Quality Control

regulatory agencies

department of health and human services

national center for devices and radiological health

food and drug administration

occupational safety and health administration

environmental protection agency

nuclear regulatory commission

2 impactful government actions

safe medical devices act (SMDA)

mammography quality standards act (MQSA)

ACR

american college of radiologists

AAPM

american association of physicists in medicine

DICOM sets standards for

images

HL-7 sets standards for

transmission of written data

goal of quality control

to calibrate and monitor technical equipment to maintain image quality and ensure radiation exposure as low as reasonably achievable

follow standards set by ACR and AAPM

ACR recommendations for digital QC

must document policies and procedures for monitoring and evaluating equipment

QC program should be designed to maximize quality and accessibility of diagnostic information

components of QC

acceptance testing

diagnosis of deviation from acceptable equipment tolerances

documentation of corrections made

acceptance testing

completed upon installation

confirms equipment is functioning according to specifications set by the vendor

establishes a baseline for performance

people involved in acceptance testing

service engineer & bio med

IS and PACS administrator

medical physicist

manager

performance monitoring

evaluates performance of generator and tube

simple to complex equipment can be used

ACR requires medical physicist to perform tests

most testing is done annually

what is used for monitoring equipment performance

ion chamber dosimeters

• gas filled chambers

newer solid-state dosimeters

• small and accurate

• measures the intensity of xr beam accurately

FDA mandates on the operating console

control booth walls

• min of 7.5 sq ft with walls 7 inches high

exposure switch

• must be fixed and a minimum of 30 inches from booth wall

control booth window

• 1.5 mm of lead equivalent

no exposure with the door open

xr must scatter 2 times before entering the control booth

tube support mechanism

strong and counterbalanced with immobilization locks

metal housing

leakage less than 100 mR/hour at 1 meter

max tabletop thickness

1 mm of Al equivalent

high voltage generators

convers low V to kV

step up transformers for kV

step down for tube filament

rectifiers convert incoming alternating V to pulsating direct current

requires less space and less power

ripple for high frequency generators

less than 1%

timer accuracy

if timer is off, mAs is off

tested annually

tested with spinning top or motorized synchronous top

back up timer

should terminate exposure after 6 seconds or 600 mA

what equipment is tested with a spinning top

single phase equipment

what equipment is tested with a motorized synchronous top

3 phase equipment

spinning top test

circular steel/lead disc with a small hole on the periphery

can indicate a problem with a rectifier or the timer

rectifier issue with spinning top test

you see ½ of the expected dots or arc angle

timer issue with spinning top test

results are off by a few dots

single phase rectified equipment with spinning top test

produces dots

1 second exposure should produce 120 dots

3 phase rectified equipment with spinning top test

produces an arc

1 second exposure should produce an image with 360 degree arc

how to calculate expectation of dots/arc for spinning top test at >1 second

multiply the seconds by the expected dots/arc for 1 sec

EX: 0.5 sec x 120 dots = 6 dots should be seen on properly working equipment

line focus principle

when target angle is less than 45 degrees, the effective focal spot is smaller than the actual focal spot

focal spot size determines

resolution

smaller FSS = better resolution

blooming

FS size increases as the tube ages and with high mA usage

causes electron cloud to swell

unwanted change in actual focal spot size during a change in exposure

how often is FSS tested

annually

tools for FSS testing

pinhole camera

star or slit camera

• line pair resolution tool

pinhole camera

CR is aligned through metal sheet with 0.03 and 0.075 mm holes

image of focal spot is displayed using a long exposure

the size of the effective focal spot is measure using a micrometer

blooming change with tube current

higher tube current = more blooming

which is preferred between slit and star cameras

slit cameras

slit camera

metal template with series of slits in groups of 3

• crosswise and lengthwise

what do slit and star cameras measure

resolution and estimated focal spot size

line pair resolution tool is used in conjunction with

slit and star cameras

line pair resolution tool measures

spatial resolution is measured in lp/mm and corresponds to a certain focal spot size

NEMA specifications for FS variations

FS smaller than 0.8mm may be 50% larger variations

FS middle 0.8mm - 1.5mm may be 40% larger variations

FS larger than 1.5mm may be 30% larger variations

what is kV and mA accuracy

stability of the relationship between kV or mA and the resulting exposure intensity

kV accuracy must be within

± 4 kVp OR ± 5% of the kV output

EX: if you expect 80 kVp and get exposure with 86 kVp is it allowed?

