Instrument Operations and Quality Control- Domain 4

field uniformity

this is the ability of a scintillation camera to produce a uniform image when a source provides uniform distribution photons over a detector.

(Clinically = the ability to produce accurate images of RPx distribution in a patient)

Intrinsic field uniformity

this type of field uniformity is done WITHOUT the collimator and monitors the condition of the sodium iodide crystal and the electronics associated with detector

Extrinsic field uniformity

this type of field uniformity is done with the collimator on and assess the camera as it is in use

Sources of gamma camera non-uniformity:

Mistuning

Uncoupling of PMT from crystal

Cracked crystal

Corrupted/ deleted/ switched-off software connection cables

Collimator defect

Radioactive contamination

X-Y misalignment

Hydroscopic damage to crystal

(Film artifacts NA)

What is mis-tuning in a gamma camera?

The photopeak of the radionuclide does not coincide with the photopeak energy window of the camera (energy windows or high voltages of PMT not set correctly)

Gas filled detectors include:

• Ionization chamber-type survey meter ("cutie pie")

• Proportional counter

• GM survey meter

**dose calibrator

Dose calibrator

this is an ionization chamber that measures radioactivity in curies or Bq

(rather than cpm)

-workhorse of nuclear medicine

-anode and cathode within the chamber

-chamber contains high pressure gas

-outside of cylinder shielded by lead

Dose calibrator operates in _____ region of voltage response graph meaning that all primary electrons created in the chamber reach the anode thou gas amplification.

Ionization

A dose calibrator is a ________ instrument - the number of electrons reaching the anode per second is integrated overtime, so that ir reaches a steady reading over a second or two.

Current-mode

The dose calibrator itself cannot distinguish between ______, so it cannot distinguish between different radionuclides

Photons of different energies

Dose calibrators can measure quantities down to about ______ to within _____.

20 uCi, 5%

Dose calibrators are not able to measure the activity of most _________ directly because they cannot penetrate the liner and wall of chamber to enter the gas space

Pure-beta emitters

GM Survey meter

Device with an audible sound system that alerts the operator to the presence of ionizing radiation

-GM region of graph (Townsend avalanche to increase ionizations)

-qualitative/ semi-quantitative indicator of presence of radiation

Geiger discharge

electrons cause further ionizations and excitations, gas molecules deexcite by UV photon emission, UV photons interact with gas molecules via photoelectric, produce more electrons

GM meters operate in _____ mode

Pulse

Units of GM meter

Counts per minute

mR/ hr

Quality control

An established set of ongoing measurements and analysis designed to ensure that the performance of a procedure or instrument is within a predefined acceptable range

Constancy

QC check in which the instruments response to a long-lived radioactive source or other unchanging/ slow-changing source of radiation is measured - reading of source should be constant from one day to the next

Linearity (not gamma cam)

QC check that describes the ability o a radiation detection instrument to respond to variable quantities in a linear fashion - usually over several orders of magnitude

-activity linearity of dose calibrator

-energy linearity of scintillation detector

Linearity (gamma camera)

The ability to reproduce a radiation distribution consisting of straight lines (bar phantom)

Accuracy

QC check in which an instrument's reading is compared to the true value for the measurement - allowing for correctness of the instrument's output or reading to be evaluated

Geometry

A collection of factors that may affect a radiation measurement

-volume of source

-type of container source is in

-physical location of source relative to detector

Survey meter daily QC

-battery test

-background reading (ensure no contamination)

-constancy using long-lived source

Accuracy and calibration of survey instruments must be completed:

Annually

GM meter is considered out of calibration if accuracy reading is not within _____% of expected.

10

Dose calibrator QC requirements

Constancy - daily

Linearity - quarterly

Accuracy - annually

Geometry - installation/ repair

Dose calibrator is considered accurate if reading is within ____% of reference standard measurement

5

Daily constancy readings of a dose calibrator are considered acceptable for within +-_____%

10

Activity linearity of a dose calibrator can be evaluated in two ways:

1. Measure short-lived source (Tc-99m) at several times points over 2-3 days, then plotting the measured activity vs time on a semilogarithmic graph

2. Use a set of commercially-available lead sleeves and a single high-activity source and compare using known attenuation factors of each sleeve

If a dose calibrator proves to have nonlinearity greater than +- ___% in any activity range, it should be serviced.

10

What is a common way a manufacturer may preform geometry testing on a dose calibrator?

