Unit V: Introduction to Gamma Camera Lecture 2: Modern Gamma Cameras & Properties http://www.petnm.unimelb.edu.au/nucmed/overview/history.html

Objectives Regarding gamma camera systems, discuss the need for the following as well as the entity responsible for providing each: Energy corrections Linearity corrections Uniformity correction map Autotuning of PMTs Describe the methods for evaluating gamma camera uniformity

Modern Gamma Cameras Most modern gamma camera systems come with two cameral linked to a moveable gantry and/or movable bed. This is to facilitate: Circular geometry Angled planar views SPECT Head to toe (whole body) imaging Most dual-head systems are variable angle 180 degrees (opposing planar and 360 degree SPECT) 90 degrees (180 degree Cardiac SPECT)

Digital Images Digitized means a discrete value is ascribed to each pulse height (no longer an infinite variety of pulse heights—they are quantitatively categorized) A MCA discriminates acceptable pulses An accepted pulse is 1 count A matrix is an electronic/digital array of square pixels used for count storage and display on a CRT, LCD, LED, or plasma monitor

Key NM Image Terms Uniformity Sensitivity Resolution

Pixels ADC: Analog to Digital Converter (For most modern gamma cameras, the ADC is at the end of each PMT) Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 362. Information Storage for Display (Base-2 system, each pixel represents a “1” or a “0.” Each pixel depth represents a power of 2.) Kai Lee, Computers in Nuclear Medicine: A Practical Approach, 2nd Ed., (Reston, VA: SNM 2005) p. 83

Important Matrix Concepts Using the above gray scale, we can see in the simple 3 X 3 matrix below that the maximum counts are in pixel nbr. 9, and the minimum are in pixel 5. 1 2 3 4 5 6 7 8 9

Causes of Non-uniformity: positioning logic error (non-linearity)

Figure 09: Energy spectra as seen by individual PMTs. Causes of Non-uniformity: Energy registration differences Figure 09: Energy spectra as seen by individual PMTs.

Nuclear Medicine and PET/CT: Technology and Techniques, 6th Ed. , P Nuclear Medicine and PET/CT: Technology and Techniques, 6th Ed., P. Christian & K. Waterstram-Rcih, Eds, (Mosby: St. Louis, 2007), Fig 3-24, p. 77.

Figure 12: Schematic of a gamma camera with energy and linearity corrections.

Uniformity Correction Map (or Sensitivity map, uniformity matrix, correction tables…) Once linearity and energy corrections have been set (by manufacturer or technician), a count skimming/adding technique is used to adjust for still-existent regional sensitivity variations remaining in the camera’s FOV. Uses statistically verifiable uniform gamma photon exposure to a camera’s FOV. Computer compares image produced by what it knows should be a uniform exposure and makes sensitivity corrections by adding and subtracting counts. Computer remembers these corrections (count additions and subtractions) as a “map” and applies it to future acquisitions usually using the same radionuclide.

Uniformity Correction Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 238.

Matrix & percent error Kai Lee, Computers in Nuclear Medicine: A Practical Approach, 2nd Ed., (Reston, VA: SNM 2005) p. 81

Matrices and Patient Procedures—reducing percent error per pixel Larger pixels (smaller matrices) are best for low-count images Ex: Renal flow (2 sec/frame): 64 X 64 GI Bleed Dynamic (30 sec/frame): 128 X 128 Trade off: will not be able to distinguish objects smaller than 2 or 3 pixels Smaller pixels (larger matrices) are best when time and activity allow for high-count images. Static bone images (500k – 1M counts per image) Will be able to distinguish small objects due to small pixels, each with low percent error.

Acquiring a Uniformity Correction Map Need statistically significant counts for each pixel in the matrix (i.e. 10,000 counts). For a 64 X 64 uniformity correction map: 64 X 64 X 10,000 = 40,960,000 (For practical purposes: 30,000,000 typically used) For a 128 X 128 uniformity correction map: 128 X 128 X 10,000 = 163,840,000 (For practical purposes: 100,000,000 typically used) Count rate needs to be low: 20,000 – 30,000 cps because of dead time losses, baseline shift, and pulse pile up.

Uniformity Map Example 128 X 128 Uniformity Correction Map 20,000 cps 100,000,000 cts / 20,000 cps = 5000 secs = 1 hour 23 minutes Extraneous sources must be eliminated (No Patients) Often done at night

Figure 11: Effect of various corrections Figure 11: Effect of various corrections. Note the significant gain in uniformity that is made by the application of energy correction. The “all corrections” image includes not only energy and linearity corrections, but also an isotope-specific flood correction Image courtesy of Virginia Mason Medical Center

PMT Drift PMT Array All PMTs are subject to “drift” More PMTs—better spatial resolution Rectangular FOV typically have 60+ PMTs All PMTs are subject to “drift” Biggest effect on camera performance Variation in gain, degradation of components Three types of drift 1. Warm-up 2. Short-term 3. Long-term

Autotuning An automatic system in which PMT high voltages and amplifier adjust to stabilize voltage outputs so corrections remain valid. Many usually automatically adjust PMT gains by comparing current photopeak with an already established value (photopeak monitoring). Other methods appear in your book.

Figure 13: Multi-layer system for optimal gamma camera performance, with the responsible party indicated at right.

General Guideline for Care & Use Non-Anger Gamma Cameras Next time: General Guideline for Care & Use Non-Anger Gamma Cameras Image from Molecular Breast Imaging. Mercy Medical Center. Available at http://mercymedicalcenter.chsli.org/molecular-breast-imaging . Accessed 28 Nov 2017