Gamma Camera - Spect QC

SPATIAL RESOLUTION 

The definition of spatial resolution outlines the intrinsic ability of the camera to accurately detect the original location of a gamma ray on an x-y plane. The standard calls for spatial resolution to be measured in both the x and y direction and to be expressed as the full width at half maximum (FWHM) and full width at tenth maximum (FWTM) of a line-spread function measured in millimeters. This measurement as performed by standards requires a special slit phantom, but extrinsic calculations may be performed for all collimators using a Tc99m point source in a capillary tube.

The source is imaged at various locations along the x and y axes and line-spread functions are generated. Manual calculations of FWHM and FWTM can then be performed. Some nuclear medicine computer systems include programs for this testing, but as long as a line-spread function can be generated, the extrinsic spatial resolution for each collimator can be measured and compared to manufacturer specifications.

Using two point sources in capillary tubes imaged at variable distances from each other can determine the limit of the camera/collimator spatial resolution. The sources are based at variable distances closer and closer together along a ruler until the two points can no longer be distinguished as distinct separate sources. The limit is the distance at which two separate points can no longer be distinguished as separate, both visually and by peak position. In addition, bar phantom images can be evaluated subjectively for the intrinsic resolution of the camera system and extrinsically for each collimator. It is important that the bar phantom have bar spacing relevant to the resolution parameter specified by the manufacturer.

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UNIFORMITY 

Intrinsic uniformity is measured with a point source at least five useful fields of view (UFOV) from an uncollimated camera, providing a < 1% variation in the flux of radioactivity viewed by the camera crystal. Most modern camera systems use some form of uniformity correction over the raw pattern because the intrinsic flood is not inherently uniform.

Uniformity is sacrificed in order to make some gain in spatial resolution by most modern camera systems . With the advent of digital processors, most modern camera systems correct for this nonuniformity with linearity correction maps and energy correction algorithms so that uniformity and resolution are both preserved. To evaluate the tuning of the PM tubes and the accuracy of the correction maps, both uncorrected and corrected uniformity flood fields should be at least visually evaluated. The standard calls for changes in count density over the UFOV to be <5%. Some camera manufactures provide software that allows for an approximation of this standard. Intrinsic flood fields should be collected and at least visually evaluated for all commonly used isotopes to ensure that corrections collected with one isotope (usually Tc99m or Co57) remain accurate for other isotope energies.

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LINEARITY 

Spatial non linearities are caused by the mispositioning of individual photon events. This displacement is caused by a limited number of PM tubes trying to locate an infinite number of events and results in a wave-like distortion over the field of view of the system. The manufacturer uses a correction algorithm to compensate for this inherent distortion. The standard uses the same phantom utilized for spatial resolution testing, i.e., the slit phantom, to collect an image for evaluation. Determination of line-spread function peak positions are then compared to an ideal grid. A subjective evaluation of spatial linearity can be done by checking the straightness of the lines in the bar phantom image obtained for spatial resolution. 

ENERGY RESOLUTION 

A camera's intrinsic energy resolution may be described as the ability of the camera to accurately identify photopeak events differing in very small energy amounts. The results are expressed as a ratio of FWHM-to-photopeak energy as a percentage. Routinely this will be 9%-11% for 99mTc for most camera systems. The data are collected with an uncollimated camera positioned at least five useful fields of view away from a point source. If possible, this data should be collected on a multichannel analyzer with at least 50 channels at the FWHM range, as recommended in the standard. 

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COUNT RATE PERFORMANCE 

All sodium-iodide detection systems have an inherent dead time which should be evaluated during acceptance testing. This is often a major parameter in the selection of a camera, especially with renewed interest in cardiac first-pass studies.

The maximum count rate can be determined by observing the count rate while a source is moved toward the uncollimated camera face, through the maximum observed count rate, until the count rate falls. This technique simulates adding small equal aliquots of activity to a source with unchanging distance from the camera. Counts per second are plotted versus distance, where distance becomes the equivalent of increasing activity . 

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MULTIPLE WINDOW SPATIAL REGISTRATION 

When multiple windows are used to create an image, the spatial resolutions of the different energies are added together in the final image. If the spatial registrations for the different energy windows are not positioned properly, the resulting image will have a loss of spatial resolution.

By imaging a source using different energy windows and comparing the separately created images, the spatial registration can be visually evaluated fig . standards require four different positions to be used and deviations of the images to be measured in millimeters. 