Incremental additions of water to a radioactive solution with measurements after each addition - 3mL syringes and 30mL vials

Limit for geometry variation of a dose calibrator is +-_____%

10

Scintillation detectors

Radiation interacts with crystal such as sodium iodide, cesium iodide, or zinc sulfide to produce very small flash of light. Electronics of instruments amplify this light pulse thousands of times in order to produce useful signal that can be processed. Most useful when detection of very small amounts of radiation is required

Scintillation detectors in NM:

Thyroid probe

Well counter

Gamma camera

PET camera

Daily QC for scintillation detectors

Calibrate high voltage supply (peaking)

Constancy/ sensitivity

Background

High voltage should be:

Nearly constant from day to day

Constancy for scintillation detectors

Long-lived source measured to ensure constant cpm each day before use

Infrequent QC for scintillation

Energy resolution - quarterly

Chi square test - quarterly

Efficiency factor - annually

Energy resolution formula

FWHM = B-A/E x 100%

Sensitivity formula

TP/(TP+FN) ??

Pulse height analysis

can be applied to a series of pulses which have been generated with a size proportional to the radiation energy.

-energy window used to identify photopeak of radionuclide being imaged

-eliminates data of lower energy (bc of scatter)

-operator chooses appropriate percentage window and centers it on the photopeak visualized on the spectrum display (sets upper and lower limits)

Spatial resolution - gamma camera

Ability of a gamma camera to reproduce the details of a nonuniform radioactive distribution

- bar phantom

-measures in mm

- ability to put counts in their correct locations

Peaking

Checking alignment of gamma cameras pulse height analyzer window with the photopeak of the gamma ray being imaged

-scatter free source

Intrinsic uniformity of gamma camera

-collimator removed

-point source placed at least 5x the latest camera dimension

-do not exceed 20K cps

Extrinsic uniformity gamma camera

-collimator on

-flat-field flood source ( water-filled or Co-57 sheet)

-quicker, no need to expose crystal

-2-6 million counts (visual)

-30-120 million counts (quantitative)

Flat-field flood source

Used to test uniformity on a daily basis

Bar phantom

Assess gamma camera spatial linearity & resolution

-5mil counts

- mm

- better than largest lines seen but not better than smallest seen

Causes of gamma camera artifact

1. Wrong energy window

2. PMT problems

3. Cracked crystal

4. Measles (water)

5. Loss of auto corrections

6. Off-peak windows

7. Poor coupling of PMTs

8. Collimator artifact

9. Fillable flood problems

10. Photographic system problems

Sensitivity

Ability of gamma camera to use gamma rays available to it

Sensitivity maps

Uniformity correction maps

Sensitivity check gamma camera

-extrinsic

-1-2 mCi Tc-99m in Petri dish diluted to 3-mm depth

-10 count measurements of 1 min each

-tests collimator more than camera

-sensitivity over time should stable to within 5%

Attenuation correction

correction for the different levels of attenuation a photon might undergo as it gets to the detector

-creating 3D image while taking into account attenuation from tissues etc

-mathematically or patient-specific (attenuation map)

Chang attenuation correction

Oldest and most simple assumes body is an ellipse and uniform attenuation/density through the slice. Attenuation through each pixel is the same. Not used in chest region

-do not consider scatter or detector response

-finds average AC for distance from edge of user-defined eclipse

Patient specific attenuation map

-specific to area being imaged

-CT scan information used to correct for attenuation of tissue is applied to SPECT emission data

-transmission based AC

-external radiation source

Transmission based attenuation correction

Patient-specific attenuation map from external radiation source (CT)

SPECT Uniformity

Intrinsic: point source, no collimators

Extrinsic: flat field flood, collimators

Center of rotation (COR)

reference data used for correcting the slight misalignment of SPECT images taken from various projections

-AOR matches COR

-point or line source

-x-axis offset measured in pixels

-sinogram for visual inspection

COR should look like

Single point of activity in transverse slices

COR artifact 360 acquisition

Donut

COR artifact 180 acquisition

Tuning fork

Causes of COR misalignment

-gravity causes normal wear

-weight of heads

COR should be checked

Weekly

3D uniformity (tomographic) SPECT

-20-cm diameter cylinder with 10-15 mCi Tc99 diluted to fill bottle

-analyzed for integral and differential uniformity

3D resolution SPECT

Jaczak phantom

-rods: resolution

-spheres: contrast

-empty portion: uniformity

SPECT artifacts

1. ray

2. ring

3. motion

4. truncation

Ray artifact

FBP reconstruction only

-very hot or cold lesion is in sufficiently corrected by applied filter

-2D

-remove if possible

Ring artifact

Any abnormalities due to errors in rotational mechanics of detectors

-carried through every projection

-"bullseye"

-flood nonuniformities or collimator defects

-COR errors, head-tilt errors

-only corrected by fixing issue

Motion artifact

Movement of 2 or more pixels during acquisition time

-depends on magnitude of movement and time when it occurs

-review cine

-motion correction programs

Truncation artifact

Transmission scanning

-part of the body is not seen in all projections

-incorrectly place counts/ count boundaries

-more likely with limited FOV collimator (cone or fan beam)

