY

effect of reducing patient dose by effect of reducing patient dose by _________ ?_________ ?

Proper collimation also improved Proper collimation also improved image contrast by reducing image contrast by reducing radiographic noise or fog caused by radiographic noise or fog caused by scatterscatter

Fog or Noise

Contrast changes with the use of a grid

Less scatter radiation & less radiographic noise – shorter scale = “better contrast”

With Grid No Grid

kVpkVp

As x-ray energy increases Photoelectric As x-ray energy increases Photoelectric and Compton interactions decrease. and Compton interactions decrease. Explain?Explain?

At 50 kVp 79% photoelectric, 21% At 50 kVp 79% photoelectric, 21% Compton & less than 1% transmissionCompton & less than 1% transmission

At 80 kVp 46% photoelectric, 52% At 80 kVp 46% photoelectric, 52% Compton & 2% transmissionCompton & 2% transmission

Pg. 225Pg. 225

How does increasing kVp How does increasing kVp affect patient dose? affect patient dose?

Patient ThicknessPatient Thickness

Imaging thick parts of the body Imaging thick parts of the body results in more scatter radiation results in more scatter radiation than thin partsthan thin parts

IMAGE TEST TOOL

Is patient thickness Is patient thickness something the radiographer something the radiographer

can control? can control?

Patient thicknessPatient thickness

Normally, NoNormally, No

Compression devices improves Compression devices improves spatial resolution by reducing spatial resolution by reducing patient thickness and bringing the patient thickness and bringing the object closer to the IR. Compression object closer to the IR. Compression also reduces patient dose and also reduces patient dose and contrast resolution contrast resolution

CompressionCompression

Improves spatial Improves spatial resolutionresolution

Reduces OIDReduces OID Reduces patient Reduces patient

dose dose Improves contrast Improves contrast

resolution resolution (reducing fog or (reducing fog or noise)noise)

CompressionCompression

Field SizeField Size

As field size increases, intensity of As field size increases, intensity of scatter radiation also increases scatter radiation also increases rapidly. Especially during rapidly. Especially during fluoroscopyfluoroscopy

Compare images: What do you think about radiographic contrast & image noise?

Control of Scatter Control of Scatter RadiationRadiation

Technologists routinely use two Technologists routinely use two types of devices to reduce the types of devices to reduce the amount of scatter radiation reaching amount of scatter radiation reaching the IRthe IR

Beam restrictorsBeam restrictors

GridsGrids

3 Types of beam-restricting 3 Types of beam-restricting devicesdevices

Aperture Aperture DiaphragmDiaphragm

Cones or Cones or

Cylinders Cylinders

Variable aperture Variable aperture collimatorcollimator

Aperture DiaphragmAperture Diaphragm

The simplest of all beam-restricting The simplest of all beam-restricting devicesdevices

Lead or lead-lined metal diaphragm Lead or lead-lined metal diaphragm attached to the x-ray tube headattached to the x-ray tube head

The opening in the diaphragm is The opening in the diaphragm is usually designed to cover just less usually designed to cover just less than the IR usedthan the IR used

DiaphragmDiaphragm

Fixed lead opening Fixed lead opening

Fixed image Fixed image receptor size receptor size

Constant SIDConstant SID

Source-to-Source-to-diaphragm diaphragm distance = SDDdistance = SDD

Cones & CylindersCones & Cylinders

Are modifications of the aperture Are modifications of the aperture diaphragmdiaphragm

AlignmentAlignmentis one difficultyis one difficultywhen usingwhen usingconesconesNow mostly Now mostly used with spinesused with spinesteeth & headsteeth & heads

Improved contrast resolution of the frontal sinuses

Variable Aperture Variable Aperture CollimatorCollimator

The most common beam-restricting The most common beam-restricting device is the light-localizing variable device is the light-localizing variable aperture collimatoraperture collimator

The first part of the collimator The first part of the collimator serves to control off-focus radiation. serves to control off-focus radiation. What is off-focus radiation?What is off-focus radiation?

Off - focus RadiationOff - focus Radiation

X-ray tubes are designed so that the X-ray tubes are designed so that the projectile e- interacts with the projectile e- interacts with the target. However, some of the e- target. However, some of the e- bounce off the target and land on bounce off the target and land on other areasother areas

This caused x-rays to be produced This caused x-rays to be produced out side the focal spotout side the focal spot

Extrafocal RadiationExtrafocal Radiation

These rays can also be called off-These rays can also be called off-focus radiationfocus radiation

Extrafocal radiation is undesirable Extrafocal radiation is undesirable because it extends the size of the because it extends the size of the focal spot, increases patient skin focal spot, increases patient skin dose & reduces image contrastdose & reduces image contrast

Off-focus radiationOff-focus radiation

Fixed diaphragm in the Fixed diaphragm in the tube housingtube housing

Using a grid Using a grid

does not reducedoes not reduce

extrafocal extrafocal

radiationradiation

First-stage entrance First-stage entrance shuttering deviceshuttering device

Has multiple Has multiple collimator collimator blades blades protruding from protruding from the top of the the top of the collimator into collimator into the tube housingthe tube housing

