rad 206 p11 fundamentals of imaging - control of scatter radiation

FUNDAMENTALS OF IMAGING

RAD 206

Control of Scatter Radiation

What is focal spot?What is effective focal spot ?Which side of the x-ray tube has better resolution ?Which focal spot size produces better resolution?Which OID produces better resolution?Which Object Thickness produces better resolution?Which Object shape produces better resolution?

OBJECTIVESBegin discussing factors that influence image detail or visibility of detail

• Scatter Radiation, factors affecting it, and there effect on Spatial & Contrast resolution

• Ways to reduce scatter & improve image quality• Primary beam restriction & Grids• Technique adjustments when using grids

WHAT IS CONTRAST

What is scatterWhat is scatter

RELATIONSHIP BETWEEN CONTRAST AND SCATTER

The relationship is INVERSED

IMAGE-FORMING X-RAYSTwo kinds of photons are responsible for the OD and contrast on an image: • Photons that pass through without interacting

and

• Photons that are scattered though Compton.

EFFECT OF SCATTER ON THE IMAGE

Which interaction produces scatter radiation ?

FACTORS CONTRIBUTING TO AN INCREASE IN SCATTER

• Increased kVpkVp (Beam Energy, Compton)

• Increased partpart thickness thickness (e.g. Dabbeh)

• Increased x-ray field sizefield size (opening Collimation to Larger Fields)

Q: what can you do to reduce scatter ?Q: what can you do to reduce scatter ?

1- KVPAt 50 kVp

7979% photoelectric, 21%21% Compton<1% transmission

At 80 kVp 4646% photoelectric, 52% 52% Compton2% transmission

RELATIONSHIP BETWEEN KVP AND CONTRAST

How does increasing kVp affect patient dose?

2- PATIENT THICKNESS

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

IMAGE TEST TOOL

Is patient thickness something the radiographer can control?

IS PATIENT THICKNESS SOMETHING THE IS PATIENT THICKNESS SOMETHING THE RADIOGRAPHER CAN CONTROL? RADIOGRAPHER CAN CONTROL?

Normally, No

But Compression Devices can be used in certain situations which reduces patient thickness and bringing the object closer to the IR, and therefore results in better spatial resolution and contrast resolution and also lesser exposure factors and patient dose

COMPRESSION

• Reduces OID• Improves spatial

resolution• Improves contrast

resolution (reducing fog or noise)

• Reduces patient dose

COMPRESSION

3- FIELD SIZEAs field size increases, intensity of scatter radiation also increases rapidly. Especially during fluoroscopy

Larger radiation field produces more scatter radiation

COMPARE IMAGES: WHAT DO YOU THINK ABOUT RADIOGRAPHIC CONTRAST & IMAGE NOISE?

INCREASE IN SCATTER

• kVpkVp

• PartPart ThicknessThickness

• Field SizeField Size

CONTROL OF SCATTER RADIATION

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

Beam RestrictorsCompression

Grids

Before Patient

On patient

After patient

BEAM-RESTRICTING DEVICESAKA LBD

1. Aperture Diaphragm

1. Cones or Cylinders

2. Variable aperture collimator

APERTURE DIAPHRAGMThe simplest of all beam-restricting devices

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

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

APERTURE DIAPHRAGMFixed lead opening

Fixed image receptor size Constant SID

Source-to-diaphragm distance = SDD

CONES & CYLINDERSAre modifications of the aperture diaphragmAlignmentis one difficultywhen usingconesNow mostly used with spinesteeth & heads

IMPROVED CONTRAST RESOLUTION OF THE L5 - S1

IMPROVED CONTRAST RESOLUTION OF THE FRONTAL SINUSES

VARIABLE APERTURE COLLIMATOR

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

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

OFF - FOCUS RADIATIONALSO CALLED EXTRA-FOCAL RADIATION

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

This caused x-rays to be produced out side the focal spot, they are undesirable because it extends the size of the focal spot, increases patient skin dose & reduces image contrast

Note: Using a grid does not reduce off-focal radiation

FIRST-STAGE ENTRANCE SHUTTERING DEVICEHas multiple collimator blades protruding from the top of the collimator into the tube housing

THE SECOND-STAGE COLLIMATOR SHUTTERSPb leaves are at least 3 mm thickThey work in pairs and are independently controlled

THE COLLIMATOR LAMP & MIRRORMust be adjusted so that the projected light field coincides with the x-ray beamMisalignment of the light field and beam can result in collimator cutoff of anatomic structures

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

What is a PBL BLD

GRIDS

QUESTIONS …?

Technique adjustments problems