physics mi diploma mi 10708-3

8/14/2019 Physics MI Diploma MI 10708-3

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Physics of

Medical ImagingDiploma of Medical Imaging

1/2007/2008

Siti Zurina Mat Noor


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Prime Exposure Factors

X-ray Panel (control board)

KVp

mA

Exposure time

Distance

Generator

X-ray circuit


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Objectives

Be able to list the four prime exposure factors

Be able to discuss mAs and kVp in relation to

x-ray beam quantity and quality

Be able to describe characteristics of the

imaging system that affect x-ray beam qualityand quantity


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Exposure Factors

Proper exposure of a patient to x-radiation is

necessary to produce a diagnostic radiograph


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Exposure Factors

(control)

The emission of x-ray photons from an x-ray tube iscontrolled by a number of factors

These factors are

Milliamperage (mA) Exposure time (s) Kilovoltage (kVp)

SID (distance (d))

These factors and filtration control the quantityand/or quality of x-ray emission

mAs


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Exposure Factors

The x-ray beam can be described in terms of

both its quality and its quantity

mAs and kVp are most important factorsresponsible for x-ray quantity and quality

Focal spot size, distance and filtration are

secondary factors


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X-ray quality & quantity

X-ray quantity is a measure of the number of x-rayphotons in the useful beam (x-ray output/intensity)

Measured in mR or mR/mAs

X-ray quality is a measurement of the penetrating

ability of the x-ray beam (x-ray energy/penetrability)

Represented/measured by the half-value layer (HVL)


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X-ray Image Formation


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kVp

kVp is the primary control of beam quality (penetrability) The ability of x-ray beam to penetrate tissue

The higher-quality x-ray beam is one with higher energy andthus is more likely to penetrate the anatomic part of interest

kVp Controls radiographic contrast

Increasing kVp, more x-rays are emitted that have higherenergy and greater penetrability

Produce more scatter radiation resulting Increase image noise

Reduce image contrast


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Body Penetration


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X-ray Beam Quality


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X-ray Image Contrast


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mA

mA determines the number of x-raysproduced and therefore the radiation quantity

Unit Ampere = 1 C/s = 6.3 x 1018 electrons/s

More electron flow through the x-ray tube,

more x-ray are produced

An increase in mA will increase the radiation doseto the patient


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X-ray Image Contrast


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X-ray Beam Penetration


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Exposure Time

Exposure times are usually kept as short as possible Not to minimize patient radiation dose but to minimize

motion blur

When exposure time is reduced, the mA must beincreased

Three-phase and high-frequency generators cannormally provide exposure times as short as 1 ms


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Exposure Time

Image blur


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Blur


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mAs

mA and exposure time are usually combined andused as one factor (mAs)

mAs controls

Radiation quantity Doubling of the mAs doubles the x-ray quantity Optical density

OD is constant for any combination of mA and exposure time thatresults same mAs

Patient dose

mAs is not influence radiation quality


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Distance

SID determines the intensity of the x-ray beam at theimage receptor and has no effect on radiation quality

Standard SID for tabletop radiography is 100cm

whereas chest examination is usually conducted at

180cm

The use of longer SID results in

less magnification,

less focal-spot blur, and

improve spatial resolution but more mAs


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Distance


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Distance and mAs

mAs (Second exposure) / mAs (first exposure)= SID2 (Second exposure) / SID2 (first

exposure)


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Exposure Factors

(Fixed)

All factors are under the control of theradiologic technologist except those fixed by

the design of the x-ray imaging system

Focal spot size

Added x-ray beam filtration is fixed

High voltage generator provides characteristic

voltage ripple that cannot be changed

Fil i d


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Filtration and

voltage ripple


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Focal spot


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Focal-spot size

Most x-ray tubes have small and large focal-spot 0.6mm/1.2mm, 0.5mm/1.0mm or 1.0mm/2.0mm

0.3mm/1.0mm for magnification radiography

Large focal spot is used for normal radiography

Produce many x-ray Provide shorter exposure time (minimize blur)

Use for thick or dense body part

Small focal spot is used in most mammography tube to checkmicrocalcification at short distance (0.1mm/0.3mm)

Fine-detail radiography Magnification radiography

Use for extremities radiography


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Focal spot


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Filtration

Inherent filtration(total 2.5mm aluminum - no control)

Properties of glass or metal envelope

Approximately 0.5 mm aluminum equivalent

The variable-aperture light-localizing collimator provides

additional 1.0mm aluminum equivalent

Mirror of collimator

Additional filter of 1.0mm of aluminum between x-ray tubehousing and collimator


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Filtration


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Filtration

Added filtration Higher filtration is used during examination of tissue with

high subject contrast such as extremities, joints and chest

Lower patient dose

As added filtration increases, x-ray beam quality and penetrability is increased

More scattered radiation

Image noise increased

Reduced image contrast


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High-voltage generation

The radiation quantity and quality produced inx-ray tube are influenced by the type of high-

voltage generator

The three type of high-voltage generators are

Single phase

Three phase

High frequency


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Tube kVp


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Transformer


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Waveform


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Voltage ripple


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Single phase

Half-wave rectification

It has 100% voltage ripple

During exposure, x-ray are produced and emitted only half

the time Radiation quantity is halved compare to full-wave

rectification

During each negative half-cycle, no x-ray are emitted

Mobile and dental x-ray are half-wave rectified


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Single phase

Full-wave rectification

The voltage waveform is identical to that half-waverectification except there is no dead time

During exposure, x-ray are continually emitted inpulses

The radiation quantity is doubled compare to half-wave rectification

The required exposure time is half that for half-wave rectification


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Three phase

6 or 12 pulse

The difference between two forms cause a detectiblechange in x-ray quantity and quality

It is more efficient than single-phase power More x-ray produced per mAs

The average energy of x-ray is higher

Higher x-ray quantity and quality The radiation emitted is nearly constant rather than pulsed


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High frequency

Is used with low-power x-ray systems

The voltage waveform is nearly constant withless than 1% ripple

Higher x-ray quantity and quality

Is used with mammography and mobile x-ray


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High-voltage generation

70/24112/3300


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Summery

Radiographic exposure factors mAs, kVp and distance

The exposure factors influence

Quantity (number of x-rays) Quality (penetrability of x-rays)

Proper selection of these factors optimize

Spatial resolution Contrast resolution

S


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Summery

IncreaseDecreaseFiltrationDecreaseDecreaseVoltage ripple

No changeDecreaseDistance

No changeIncreasemAsNo changeIncreaseExposure time

No changeIncreasemA

IncreaseIncreasekVpQualityQuantityFactor increased

Ch kV


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Change kVp

Ch kV


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Change kVp

Test


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est

your understanding

What is the electron flow from cathode toanode when the 500-mA station is selected?

At 200 mA the entrance skin exposure (ESE)is 752 mR (7.5 mGy). What will be the ESE at

500 mA if no change in exposure time?

A radiographic technique calls for 600 mA at200 ms. What is the mAs?

Test


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your understanding

A radiograph of the abdomen requires 300 mA and500 ms. The patient is unable to hold his breath,

which results in motion blur. A second exposure is

made with an exposure time of 200 ms. Calculate

the new mA that is required?

An examination requires 100 mAs at 180cm SID. If

the distance is changed to 90cm SID, what should

be the new mAs?


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Thank You

physics mi diploma mi 10708-3

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