The second-stage collimator The second-stage collimator shuttersshutters

Pb leaves are at Pb leaves are at least 3 mm thickleast 3 mm thick

They work in They work in pairs and are pairs and are independently independently controlledcontrolled

The collimator lamp & mirrorThe collimator lamp & mirror

Must be adjusted Must be adjusted so that the so that the projected light projected light field coincides with field coincides with the x-ray beamthe x-ray beam

Misalignment of Misalignment of the light field and the light field and beam can result in beam can result in collimator cutoff of collimator cutoff of anatomic anatomic structuresstructures

• Always keep the collimated area smaller than the size of the cassette

• What is a PBL

Total FiltrationTotal Filtration Filtration review…Filtration review… Total Filtration = _______________ + Total Filtration = _______________ +

_____________?_____________?

The collimator assembly is usually The collimator assembly is usually equivalent to approximately _______ mm Al equivalent to approximately _______ mm Al filtration.filtration.

Minimum filtration for tubes that can Minimum filtration for tubes that can operate about 70 kVp is _______ mm Al or operate about 70 kVp is _______ mm Al or equiv.equiv.

The GridThe Grid

Contrast & Contrast Contrast & Contrast ResolutionResolution

Two devices are used to reduce Two devices are used to reduce Compton effect beam-restricting Compton effect beam-restricting devices and radiographic gridsdevices and radiographic grids

Beam-restricting devices effects Beam-restricting devices effects what reaches the patient. Grids what reaches the patient. Grids effect the remnant beam effect the remnant beam

Contrast & Contrast Contrast & Contrast ResolutionResolution

Contrast = the comparison of areas Contrast = the comparison of areas of light, dark and shades of gray on of light, dark and shades of gray on the imagethe image

Contrast Resolution = the ability to Contrast Resolution = the ability to image adjacent similar tissuesimage adjacent similar tissues

Beam-restricting devicesBeam-restricting devices

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

Table pg. 237

Effects of Scatter Radiation Effects of Scatter Radiation on Image Contrast on Image Contrast

Contrast is the degree of difference Contrast is the degree of difference in OD between areas of an imagein OD between areas of an image

If you could only capture If you could only capture transmitted, unscattered x-rays, the transmitted, unscattered x-rays, the image would be very sharpimage would be very sharp

The corresponding bone-soft tissue The corresponding bone-soft tissue interface, would be very abrupt, and interface, would be very abrupt, and therefore the image contrast would therefore the image contrast would be highbe high

GridsGrids

Are very effective device for reducing Are very effective device for reducing scatter radiationscatter radiation

The grid is a series of sections of The grid is a series of sections of radiopaque material (grid strips) radiopaque material (grid strips) alternating with sections of radiolucent alternating with sections of radiolucent material (interspace material)material (interspace material)

The grid is designed to transmit only x-The grid is designed to transmit only x-rays that are traveling in a straight line rays that are traveling in a straight line from the source to the IRfrom the source to the IR

Grids “clean up” scatter Grids “clean up” scatter radiationradiation

A high quality A high quality

grid can attenuate grid can attenuate 80 –80 –

90 percent of scatter90 percent of scatter

radiationradiation

Grid StripsGrid Strips

Should be very thin and have high Should be very thin and have high photon absorption propertiesphoton absorption properties

Lead is most commonLead is most common

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

Interspace MaterialInterspace Material

Used to maintain precise separation Used to maintain precise separation between the delicate lead stripsbetween the delicate lead strips

Aluminum or Plastic FiberAluminum or Plastic Fiber

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

Grid RatioGrid Ratio

3 important dimensions on a grid: The 3 important dimensions on a grid: The thickness of the grid strips, the width of thickness of the grid strips, the width of the interspace material, and the height the interspace material, and the height of the gridof the grid

The grid ratio is the The grid ratio is the HEIGHTHEIGHT of the grid of the grid divided by the divided by the INTERSPACE WIDTHINTERSPACE WIDTH::

Grid ratio = Grid ratio = h h

DD

h = height of the grid, T = h = height of the grid, T = thickness of the grid strip, D = thickness of the grid strip, D =

width of the interspace materialwidth of the interspace material

Grid RatioGrid Ratio

High-ratio grids are more effective in High-ratio grids are more effective in cleaning up scatter radiation than low-ratio cleaning up scatter radiation than low-ratio gridsgrids

The angle of deviation is smaller for high-The angle of deviation is smaller for high-ratio grids. (the photon must be traveling in ratio grids. (the photon must be traveling in a straighter line to make it through the grid)a straighter line to make it through the grid)

However, the higher the ratio the more However, the higher the ratio the more radiation exposure necessary to get a radiation exposure necessary to get a sufficient number of x-rays through the grid sufficient number of x-rays through the grid to the IRto the IR

The higher the ratio the The higher the ratio the straighter the photon must straighter the photon must

travel to reach the IRtravel to reach the IR Grid ratios Grid ratios

rangerange

from 5:1 to 16:1from 5:1 to 16:1 Most commonMost common

8:1 to 10:18:1 to 10:1 A 5:1 grid will A 5:1 grid will

clean up 85% clean up 85%

16:1 clean up 97%16:1 clean up 97%

Grid FrequencyGrid Frequency

The number of grid strips or grid The number of grid strips or grid lines per inch or centimeter lines per inch or centimeter