• 6 kVp is > the 4 allowed, so no

• 6/80 = 0.075 7.5% which is higher than 5%, so no

how is kV accuracy measured

old

• WI test cassette

new

• digital kVp meter

• more accurate and easier

linearity

sequential increases in mA should produce the same increase in the exposure rate

linearity requirement

exposures must not vary more than ± 10%

reproducibility

output intensity must be consistent from one exposure to the next

same technique should produce the same exposure

reproducibility requirement

must be within ± 5%

gassy tube

tube with fluctuations in mA readings

half-value layer is an expression of

beam quality

filtration of the primary beam

filter absorbs low energy xrs

• improves image quality

• reduces pt dose

• increases average beam energy

expressed in Al/Eq

how is filtration measured

computerized dosimeter provides a HVL readout

NCRP requirement for filtration

for equipment operating above 70 kVp, minimum of 2.5 mm Al/Eq total filtration is required

collimation

beam restrictors limit the size of the XR field

improves image quality

• less scatter

• improved contrast

reduces pt dose

should never be larger than the IR

collimation congruency

measures how well a collimator regulates

NCRP requirement for collimation congruency

accurate within ± 2% of the SID

tested annually

how is collimation congruency tested

8 or 9 penny test tool

if variance is smaller than the penny it is acceptable

central ray accuracy is measured with

beam alignment tool

• plastic cylinder with metal washers or BBs

central ray accuracy requirement

must be accurate within ± 1% of the SID

• 40 inch SID = 0.4 inch variance

• 72 inch SID = 0.72 inch variance

tested annually

distance accuracy is measured by

measuring devices and detents are checked with a tape measure

tube angles are checked with a protractor

distortion is checked with

wire mesh test

wire mesh test

mesh is placed on cassette

dark areas indicate poor contrast

can show pincushioning in fluoro

pincushion distortion

warped image appearance from curved surface of input screen to flat surface of output screen

exposure requirements in fluoro

must not exceed 10 mR/min at tabletop

entrance dose must not exceed 5R/min

how are exposure requirements tested in fluoro

physicists measures output with a calibrated ion chamber

artifacts in fluoro

veiling glare

vignetting

distortion

• pincushion

• s-distortion

required thickness for primary barrier of the housing in fluoro

2 mm Pb/Eq

required thickness for bucky slot cover in fluoro

0.25 mm Pb/Eq

required thickness for fluoro curtain

0.25 mm Pb/Eq

timer requirement for fluoro

5 minutes

sourced to table distance requirement for stationary fluoro units

15 inches

sourced to table distance requirement for mobile fluoro units

12 inches

field uniformity checks for

checks the image visually for defects

ghosting

inadequate CR erasure

• loss of lamp intensity or erasure time too short

residual charge is trapped in DEL of digital IR

off-focus/STEM radiation

examples of CR quality control tests

dark noise

uniformity

spatial accuracy

erasure throroughness

dark noise is AKA

intrinsic noise

dark noise test

scans an unexposed plate

should come out as a uniform lighter gray color

uniformity test

expose a plate and run it through the scanner

should come up as a uniform dark image

spatial accuracy test

wire mesh is placed on the CR and exposed and ran through the scanner

should have no distortion

erasure thoroughness test

expose a CR with a step wedge, run through scanner and erase, run through scanner again

after the second time through the CR shouls be blank

AAPM proposed standard for exposure index

to use deviation index numbers

what is the weakest link in the imaging chain

the monitor

ACR and FDA classify monitors as

primary monitors

secondary monitors

primary display monitor

diagnostic monitor

used by the radiologist for interpretations minimum of 2K (2048×2560) required

• minimum of 4K for mammography

secondary monitors

technologist review stations

minimum of 1K (1024×1280) required

not considered diagnostic quality

types of viewing monitors

cathode ray tube

flat panel

• LCD or Plasma

how often must viewing monitors be cleaned

monthly

cathode ray tube monitors

resolution deteriorates over time (blooming)

higher reflectance

better luminance

lower resolution

• pincushion, barrel, and skew distortions

• veiling glare cause by scatter of the phosphor layer

flat pannel liquid crystal display (LCD)

layer of liquid crystals sandwiched between two transparent electrodes

used for laptops and flat panel monitors

advantages of flat panel LCD

uses less power

better resolution and contrast

faster refresh rate

higher level of luminance longer

no distortion from:

• pincushion

• barrel

• skew

• veiling glare

drawbacks of flat panel LCD

limited viewing angle and more easily damaged

flat panel plasma monitors

acctive array of tiny fluorescent bulbs emit light from each pixel location

advantages of flat panel plasma monitors

thinner than an LCD

higher luminance output

wider viewing angle than LCD

drawbacks of flat panel plasma monitors

more expensive

shorter life

ACR remmonendations for flat panel plasma monitors

monthly performane evaluation

AAPM recommendations for flat panel plasma monitors

acceptance testing evaluation

annual physicist evaluation

daily/monthly/quarterly evaluation by QC tech

types of quality testing on display monitors

luminance

illuminance

resolution

noise

dead and stuck pixels

luminance for display monitors

ability to display different shades of brightness

rate of light emitted from a source

ACR requires min brightness of 250 lumens

illuminance for display monitors

rate of light striking the surface

max ambient lighing in a reading room must be less than ¼ normal lighting