Lecture 1: Development of the Analog Gamma Camera Unit V: Introduction to Gamma Camera Lecture 1: Development of the Analog Gamma Camera http://www.petnm.unimelb.edu.au/nucmed/overview/history.html

Objectives Describe the major components of the Anger gamma camera and the development and concept of positioning logic, including the requirement for energy normalization Discuss later improvements to the Anger camera’s design

Circa 1950s Rectilinear Scanner http://www.petnm.unimelb.edu.au/nucmed/overview/history.html

Anger Camera—circa 1958 http://www.rsna.org/Publications/rsnanews/jan06/people_jan06.cfm

Hal Anger’s original plan using 7 PMTs Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 253.

Cross-sectional Schematic of Gamma Camera PMTs NaI (Tl) Crystal

What we want to get is…

But the schematic says we’ll get…

Because of its particulate, high energy, nature, we can only use High “Z” material to “absorb” gamma radiation (i.e. lead).

… adding a multi-hole collimator to the exterior-facing portion of the large scintillation crystal

Collimators…”discriminate based on direction of flight…” (NM & PET/CT, 7th Ed). Collimators allow only gamma photons traveling in a desired direction to be detected. Collimators themselves cannot discriminate different photon energies from one another.

Position Logic Circuitry

Position Logic Circuitry with normalization of event energy Sum circuit totals all output to create the Z pulse The Z pulse is sent to the PHA and the division circuit to normalize X and Y output for positioning

Related positioning logic concept: edge packing Picture from: http://www.medimaging.gr/cd/pages/par4.htm accessed 11/11/12

NaI(Tl) Crystals Usually 3/8 to 5/8 inch thick Hygroscopic so hermetically sealed Surrounded by inward reflective cover

Thicker crystal : more sensitive crystal Gamma photon Less likely gamma photon will pass through undetected But PMTs will see it more as a foggy diffuse light

Thinner crystal : better resolution Gamma photons But PMTs will see a gamma photon as a more distinct point of light More gamma photons will pass through undetected

3/8 inch crystal required to efficiently detect gamma photons up to 200 keV Most modern gamma cameras have 38 x 50 cm rectangular FOVs

More resolute thicker crystals are possible for higher photon energies. Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 69.

Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) pg 69.

Putting It All Together

Putting It All Together Collimator – Only allows direct path photon to interact with NaI(Tl) crystal.

Putting It All Together Crystal – Emits visible light when energy from gamma photon is deposited within in

Putting It All Together Light Pipe— Anger’s original design included a thick clear plastic sheet between crystal & PMT. The photoemissive material is thicker at the center of the photo anode resulting in making the PMTs shapes visible on a uniform image. This was an attempt to reduce this effect. Newer cameras correct this digitally and no longer use light pipes.

Putting It All Together PMTs – Generate electric pulse in response to visible light from crystal. (Closest PMT to light generates greatest pulse) Modern cameras have an array of 60-120 PMTs More recently produced systems include software for thresholding PMT responses for more accurate positioning

Putting It All Together Position Logic Circuit – Compares PMT pulses and assigns a location on an image matrix that corresponds with the event detected by the crystal.

Putting It All Together Position Logic Circuit – Sends location signals in terms of x and y values

Putting It All Together DIVISION CIRCUITRY Z Pulse Sum of PMT pulses generated by a detection event (represents total energy deposited on the crystal). The Z pulse is also used in the “Division Circuitry” to “normalize” the X & Y positioning signals.

Putting It All Together Pulse Height Analyzer- As with any scintillation detector, allows us to select pulses of a certain height and reject others.

-y +y +x -x Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 253 & 255.

Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 255.

Multiformatter High quality CRT Can direct image to multiple spots on film 2, 4, 8, 16, 30 images on one film Works great, but NO MISTAKES! Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 255.

Variable Persistance Scope (P-Scope) More commonly digitized now Screen is “refreshed” based on time or counts Paul Early, D. Bruce Sodee, Principles and Practice of Nuclear Medicine, 2nd Ed., (St. Louis: Mosby 1995), pg. 256.

Next time: Modern Gamma Cameras