Chapter 15 (mAs) - Wk 2

What is mA/milliamperage?

• A measure of the QUANTITY of electrical current flowing through a circuit (applied to a filament)

• mA control on console controls a series of resistors in the filament circuit

  • less resistance = fewer resistors selected = greater flow rate of electrons = more current, more friction, more heat, hotter filament = more electrons ‘boiled’ off of the filament wire (thermionic emission) = higher mA

• Rate representing the number of electrons passing down a wire per second

• Located on tech’s console (we select mA with every exposure)

• How many electrons in cloud determines how many photons for X-Rays

What does the filament determine?

Small vs large filament determines size of focal spot

What happens when electrons get boiled off?

• the Electron cloud flies across the tube to the anode

• Strikes anode and creates X-Rays

• Thermionic Emission

mA controls electrical _________

quantity

mA controls _____ _____ of electrons flowing across tube

intensity rate

What does the quantity of electrons flowing across the X-Ray beam controlled by?

• DIRECTLY controlled by the mA station set at the console

• 2x mA = 2x flow of x-rays emitted in the beam

How to solve for mAs?

mA (milliamperage) x s (seconds)

What does s mean in mAs?

• Seconds

• The amount of time an x-ray is on

What does mAs mean?

• Total amount of electricity used during an exposure (total intensity of the entire x-ray exposure made)

mAs is ____ proportional to patient exposure

directly

Why are mAs considered the primary control for exposure?

• Preferred way to control the x-ray beam exposure because mAs only affects quantity

What is the Law of Reciprocity?

• IR exposure remains unchanged as long as the total mAs remains unchanged

• ma1 x s1 = ma2 x s2

The law of reciprocity is _____ proportional

inversely

• when holding total mAs constant, if we decrease the exposure time, we must increase the mA

What is quantum mottle?

• When the IR does not receive enough X-Ray photons exiting the pt to form a diagnostic radiographic image

• Results in a grainy appearance

• UNDEREXPOSURE

What is quantum mottle most commonly caused by?

Insufficient mAs

• but can be caused by anything leading to insufficient photons reaching the IR

  • insufficient kVp to penetrate the part

  • rectifier failure in the X-ray machine

How do we correct the quantum mottle?

• Reduce Noise

• Increase the Signal (mAs or possibly kVp)

  • increase the mAs… primary control of beam exposure… more electrons/photons available = increases useful signal

  • insufficient kVP can cause mottle… no quantity of electrons can fix insufficient penetration of the part. However, insufficient mAs is usually the culprit of quantum mottle

What do mA’s affect?

• Quantity of beam

• Intensity of the exposure at the IR

  • image brightness

  • image noise/quantum mottle

What do mA’s not affect?

• Penetration of beam (kVp)

• Subject contrast

• Image contrast

• Geometric factors

  • sharpness of recorded detail

  • magnification

  • shape distortion

What is the relationship between exposure time & motion unsharpness?

• Long exposure times are not the direct cause of motion = the patient is

• Exposure times are a contributing factor to motion

  • Can decrease the likelihood of motion through technique factors

  • Shorter exposure times = decrease likelihood of motion

mA in the x-ray tube is the unit of measure describing?

Tube current

Beam intensity is ____ proportional to the mA

Directly

How is the x-ray beam changed when the mA is increased by a factor of 2?

Beam quantity increases by a factor of 2

What would double the quantity of photons in the x-ray beam?

• Double the exposure time, but do not change the mA

• Double the mA, but do not change the exposure time

mA x s = mAs

Receptor exposure is ____ proportional to mA

Directly

Increasing mA results in _____ beam quantity

Increased

To reduce the patient dose by 1/2, what must be done to the mA if all other factors are held constant?

Reduce mA by 1/2, directly proportional

An x-ray is taken using 200 mA at 0.5 seconds and results in 0.10 mGy of patient exposure. Assuming that all of their factors remain constant, which mA settings would result in the patient exposure of 0.2 mGy?

400mA

Proportional

Exposure time is the amount of time that:

Electrons are flowing through the x-ray tube

What is the unit of measure for exposure time?

S Seconds

What is the unit of measure for tube current?

Milliamperage (mA)

What property of the x-ray beam is influenced by changes to the MilliampEreseconds (mAs)?

Intensity

An exposure is made Using 100 mA And 0.05 Seconds. How will the beam intensity change if the exposure is repeated using 200 mA And 0.025 s?

Beam intensity remains the same

An exposure is made using 100 mA And 0.05 seconds. How will the patient dose change at the exposure is repeated using 200 mA And 0.025 s?

Patient dose remains the same

What is an important benefit of using a short exposure time?

Decreased motion blur

The default technique for the lateral projection of the thoracic spine is 400 mA, 0.1 seconds, 80 kg voltage, peak, and 40 inches sourced to image distance. How should the technology modify this technique to blow the ribs and increase the visibility of the spine while maintaining the same receptor exposure?

Increase s, decrease mA

The total mAs is calculated as the product of which exposure factors?

Tube current and Exposure time

An exposure is made using 70 KVP, 30 mA, And a 0.1 S exposure time. Calculate the mAs.

30 mAs

x-ray beam quantity refers to the:

Number of futons in the x-ray beam

What is synonymous with an x-ray beam quantity

Intensity

Which two measurements are commonly used to describe the quantity of radiation in the x-ray beam?

Air KERMA, Exposure