29_-_Nuclear_Imaging_II.ppt

Nuclear Imaging:Scintillation Camera II

Factors affecting performance

Operation and QA

Image formation

Design factors determining performance

• Intrinsic spatial resolution and intrinsic efficiency

• Collimator resolution and collimator efficiency

• System spatial resolution and efficiency

• Spatial linearity and uniformity

Intrinsic spatial resolution

• Only a small number of photoelectrons is generated in any one PMT after an interaction in the camera’s crystal

• The pulses from the PMTs after an interaction contain significant random errors– Cause errors in the X and Y signals produced by

position circuit of camera and the energy (Z) signal

• These random errors limit both the intrinsic spatial resolution and the energy resolution of the camera

Intrinsic resolution (cont.)

• Energy of incident x- or gamma rays determines amount of random error in the event localization process– Higher energy photons provide lower relative random

errors and therefore superior intrinsic spatial resolution

– Above 250 keV, improvement in spatial resolution due to more visible light photons is largely offset by increased likelihood of scattering in the crystal before photoelectric absorption

Intrinsic resolution (cont.)

• QDE of the PMTs in detecting visible light photons produced in the crystal is most significant factor limiting intrinsic spatial resolution

• Size of PMTs also affects spatial resolution– Using PMTs of smaller diameter improves

spatial resolution by providing better sampling of the light emitted following each interaction in the crystal

Intrinsic resolution (cont.)

• A thinner NaI crystal provides better intrinsic spatial resolution than a thicker crystal– Permits less spreading of the light before it

reaches the PMTs– Reduces likelihood of incident x- or gamma ray

undergoing one or more Compton scatters in the crystal followed by photoelectric absorption

Intrinsic efficiency

• Determined by the thickness of the crystal and the energy of the incident x- or gamma rays

• Design compromise between intrinsic efficiency of the camera, which increases with crystal thickness, and intrinsic spatial resolution, which degrades with crystal thickness

Collimator resolution

• Collimator spatial resolution of multihole collimators is determined by geometry of the holes

• Spatial resolution improves as the diameters of the holes is reduced and the lengths of the holes (thickness of the collimator) are increased

• Changing hole geometry to improve spatial resolution generally reduces the collimator’s efficiency

• Resultant compromise is single most significant limitation on scintillation camera performance

Parallel-hole collimators

• Spatial resolution of parallel-hole collimator decreases linearly as collimator-to-object distance increases– Also one of the most important factors limiting

scintillation camera performance

• Efficiency of a parallel-hole collimator is nearly constant over the collimator-to-object distances used for clinical imaging– Number of photons passing through given hole decreases

with square of distance, number of holes through which photons can pass increases with square of distance

Effect of increasing collimator-to-object distance

System spatial resolution and efficiency

• System spatial resolution, corrected for magnification, is degraded as the collimator-to-object distance increases for all types of collimators

• System efficiency with parallel-hole collimators is nearly constant with distance

• System efficiency with pinhole collimator decreases significantly with distance

Spatial linearity and uniformity

• Spatial nonlinearity is caused by nonrandom mispositioning of events

• Mainly due to interactions being shifted in the resultant image toward the center of the nearest PMT by the position circuit of the camera

Spatial linearity correction

• Modern cameras have digital circuits that use tables of correction factors to correct each pair of X- and Y-position signals for spatial nonlinearities

• Corrections are added to the uncorrected X and Y values

Nonuniformity

• First major cause of nonuniformity is spatial nonlinearities

• Systematic mispositioning of events imposes local variations in the count density

• Spatial nonlinearities that are almost imperceptible can cause significant nonuniformities

• Effectively corrected by linearity correction circuitry

Nonuniformity (cont.)

• Second major cause is that the position of the interaction in the crystal affects the magnitude of the energy (Z) signal

• May be caused by local variations in the crystal in the light generation and transmission to the PMTs and by variations in the light detection and gains of the PMTs

• Fraction of interactions rejected by energy discrimination circuits will vary with position in the crystal if regional variations are not corrected

Nonuniformity (cont.)

