Ch 11 & 13

Question 1

Explain relationships between mA, exposure time, mAs, and x-ray emission

Answer 1

mA is a measurement of x-ray tube current or the number of electrons crossing the tube from cathode to anode per second. It is directly proportional to tube current. Exposure time is also directly proportional to the number of electrons crossing the tube and is therefore directly proportional to the number of x-rays created. The number of x-rays created at the target is the product of tube current and exposure time and is described in the unit mAs. mAs is the primary controller of x-ray quantity which is directly proportional to mAs

Question 2

The density on an x-ray film should remain the same from different mA and time selections (intensity and duration) provided the mAs totals remain unchanged

Answer 2

Reciprocity law

Question 3

Calculate mAs when given mA and exposure time

Answer 3

Multiply mA by exposure time (mAs = mA x s)

Question 4

Calculate mA when given mAs and exposure time

Answer 4

Divide mAs by exposure time (mA = mAs/s)

Question 5

Calculate exposure time when given mAs and mA

Answer 5

Divide mAs by mA (s = mAs/mA)

Question 6

Explain relationship between kVp and x-ray emission

Answer 6

kVp controls beam quality : energy and penetrability. Increasing kV on the x-ray control panel causes and increase in the speed and energy of the electrons applied across the x-ray tube. X-ray quantity is approximately directly proportional to the square of the ratio of the change in kVp meaning that as kVp is doubled the amount of x-ray photons increases approximately four times

Question 7

An increase in kVp by 15% will cause a doubling in exposure (the same effect as doubling the mA or exposure time)

Answer 7

15% rule

Question 8

Calculate new kVp value needed to maintain density/image receptor exposure when changes made in mAs using 15% rule

Answer 8

Decrease kVp by 15% and double mAs

Increase kVp by 15% and halve mAs

Question 9

Explain relationship between distance and x-ray emission

Answer 9

As distance (SID) increases beam intensity decreases and vice versa

Question 10

The intensity of radiation at a given distance from the point source is inversely related to the square of the distance

Answer 10

Inverse square law

Question 11

Calculate x-ray emission (mR) when distance changed

Answer 11

I1/I2 = D2^2/D1^2

Question 12

Calculate mAs needed to maintain density/image receptor exposure when changes made in distance using exposure (density) maintenance formula

Answer 12

mAs1/mAs2=D1^2/D2^2

Question 13

Explain relationship of entrance skin exposure to other measurement points

Answer 13

The maximum exposure to the body is calculated at minimum SOD at the skin entrance. It is better to overestimate the exposure

Question 14

Calculate mR/mAs from calibration exposure total

Answer 14

Apply the inverse square law and derive mR1 from mR/mAs chart and the SOD from SID and object to receptor distance (OID)

Question 15

Describe typical entrance skin exposures for common radiographic procedures

Answer 15

Diagnostic radiography is the greatest source of ionizing radiation exposure for the public. Radiography of the lumbar spine, pelvis and hips has the highest ESEs

Question 16

Discuss methods of reducing patient dose through effective communication

Answer 16

Radiographers must appear confident to gain a patient’s confidence which results in more cooperation