Molecular ImagingLecture 2

PET/CT NUCLEAR IMAGINGBY

Dr. H. Hawesahhawesa@ksu.edu.sa

Molecular Imaging

• Def: “ is a new biomedical research discipline enabling the visualization , characterization, and quantification of biologic process taking place at cellular and subcellular level within intact living subjects including patients”.

• Molecular imaging Includes: PET, US, MRI, CT

Positron Emission Tomography

• A technique involves detection of two gamma photons (each 115 KeV).

• Positron emitting radionuclides are cyclotron produced (located close to PET scanner).

• Fluorine-18 [T½=110 min] is commonly used (<2 hours).

• PET allows high quality, quantitative Imaging.

• Availability of radiotracers through commercial distribution network.

• Mobile PET Imaging services.• Payment approved by medical insurance

companies.• New generation of dual imaging modality

(PET/CT).• Currently>1,000 PET/CT scanners are being sold

worldwide than PET alone.• One PET/CT scanner for each 1 million

population.

Difference between PET & SPECT

• No need for conventional collimation (electronic).

(PET has better sensitivity due to electronic collimation).

• Attenuating path through patient is independent of the exact location of the annihilation event.

Basic Physics of PET

Positron Decay & annihilation• P→ n + β⁺ + v• Β⁺ emitted from the nucleus . Its travels a

short distance, losing energy.• Β⁺ interact with e.• Both β⁺ + e are annihilated.• 2 gamma photons are produced.• 511 KeV (~ 180 ). ̊

Coincidence Imaging

Raw Data & image reconstruction

Positron Radionuclide for PETRadionuclide T½ Maximum

range (mm)Mean range Emax Energy

(KeV)

¹¹C 20 min 5.0 0.3 960

¹³N 9 min 5.4 1.4 1200

¹⁵O 2 min 8.2 1.5 1740

¹⁸F 110 min 2.4 0.2 663

⁸²Rb 75 sec 15.0 2.6 3400

Choice of PET Detector

Characteristics of PET detectors:1. High stopping power [High] -( efficiency of the detector to absorb the total energy

of 511 KeV).2. Higher light output (better energy resolution).-light output per KeV of photon energy.3. Decay time of light-(short decay time = higher efficiency of the detector).

PET scintillation crystal

- BGO = Bismuth Germanate- LSO = Lutetium Oxyorthosilicate- Na(TI) = Sodium Iodide

- GSO = Gadolinium Oxyorthosilicate- YSO = yttrium Oxyorthosilicate (not used in PET technology)

- BaF₂ = Barium Fluoride ( shortest decay time, rarely used)

Detectors

• Axial FOV is defined by width of the array of the rings

• No. of rings 18-32 (depend on manufacturer).• No. of detectors per ring ranges in thousands

More no. of detectors/ring →More PMTs(better spatial resolution) Width of detector elements (small element, better resolution) [3-5 mm in modern

PET).

Detector configuration in PET

Typical Configuration of PET Detector

• 8×8 elements (64 elements)connected to 4 PMTs

• 4×4 elements (16 elements/ 1 PMT)• No. of crystals ( 9,000-18,000)• Spatial resolution (5-7 mm)

Coincidence events (T,S,R)True (T), Scattered (S), Random (R)

Acquisition mode • 2D: (+ collimators)- Detectors separated by lead septa (collimator)- Sensitivity to true events is decreased - Scattered events is reduced. • 3D: (- collimators)- Without collimator or septa- Sensitivity to true events ↑ by 5 times- No. of Random events increased - Scattered events are increased- Require 3D reconstruction algorithm

Performance Parameters of PET Scanner

• Performance parameters:1. Resolution2. Sensitivity3. Noise4. Contrast5. Scattered radiation

Spatial resolution

1. Positron range (e.g. in water)(F-18 with Emax of 640 KeV, B range = < 1mm)Contribution = 0.2 mm on FWHM(Rb-82 with Emax of 3,350 KeV, B range = 10mm )Contribution = 2.6 mm on FWHM2. Noncolinearity of annihilation photon- Angle of noncolinearity = ± 0.25°Contribution = 1.5-2.0 mm on FWHM for 30-90 diameter3. Small crystals grouped in block detectorContribution= ↓ resolution due to miss positioning of events in 4

PMTs

sensitivity• It’s a measure of counting efficiency of a PET scanner• Sensitivity depend on :1. Geometric efficiency

Distance between the source & the detector Diameter of the ring Number of detectors in the ring

2. Detection efficiency (detector material, scintillation decay time)- LSO & GSO detectors are preferred to BGO ( better detector efficiency3. PHA window4. Dead time of the system- Sensitivity is ↑ at the center of FOV & gradually ↓ toward the periphery- As no. of rings ↑, sensitivity ↑- In 3D mode , sensitivity ↑ by factor of 4to 8 compared to 2D mode [in 3D,

scatter and random event ↑ significantly]

Noise

• Image noise is the random variation in pixel counts across the image

• Noise is given by [(1/√N)×100] N=total counts in pixel

• Noise can be reduced by:- Acquiring data for a long time- Injecting more radiopharmaceutical - Improving the detection efficiency of the PET

scanner

Contrast

• It’s a measure of the delectability of an abnormality relative to normal tissue

• Factors affect the contrast:1. Count Density2. Scattered radiation3. Size of the lesion4. Patient motion

Scatter Fraction (SF)

• Scatter contribution ↑ with: - density & depth of the body tissue - window width • Scattered radiation ↑ the background to the image,

thus ↓ the image contrast• In 2D mode, septa removes scattered events • SF is a parameter used to check the PET performance • SF = Cs / Cp Cs=scattered counts rate, Cp=prompt counts rate

• The lower the SF, the better the image quality.

Summary

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