Topic 16: tomographic nuclear imaging

Question 1

Describe the structure of Anger camera

Answer 1

• Collimator
• Camera head = crystal + photomultiplier tubes
• Electronics
• Computers = image acquisition + image processing

Question 2

Describe the collimator

Answer 2

• Location : infront of crystal
• Restricts rays from source = each point in image corresponds to point in source
• Thousands of precisely aligned holes
• Allows only photons traveling directly along axis of each hole = reach crystal = photons in other directions absorbed by septa between holes
• Without collimator = image unclear

Question 3

Describe types of collimators

Answer 3

• Depending on phorons of different energies
• Types = magnify/reduce image
• Can choose between imaging quality + imaging speed

Question 4

Describe the camera head

Answer 4

• Contains = crystal + PMT + electronics
• Head housing = protects internal components
• Thin layer of lead
• Gantry supports heavy camera head

Question 5

Describe the crystal in the camera head

Answer 5

• Large slab = thallium-doped NaI crystal
• Thickness = affects resolution + sensitivity
• Thicker = higher sensitivity + lower resolution = gamma rays scattered + absorbed farther from point of entering crystal

Question 6

Describe PMTs in camera head

Answer 6

• 60+ attached to back surface of crystal with light-conductive jelly
• Amount of light received by PMT = proximity of tube to site of interaction of γ + crystal
• PMT closest = greatest number of photons + output pulse
• PMT farthest = fewest photons + smallest pulse output

Question 7

Explain how PMT help form images

Answer 7

• Image formed = points corresponding to PMT with highest output at each interaction = number of resolvable points depends on total number of PMTs = no more detail than number of PMTs
• Positioning algorithm = improves resolution via combining signals from adjacent tubes

Question 8

Describe how pulse output of PMT is digitalized

Answer 8

• Pulse > digital = analog-to-digital converter = ADC
• Digital values transmitted = positioning algorithm
• Algorithim = knows location of each PMT = estimates site of γ ray interaction in crystal via weighing in digital value of amount of light per PMT

Question 9

Describe the Z-pulse

Answer 9

• Sum of digital outputs from all PMTs = proportional to energy of γ photon on crystal
• Pulse height analyzer accepts = only Z-pulse corresponding to γ energy of interest
• Accepted Z-pulse = location stored

Question 10

Describe the computer

Answer 10

• Use = acquisition + storage + processing data
• Image data = stored digitally
• Each Z-pulse accepted = count added to storage corresponding to x/y location = determined by positioning circuit
• Data storage = visualized as matrix
• Each position in matrix = correspond to pixel = unique address of row/column of location
• Data digitized via assigning matrix position to each accepted photon

Question 11

What happens when there is a greater number of pixels?

Answer 11

• Each pixel smaller for given field of view
• Better preserved resolution of image

Question 12

Describe SPECT

Answer 12

• Single-photon emission computed tomography
• Cameras get multiple planar views of radioactivity in organ
• Data = processed mathematically = create cross-sectional views
• Uses single photons from γ emitting radionucleotides

Question 13

Give radionucleotides used in SPECT

Answer 13

• 99mTc = technetium
• 111In = indium
• 123I = iodine

Question 14

Give the features of SPECT camera design

Answer 14

1) Head can rotate about patient = multiple views
2) Has computer = integrates multiple images = produce cross-sectional views

Question 15

Give the uses of SPECT

Answer 15

• Diagnose + monitor
  1) Brain disorders
• Dementia
• Seizures/epilepsy
• Head injury
• Clogged blood vessel
  2) Heart problems
• Clogged CA
• Reduced pumping efficiency
  3) Bone disorders
• Bone healing
• Cancer progression

Question 16

Describe PET

Answer 16

• Positron emission tomography
• Uses radionucleotides = positron emittors
• Annihilation event = positron collide with e- = 2 γ photons = 511 keV each
• γ photons travel in opposite directions
• PET uses complete detector rings = PMTs + conversion layer + collimator

Question 17

Define coincidence detection

Answer 17

• PET scanners designed to detect both photons simultaneously
• 2 γ photons expected = if 1 only detected scanner knows it’s not valid signal = rejected as noise

Question 18

Explain the detection of simultaneous γ photons

Answer 18

• When 2 detectors on opposite ends detect γ = single event = imaginary straight line between = direction of projection
• PET only accepts 2 γ + reject noise = PET images clearer + better signal-noise ratio than SPECT
• Electronics = ultrafast = detect simultaneous
• When detector detects = opens small time window = matching photon detection = if no match then detected as noise

Question 19

When is noise detected?

Answer 19

• 2 independent events occur
• Photon scattered + changes detection

Question 20

Give the uses of PET

Answer 20

• Inspect blood flow + O2 intake + metabolism of organs/tissues
  DETECT:
• Cancer
• Brain disorders
• CNS problems
• Heart problems
  CELLULAR LOOK:
• Brain tumors
• CAD
• Memory disorders
• Seizures

Question 21

Describe multi modality imaging

Answer 21

• SPECT/PET = location of radiophamaceutical accumation = activity of organ
• Structural imaging = CT/MRI
• Patient positioning = carefully reproduced between studies = if not causes inaccuracies
• To correct positioning errors = fused manually/software/both
• Dual scanners not widely used due to high costs

Question 22

Give dual modality scanners

Answer 22

• SPECT/CT
• SPECT/MRI
• PET/CT
• PET/MRI