-bc this type of imaging assumes that the entire object is in the FOV from all angles

Attenuation correction in PET

-more of a problem: both photons must be detected for the event to count

-head attenuates 85%, body attenuates 95%

-can be solved exactly - does not depend on location of RPx

-depends on body's thickness

-can be directly measured with CT

Recommended Daily QC for PET systems

PMT baseline check

PMT gain adjustment

blank scan

uniform cylinder/ point source scan

blank scan

this is performed on a PET scanner daily and uses a long lived source (ex Germanium 68) to produce sinograms for the detector pairs in each ring

-air scan / reference scan

-siograms will display any equipment failures

-visual inspection and quantitative data

-non-CT systems use this for attenuation correction

reccomended weekly QC for PET systems

uniformity

well counter calibration

coincidence timing

energy window calibration

coincidence timing calibration

this QC adjusts for the timing differences in the event detection circuitry and is done weekly

-generates corrections for coinidence timing between detector blocks

-also used to measure TOF

reccomended quarterly QC for PET systems

preventative maintenace

detector efficiency/ normalization

cross calibration

reccomended annual QC for PET systems

NEMA NU 2-2007 testing:

-spatial resolution

-sensitivity

-intrinsic scatter

-scatter correction

-count rate performance

update well counter calibration & normalization factors

normalization scan

evaluation of normalization factors that are applied to every study

-most important QC procedure

-must be high quality with little noise

-rod source or Ge-68 cylinder

-scan with low dead time and no pulse pike up

-compare to older version to find statistical differences

-can take 6hr

the normalization scan corrects for several issues that arise from the ____ of PET systems.

multicrystal

multi-ring geometry

important job of normalization in PET is to:

normalize the individual detectors so that they generate a uniform response to a uniform source

cross calibration in PET

-vital to numerical results generated in clinical studies

-measure source in well counter (cpm) and compare to measurement in tomograph

= count rate per voxel

artifacts in PET

electronic malfunction

ray artifact

motion

misregistration

bed-overlap

hot-spot

partial volume

ray artifact

due to FBP processing and corrected using ramp filters

(same as SPECT)

misregistration artifact

occurs because CT and PET acquisition is not completed simultaneously - if patient moes between change in bed position or between the two scans

-AC is not applied correctly

bed overlap artifact

horizontal photpenic stripes seen at junction of bed positions if normalization correction factors are not applied correctly = redo normalization and scan again

hot spot artifact

positron emitting source is used for attenuation correction and applied correction factors are not s large as they need to be

-high dead time in areas

-WB scans around bladder

co-registration

Spatial adjustment of two images obtained by different imaging modalities (e.g., PET or SPECT and CT or MRI). Their combined image is usually visualized using a fused display.

image contrast in CT

visualization of small differences in attenuation between normal and abnormal tissues

-CT can differentiate as low as 0.5%

-contrast is based on tissue density, mA, noise

spatial resolution in CT

ability to resolve closely spaced objects

-bar phantoms

-smallest line pair that can be visualized

-affected by size and spacing of detectors, focal spot, reconstruction matrix, filter

noise in CT

quantum mottle - noise resulting from finite number of x-rays used to make exposure

-CT: directly related to number of xrays detected by each detector

-fewer xrays = greater noise

-affected by imaging parameters

uniformity in CT

how uniform image of homogenous material appears

-water phantom to evaluate for differenes in HU caused by noise

-checks for cupping and beam hardening

daily QC for CT

tube warm up

verfiy tube output & detector response to vairous kVp and mA

uniformity using water phantom

artifacts in CT

beam hardening

attenuation correction artifacts in fusion imaging

noise

scatter

pseudoenhancement

motion

partial volume

motion*

beam hardening

The phenomenon whereby low-energy photons are absorbed as the x-ray beam passes through an object, resulting in an increase in the average photon energy of the beam.

-more low energy photons are asorbed, leaving only higher energy

Radiology Information System (RIS)

A system that collects, stores, and provides information on radiological tests. Main functions include:

registraion/ patient info

scheduling

workflow/ resource management

patient & study tracking

reporting

billing

-large linked databases linked by accession numbers

Picture Archiving and Communication System (PACS)

An integrated computer system that obtains, stores, retrieves, and displays digital images (in healthcare, radiological images)

-displays from all imaging modalities

-all image data must share same format (DICOM)

-biggest challenge s storage space

intravenous infusion pump

A device used to regulate the flow or rate of intravenous fluid

ECG monitor

records electrical activity of the heart for 12 second intervals

pulse oximeter

an external monitor placed on the patient's fingertip or earlobe to measure the oxygen saturation level in the blood

defibrillator

a device that delivers an electric shock to the heart to restore its normal rhythm

glucose meter

device that measures blood glucose concentration