The higher the frequency the more The higher the frequency the more strips and less interspace material strips and less interspace material and the higher the grid ratioand the higher the grid ratio

As grid frequency increases, patient As grid frequency increases, patient does is increase because more does is increase because more scatter will be absorbedscatter will be absorbed

Grid FrequencyGrid Frequency

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

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

Higher frequency with the same Higher frequency with the same interspace distance reduces the interspace distance reduces the

grid effectivenessgrid effectiveness

Grid PerformanceGrid Performance

The principal function of a grid is to The principal function of a grid is to improve image contrastimprove image contrast

Contrast Improvement FactorContrast Improvement Factor (k)(k) = the = the ratio of the contrast of a radiograph ratio of the contrast of a radiograph made with a grid to the contrast of the made with a grid to the contrast of the radiograph made without a grid. A radiograph made without a grid. A contrast improvement factor of 1 contrast improvement factor of 1 indicates no improvements indicates no improvements

The higher the grid ratio & frequency The higher the grid ratio & frequency the higher the kthe higher the k

Bucky FactorBucky Factor

Using grids require more patient Using grids require more patient dose. Why is this?dose. Why is this?

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

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

Bucky Factor or grid Bucky Factor or grid factorfactor

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

The Bucky factor increases with The Bucky factor increases with increasing kVpincreasing kVp

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

Selectivity or ability to Selectivity or ability to “clean up”“clean up”

the heavier the grid the more Pb it the heavier the grid the more Pb it containscontains

Grid Types Grid Types

Parallel Grid – simplest type of gridParallel Grid – simplest type of grid All the lead strips are parallelAll the lead strips are parallel Only clean up scatter in one Only clean up scatter in one

direction (along the axis of the grid)direction (along the axis of the grid)

Easy to make, however can cause Easy to make, however can cause grid cutoff with short SID’s. grid cutoff with short SID’s.

Grid cutoffGrid cutoff Distance to cutoffDistance to cutoff SIDSID Grid ratioGrid ratio With decreasing With decreasing SID more potential SID more potential for grid cutofffor grid cutoff IR size will alsoIR size will alsoInfluence grid Influence grid

cutoffcutoff

Grid Cutoff – Parallel grid

THE HIGHER-RATIO THE MORE CUTOFF POTENTIAL

Crossed GridCrossed Grid

Have lead strips running along the Have lead strips running along the long and short axes of the gridlong and short axes of the grid

Made by placing two parallel grid on Made by placing two parallel grid on top of each othertop of each other

Crossed GridCrossed Grid

Have twice the grid ratio as linear Have twice the grid ratio as linear

gridsgrids

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

Focused GridFocused Grid

Designed to minimize grid cutoffDesigned to minimize grid cutoff Lead strips are aligned with the Lead strips are aligned with the

divergence of the x-ray beamdivergence of the x-ray beam

Each focused grid must be identified Each focused grid must be identified with the appropriate SIDwith the appropriate SID

Wrong SID = Grid cutoffWrong SID = Grid cutoff

Focused grid have a little SID Focused grid have a little SID latitude (eg. 100cm grid could be latitude (eg. 100cm grid could be

used at 90cm – 110cm)used at 90cm – 110cm)

Moving GridsMoving Grids

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

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

Focused grids are usually Focused grids are usually used as moving gridsused as moving grids

The grid is placed in a holding The grid is placed in a holding mechanism that begins moving just mechanism that begins moving just before the x-ray exposure and before the x-ray exposure and continues moving after the exposure continues moving after the exposure endsends

2 types of movement Reciprocating 2 types of movement Reciprocating & Oscillating& Oscillating

Grid MotionGrid Motion

Reciprocating = moves several times Reciprocating = moves several times about 2cm back and forth during the about 2cm back and forth during the exposureexposure

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

Most grids are moving. Except for Most grids are moving. Except for portable imaging portable imaging

Grid ProblemsGrid Problems

Increased OID, especially with Increased OID, especially with moving gridsmoving grids

The biggest problem with grids is The biggest problem with grids is misalignmentmisalignment

GRID PROBLEMS RESULT IN:UNDEREXPOSED IMAGEOR UNDEREXPOSEDEDGES OF IMAGE

Grid Problems – Off LevelGrid Problems – Off Level

Grid Problems – Off Grid Problems – Off CenterCenter

A problem with focused & crossed gridsA problem with focused & crossed grids

Grid Problems – Grid Problems – Off FocusOff Focus (wrong SID)(wrong SID)

Grid Problems – Grid Problems – Upside-Upside-Down Down

A problem with focused & crossed gridsA problem with focused & crossed grids

Grid SelectionGrid Selection

Patient Dose Patient Dose Pg 241 – mAs changesPg 241 – mAs changes

ExamExam Detail requiredDetail required Part thicknessPart thickness Desired technique (kVp)Desired technique (kVp) Equipment availabilityEquipment availability

Questions….?

Questions …?

• Technique adjustments problems