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Floods – Uniformity Testing• Types of floods
– Co57• Impregnated plastic• Old source vs. the new source
– Refillable flood• Adding the Tc99m• Mixing and spilling• It will bow if its too big
– Point source at least 5 diameters away from the surface– Key Points
• Setting the peak• How many millions of counts? • Size of the detector• Complete daily• Amount of activity
Corrected Vs. Uncorrected• Setting your correction matrix
– Depends on SPECT
– 30 M vs. 120 M vs. the size of the head
– Do one for each radiopharmaceutical
• Correction will either subtract or add counts• Take un/corrected flood and bars• Correction for each PMT is no more than 10%• Acquire at least once or month or when needed
Intrinsic Uniformity• Determine Integral and Differential
Uniformity– Remove collimator place a 3mm Pb “ring”
around the UFOV (example next side)– Set 20% window 99mTc with source (~200
μCi) 5 UFOV away– Set matrix at 64 x 64 to collect no more than
20,000 cps– Acquisition - center pixels must collect
4000counts/pixe; (~16 M count flood)– Complete a 9 pt smooth and calculate the
following
What are we looking at?
• Integral uniformity looks at maximum deviation for both CFOV and the UFOV
• Differential uniformity looks at the maximum change of counts over a range of any 5 pixels in every column and row
• Excessive changes in any of the numbers is an indication non-uniformity problems
• Time to call service guy!
Flood Sources
Co57 Flood Source
•Rectangular or Circular•5 to 20 mCi•Non-uniformity at <1.0%•No mixing required – reduces radiation exposure•Expensive since it must be repurchased based on Co57 decay
Refillable Flood Phantom
•Rectangular or Circular•Different sizes to fit a specific type of detector•Inject mCi amount of Tc99mand mix in the water filled container•Concerns
•Mixing can get messy•Radiation exposure when mixing•Air bubble will distort the image
Typical Bars – Four Quadrant
•Multiple types of phantoms are available to match the size of the detector•Bars contain Pb embedded in plastic.•Bars attenuate the gamma radiation being emitted from the flood source•Bars vary in size within each quadrant•Is usually done with the collimator on •Known as an extrinsic bar •Tests system resolution •Imaging system should resolve between the 3rd and 4th smallest quadrant•Smallest bars are 2 mm, however, it depends on the type of 4 quadrant bar
purchased
Setting Up The Bar Phantom
Comments
• How would imagine resolution be effected by LEHS vs. LEHR collimation?
• How does increased counts effect image resolution?
• How often is this procedure done on the imaging system?
• Why are the bars rotated every time this procedure is done?
Other Types of Bar Phantoms
BHR Test Pattern•Lead plate that is 3.2 mm thickis placed between 2 sheets of Plastic•Orthogonal-holes are then drilledin an array pattern•Emission source can be extrinsicor intrinsic (intrinsic usually done)•Clearly defines system resolution
•Over the entire camera•Linearity also identified
PLES (Equally Spaced Parallel-Bar)Phantom• Checks linearity and resolution•Phantom is placed with flood sourceon top (just like the Bar and Flood)•5 mm and 3 mm bars are available•Linearity is the key
•X and Y axis- linearity is tested•Looking for a straight line•Lines should not be wavy
More On Bars• Do weekly and rotate each week
• How many counts?– Size of detector– Increased counts improves resolution– Collimation
• Changes in resolution can be seen and identifies a loss of system resolution
• Intrinsic or extrinsic?
Systems Uniformity/Linearity/Resolution
Uncorrected FloodAnd Bars
4000k/image
Corrected FloodAnd Bars
4000k/image
Edge Packing
• Intrinsic flood uses a mask on top of the crystal to eliminate this artifact
• Extrinsic flood may show this artifact
Setting The Peak
• Centering the peak is essential
• Off peaking causes imaging artifact
• What would happen if you imaged a patient with Co57 with Tc99m on board?
Example Of Being Off-Peak
• Creation of lesions in the liver
• Could this lead to an inappropriate diagnosis?
Distance And ResolutionThese images show the effectof distance on image resolution
•Surface vs. 1 inch
•2 inch vs. 3 inches
What conclusion can you makewhen it comes to imaging a patient?Does count density also add to this issue?
Problem With A PMT
• What causes this artifact?
• How do both image sets differ?
• How might this effect the a patient’s diagnosis?
Problems With Liquid Filled Floods
• Image on the left shows a flood that was note mixed completely
• Image on the right shows an air bubble in the flood
Collimator Damage
• Image on the right indicates damage to the collimator septa
• Image on the left shows the effects of septa damage on a diagnostic study
Types Of Bars
A – Parallel-line equal-space (PLES) phantom: Small lead bars Test Camera’s spatial resolution. Two transmission images are Taken at right angles. Consider the Moire patternB – Four quadrant bar phantom: Show wholes of equal size. The smallest bars can go down to 2 mm in size.C – Orthogonal-hole (OH) phantomComes in different sizes (0.64 cm, 0.48 cm, and 0.32 cm Diameter spaced at 1 cm, 0.96 cm, and 0.64 cm respectively.Matching the lower end of system resolution is important.
Results Of Excessive Activity
• Activity from the source should never exceed 20k/second
• The image on the left shows the results of excessive activity
• The image on the right is the same, however, the activity has been reduced to less than 20k per second
Intrinsic Uniformity• With a point source collect 64 x 64 flood where
the center pixel receives a minimum of 4000 cts (16 million count flood)
• Filter with a 9 point smooth• Integral uniformity is the maximum pixel
deviation• Differential uniformity max change of cts over
range of any 5 pixels in all rows and in all columns
Key Points • Identifies the amount of nonuniformity with all
areas of the flood• Variations between un/corrected should be no
greater than 15%• Should be done weekly to determine when the
camera needs a PM• “Measles” occur when water gets into the crystal • Floods should be done with the collimator on to
see if there are any defects in it• A crystals may crack when there is change of 8
degrees F• Room temperature should range between 40 and
110 degrees F
Measles
• Image is off-peak• Loss of the hermetic
seal causes water to enter and dissolve the crystal
System Sensitivity• Is a parameter of camera’s ability to efficiently
detect the incident gamma with the collimator on• Changes in sensitivity result from
– Nonuniformities in the crystal – Decreased energy resolution– Incorrect collimation– Improper setting of the PHA
• Recording the time it takes to collect a Co57 flood corrected for decay will identify any change in system sensitivity
System Spatial Resolution• Is defined as camera’s ability to accurately
determine the original location of a gamma ray on an X-Y plan with collimation
Moiré Pattern
• http://www.sandlotscience.com/Guided_Tours/Tour1/Tour_5.htm– When a bar phantom is not lined (oriented) up
septa in collimator this patter will occur– Bars travel at angle to the line of wholes
(septa) in the collimator– This can be further seen on the next two slides
Moiré ContinuedMoiré seen on barImages
Bands representthe bars and theholes represent the septa
Images across From each otherIdentify the pat-tern
Moiré Continued
This shows a different moiré pattern because of the changeIn rotation of the bars to the collimator septa
Instrumentation and QC
Consider the following procedures that involve QC and instrumentation• Uniformity Matrix• Floods and Bars• Other types of Bars - what is there purpose?• Linearity• Off peak or on peak• Effects of Distance on image resolution• Count density• PMT artifacts• Crystal artifacts• Collimator artifacts• Flood field artifacts