Physics Mi Diploma Mi 10708-3

Physics of Medical Imaging Diploma of Medical Imaging 1/2007/2008 Siti Zurina Mat Noor

Prime Exposure Factors • X-ray Panel (control board) – KVp – mA – Exposure time

• Distance • Generator • X-ray circuit

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 quality and quantity

Exposure Factors • Proper exposure of a patient to x-radiation is necessary to produce a diagnostic radiograph

Exposure Factors (control) • The emission of x-ray photons from an x-ray tube is controlled by a number of factors • These factors are mAs

– Milliamperage (mA) – Exposure time (s) – Kilovoltage (kVp) – SID (distance (d))

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

Exposure Factors • The x-ray beam can be described in terms of both its quality and its quantity – mAs and kVp are most important factors responsible for x-ray quantity and quality – Focal spot size, distance and filtration are secondary factors

X-ray quality & quantity • X-ray quantity is a measure of the number of x-ray photons 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)

X-ray Image Formation

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 and thus is more likely to penetrate the anatomic part of interest – kVp Controls radiographic contrast

• Increasing kVp, more x-rays are emitted that have higher energy and greater penetrability – Produce more scatter radiation resulting – Increase image noise – Reduce image contrast

Body Penetration

X-ray Beam Quality

X-ray Image Contrast

mA • mA determines the number of x-rays produced 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 dose to the patient

X-ray Image Contrast

X-ray Beam Penetration

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 be increased • Three-phase and high-frequency generators can normally provide exposure times as short as 1 ms

Exposure Time • Image blur

Blur

mAs • mA and exposure time are usually combined and used 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 that results same mAs

– Patient dose

• mAs is not influence radiation quality

Distance • SID determines the intensity of the x-ray beam at the image 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

Distance

Distance and mAs • mAs (Second exposure) / mAs (first exposure) = SID2 (Second exposure) / SID2 (first exposure)

Exposure Factors (Fixed) • All factors are under the control of the radiologic 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

Filtration and voltage ripple

Focal spot

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 check microcalcification at short distance (0.1mm/0.3mm) – Fine-detail radiography – Magnification radiography – Use for extremities radiography

Focal spot

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 tube housing and collimator

Filtration

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

High-voltage generation • The radiation quantity and quality produced in x-ray tube are influenced by the type of highvoltage generator • The three type of high-voltage generators are – Single phase – Three phase – High frequency

Tube kVp

Transformer

Waveform

Voltage ripple

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

Single phase • Full-wave rectification – The voltage waveform is identical to that half-wave rectification except there is no dead time – During exposure, x-ray are continually emitted in pulses – The radiation quantity is doubled compare to halfwave rectification – The required exposure time is half that for halfwave rectification

Three phase • 6 or 12 pulse • The difference between two forms cause a detectible change 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

High frequency • Is used with low-power x-ray systems • The voltage waveform is nearly constant with less than 1% ripple • Higher x-ray quantity and quality • Is used with mammography and mobile x-ray

High-voltage generation Generator type

Ripple Relative Equavilent technique (%) quantity (kVp/mAs) Chest Abdomen

Half-wave

100

100

120/20

74/40

Full-wave

100

200

120/20

74/40

260

115/6

72/34

280

115/4

72/30

300

112/3

70/24

Three-phase, 6 14 pulse Three-phase, 12 4 pulse High frequency <1

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

Summery Factor increased

Quantity

Quality

kVp

Increase

Increase

mA

Increase

No change

Exposure time

Increase

No change

mAs

Increase

No change

Distance

Decrease

No change

Voltage ripple

Decrease

Decrease

Filtration

Decrease

Increase

Change kVp

Change kVp

Test your understanding • What is the electron flow from cathode to anode 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 at 200 ms. What is the mAs?

Test your understanding • A radiograph of the abdomen requires 300 mA and 500 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?

Thank You