Determines if multi-energy radionuclides are being recorded in their correct energy peaks at the right location. Sometimes an imaging system will mis-register the different photo-peak's

extrinsic

Place 1 to 2 drops activity in 4 locations on a piece of cardboard , Acquire each energy peak separately for the same amount of time , Subtract one peak from the other and if there is any superimposition remaining then the system has a mis-registry found via the different energy gammas

Given a multiple peak radionuclide (67Ga) the above shows that the highest counts for the 93 keV window is at pixel 91 and 141, however, for 300 keV the pixels maximum peaks change to 90 and 140 ,If the highest counts are not within all three (same) pixels then there the system is mis-registering

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SENSITIVITY 

Camera sensitivity is measured with a collimator in place. The calculation is performed as counts per unit time per unit source activity. To follow the standard, a syringe of activity that will produce no appreciable dead time is measured in a dose calibrator before the source is transferred to a petri dish, and the empty syringe is measured after transfer in order to calculate the activity placed in the dish. The petri dish source is then imaged for a set total count and the time is recorded. Each collimator should be evaluated in this way during the acceptance test with the appropriate radionuclides for the collimator used in each case. 

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Peaking

Intrinsic or extrinsic (scatter free source) , Source should cover the entire FOV, but that may not be practical (consider your distance) , Never peak off a patient (too much scatter) ,The camera should be peaked every morning on the energy peaks of all radionuclides being used that day ,More modern systems have auto-peaking where the peak is split in two and adjusted until the counts on either side are equal , Energy resolution should be compared with manufacturer's guideline (most systems to not have this capacity) , HV does fluctuate so expect the peak to move a few keV in either direction , Multi-head cameras require greater adjustments, because the heads must be "in tune" with each other. In addition there is less tolerance when two or more heads are being used to acquire the data. Consider with slight changes in keV from one head to the other Photopeak adjustment and the effects on the image

A - decreases the amount of scatter, however, too much to the right will cause the PMTs to show up hot , B - is a normal window with LLD and ULD are set appropriately , C - asymmetrical shift to the left causes the PMTs to appear cold surround by a hot background

Off peaking may help resolve camera issues: evaluates detector to see if it needs to tuned, may show loss of optical coupling, and even identify crystal deterioration

1. What might occur if a bone scan was acquired with peak A
2. What might occur if a liver scan was acquired in peak C
3. Modern gamma cameras are able to correct for asymmetric windows .

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Uniformity Flood

Is the QC procedure to evaluate the camera's ability to operate correctly. This procedure can be done intrinsic or extrinsic

A- Intrinsic flood

With the collimator off acquire a <1.0 mC 99mTc source, in a syringe. Some cameras systems may require as little as 250 μCi (or even less)

The source should be placed in the center of the detector at a distance of at least 5 FOVs. This will causes <1% variation of photon flux within the entire areas of the crystal's surface To prevent deadtime from occurring the rate should not exceed 20k cps , Some systems require a lead mask placed around the edge of the crystal which prevents gamma rays from interacting with the edge packing. What does edge packing look like? It will be a hot rim seen at the edges of the FOV, where the rest of the crystal will show significantly less uptake

Some dual-headed systems require an additional test where a low activity point source is placed between the heads (may require 100 to 250 μCi). This creates a large non-uniform spot which is required prior to taking the intrinsic flood source . Finally, make sure that there is no other radioactive source interfering with the intrinsic flood. The above is and example of this problem

B- Extrinsic Flood

Using either a Co57 sheet or a refillable flood with 5 - 15 mCi of Tc99m place it on the collimator's surface.

If there are 2 heads then place 4 Styrofoam cups on the bottom and 1 at the top. This will assure equal distance between both heads

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Two quantification formulas

1.Integral uniformity evaluates the entire flood at the pixel level and finds the pixel with the least counts and the one with the most counts.

Apply those numbers to the above formula. This gives you an overall picture on how uniform the flood is. Over time, if the % slowly increases then service dude needs to called in for a PM

2. Differential uniformity looks a 5 pixel segment, either vertically or horizontally and determines what the counts are in each pixel.

The one with the most and less counts are placed into the formula. This value shows non uniformity over a small segment of the camera

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Bar Phantoms

Bar size assessment is only semi-quantitative, since it does not pinpoint the exact resolution, it only shows resolution to the smallest bars Linearity can also be evaluated by eyeballing the imaging system's ability to duplicate a straight line.

A 0.4mm deviation in linearity will produce 8% nonuniformity

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Count-Rate and deadtime methods

Count - Rate

Using Two source method for determining 20% loss

Attain two sources of Tc99m that have similar amounts of activity which generate ~20k counts each (define as R1 and R2) , The combined sources should produce approximate 20% count loss. Confirm the actual amount with the manufacturer ,Acquire counts for 100 seconds with some type of scatter medium between the source(s) and the detector.

Apply the recorded data to the formula. Twenty percent loss can then be verified with the vendor's benchmark

Deadtime loss

Using Copper plate method 

Remove the collimator and acquire a 20 mCi source of 99mTc and place it 150 cm away from camera surface , Take 25 copper plates (1-mm in thickness) and place all of them on the crystal and between the source , Preset time (~1 minute) and measure/record the counts. Then remove 1 copper plate at a time and repeat/record the acquisition

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