• Third major cause is local variations in the efficiency of the camera in absorbing x- or gamma rays, such as manufacturing defects or damage to the collimator

• May be corrected by acquiring an image of an extremely uniform planar source– Correction factor determined for each pixel by dividing

the average pixel count by that pixel count– Each pixel in clinical image multiplied by its correction

factor to compensate for this cause of nonuniformity

Effects of scatter and attenuation

• Ideal nuclear medicine projection image would be a 2D projection of the 3D activity distribution in the patient– Number of counts in each point in the image would be

proportional to the average activity concentration along a straight line through the corresponding anatomy of the patient

• Not ideal due to:– Attenuation of photons in the patient– Inclusion of Compton scattered photons in the image– Degradation of spatial resolution with distance from

collimator

Operation and routine QA

• Energy discrimination windows must be adjusted to center them on the photopeak or photopeaks of the desired radionuclide

• “Peaking” may be done manually by adjusting the energy window settings while viewing the spectrum or automatically by the camera

• Should be peaked before first use each day and before imaging a different radionuclide

• Small source used to peak camera; radiation emitted by the patient would have a large scatter component

Operation and routine QA (cont.)

• Uniformity of the camera should be assessed daily and after each repair

• May be made intrinsically by using a Tc-99m point source, or system uniformity may be evaluated using a Co-57 planar source or a fillable flood source

• Images must contain enough counts that quantum mottle does not mask uniformity defects

• Uniformity test will reveal most malfunctions of a scintillation camera

Other QA

• Spatial resolution and spatial linearity should be assessed at least weekly

• Efficiency of each camera head should be measured periodically

• Complete evaluation at least annually– Include multienergy spatial registration and

count-rate performance

Frame mode image acquisition

• Before acquisition begins, a portion of the computer’s memory is designated to contain the image

• All pixels within this image are set to zero• After acquisition begins, pairs of X- and Y-

position signals are received from the camera, designating a single pixel in the image

• Once count is added to the counts in that pixel• As many pairs of position signals are received, the

image is formed

Types of frame-mode acquisition

• Static – single image acquired for either a preset time interval or until total number of counts in image reaches preset number

• Dynamic – a series of images is acquired one after another, for a preset time per image– Used to study dynamic processes, such as the first

transit of a bolus of radiopharmaceutical through the heart or extraction and excretion of a radiopharmaceutical by the kidneys

Gated frame-mode acquisition

• Some dynamic processes occur too rapidly for effective portrayal by dynamic image acquisition

• If process is repetitive, gated acquisition may permit acquisition of an image sequence that accurately depicts the dynamic process

• Requires a physiologic monitor that provides a trigger pulse to the computer at the beginning of each cycle of the process being studied (e.g., an ECG monitor for a cardiac study)

List-mode acquisition

• Pairs of X- and Y-position values are stored in a list instead of being immediately formed into an image

• Periodic timing marks are included in the list• If a physiologic monitor is being used, trigger

marks are also stored in the list• Allows greater flexibility in how the X and Y

values are combined to form an image• Generates large amounts of data; data must be

subsequently processed for viewing

Regions of Interest

• A region of interest (ROI) is a closed boundary that is superimposed on the image

• May be drawn manually or may be drawn automatically by the computer

• Sum of all counts in all pixels in the ROI is an indication of the activity in the corresponding portion of the patient

Time-Activity Curves

• To create a time-activity curve (TAC), a ROI must first be drawn on one image of a dynamic or gated image sequence

• Same ROI then superimposed on each image in the sequence by the computer and the total number of counts within the ROI is determined for each image

• Counts within ROI are plotted as a function of image number

• Curve is an indication of activity in corresponding portion of patient as a function of time

Left ventricular ejection fraction

• LVEF is a measure of the mechanical performance of the left ventricle of the heart

• Defined as the fraction of the end-diastolic volume ejected during a cardiac cycle:

where VED is the end-diastolic volume and VES is the end-systolic volume of the ventricle

ED

ESEDV

V - V LVEF

Crosstalk

• Number of counts in left-ventricular ROI estimated as:

• Thus:

ROIcrosstalk in Pixels

ROI LVin PixelsROIcrosstalk in Counts crosstalk Counts

crosstalk CountsED Counts

ES CountsED Counts LVEF