PET. Technical aspects

Transcription

1 PET Technical aspects

2 15 N 15 O Detector 1 β+ Detector 2 e-

3 Evolution of PET Detectors CTI/Siemens

4 15 N 15 O Detector block 1 β+ Detector block 2 x e- x y y location line of response Constant fraction discriminator Constant fraction discriminator

5 PET Detectors Most modern PET system use a different detector technology where a large number of scintillation crystals are coupled to a smaller number of PMTs. For a given crystal, the lower the cross section is the shorter the length of the crystal has to be due to the light transport.

6 Scintillator Materials NaI (Tl) BGO GSO LSO LYSO LaBr 3 Density [g/ml] /µ [cm] ~2 Index of Refraction Hygroscopic Yes No No No No Yes Rugged No Yes No Yes Yes Yes Peak Emission [nm] Decay Constant [ns] Light Output >100 Energy Resolution <

7 PET Detectors In the block detector, a matrix of cuts are made into a solid block of scintillator material to define the detector elements. The depth of the cuts are adjusted to direct the light to the PMTs. The light produced in each crystal, will produce a unique combination of signals in the PMTs, which will allow the detector to be identified. The more light output the more crystals can be identified.

8 PET Detectors To identify the detector elements, the two following ratios are calculated (similar to the logic used in scintillation cameras). A+ B C D X = A + B + C + D A B+ C D Y = A + B + C + D Below is the flood response (i.e., X and Y density distribution when exposed to a flood source of 511 kev photons) for a block detector from the ECAT EXACT HR. Element (4,4) Element (5,3)

9 The Technology : HiRez Standard Detector 6.4 mm x 6.4 mm 64 crystals/block 144 blocks/scanner 9216 crystals/scanner 3.4 mm slice width 47 slices HI-REZ Detector 4.0 mm x 4.0 mm 169 crystals/block 144 blocks/scanner crystals/scanner 2 mm slice width 81 slices

10 Detector block 1 Detector block 2 x x y y location 12 ns AND line of response coincidence? Constant fraction discriminator Constant fraction discriminator

11 Scintillator Materials NaI (Tl) BGO GSO LSO LYSO LaBr 3 Density [g/ml] /µ [cm] ~2 Index of Refraction Hygroscopic Yes No No No No Yes Rugged No Yes No Yes Yes Yes Peak Emission [nm] Decay Constant [ns] Light Output >100 Energy Resolution <

12 Detector block 1 Detector block 2 x x y y location 12 ns AND MCA line of response coincidence? Energy spectrum Constant fraction discriminator Constant fraction discriminator

13 Scintillator Materials NaI (Tl) BGO GSO LSO LYSO LaBr 3 Density [g/ml] /µ [cm] ~2 Index of Refraction Hygroscopic Yes No No No No Yes Rugged No Yes No Yes Yes Yes Peak Emission [nm] Decay Constant [ns] Light Output >100 Energy Resolution <

14 Event Types True Event Scattered Event Random Event Multiple Event

15 Random Coincidences Because of the finite width of the logic pulses that are fed into the coincidence circuit, there is a probability for random or accidental coincidences between unrelated events. True Coinc. Single Event Random Coinc. Detector 1 Detector 2 Time τ True Coinc. Single Event

16 Random Coincidences If N 1 and N 2 are the individual average count rates of detector 1 and 2, respectively, then it can be shown that the random coincidence rate for the pair of detectors is: N R = 2τ N 1 N 2 where 2τ is the coincidence window (or τ is the width of the singles pulses)

17 Random Coincidences Randoms can be estimated by adding a second coincidence circuit. The logic pulse from one detector is delayed beyond the time resolution of the detector pairs. This does not require any knowledge of the coincidence time window, but is not as statistically accurate as the singles method Amp./ PHA Amp./ PHA Coinc. Delay Prompts Coinc. Randoms

18 Estimation of Random Coincidences Singles Method Monitor singles for each detector Statistically accurate Requires accurate knowledge of τ Dead-time different between prompt coinc. & singles Delayed Coincidence Method Separate coincidence circuitry/processor Poor statistical accuracy (improvement with variance reduction)

19 Scatter Scatter can be reduced by better energy resolution more collimation (2D)

20 Signal-to-Noise True Coincidences ~ Activity Good events! / ~ T S N T

21 Signal-to-Noise Random Coincidences ~ Activity 2 Can be accurately corrected for Correction increases image noise Detector material dependent S/ N ~ T T + 2R

22 Signal-to-Noise Scattered Coincidences ~ Activity Reduces Image Contrast Requires correction Analytical estimation Correction increases image noise S/ N ~ T T + S+ 2R

23 Signal-to-Noise Multiple Coincidences: ~ Activity 3 Never saved Source of Dead time

24 Noise Equivalent Count Rate - NEC The correction for random and scattered events increases the noise in the net true events The correction therefore reduce the effective true count rate The NEC describes the equivalent true count rate in the absence of random and scattered events: NEC = T T + S + 2kR ~ S / N

25 Count Rate [cps] Trues Prompts Randoms NEC Specific Activity [MBq/ml]

26 Spatial Resolution The spatial resolution in PET is primarily determined by: Detector size Physics of positron decay System geometry Detector material

27 Spatial Resolution For a source placed at the midpoint between two scintillation detectors with a width w d, the geometric line spread function has a triangular shape with a FWHM of w d /2. w d FWHM = w d 2

28 Spatial Resolution -Tangential For sources located between the midpoint and the detector surface the LSF will have a trapezoidal shape with width varying from w d /2 (at the center) and w d at the detector surface.

29 Spatial Resolution - Radial Transaxial Resolution or ECAT EXACT HR Axial section Radial FWHM (mm) 8 Tangential FWHM tang FWHM rad R= R (cm)

30 Depth-of-Interaction cm etector eparate

31 Depth of Interaction Decoding Techniques Multiple layers of scintillators with different decay constants. Wong, 1985; Carrier et. al., 1987; Casey et. al Additional light readouts on the scintillator. McIntyre et. al., 1980; Moses et. al., 1991 Modulation of light output Rogers et. al., 1995

32 30 mm 2x15 mm 3x10 mm 4x7.5 mm 30 cm 40 cm UCLA School of Medicine

33 DOI Detectors Siemens Molecular Imaging

34 SSPM/SiPM UCLA School of Medicine

35 SSPM/SiPM Counts ns FWHM Channel UCLA School of Medicine

36 Avalanche Photo Diodes (APD) vs. Photo Multiplier Tubes (PMT) PMTs Size 10-52mm dia. APDs 5x5mm Gain Up to 10 6 Up to 200 Rise time ~1 ns ~5 ns QE 20% 70% Magnetically sensitive insensitive

37 Spatial Resolution Although the most energetic positrons can travel several mm before annihilating, only a few of these are emitted. The average positron energy emitted is approximately 1/3-1/2 of the maximum energy. The total path length the positrons travel is not along a straight path. Through inelastic interactions with electrons in the positrons path is deflected. The distance from the mother nucleus is therefore much shorter.

38 Positron Range From Levin & Hoffman PMB 44, 1999

39 Resolution loss due to the positron range Isotope β + Energy FWHM FWTM [MeV] [mm] [mm] 18 F C N O m Rb

40 Positron Range ~65% ~50% 18 F 635 kev 124 I 1.53 & 2.14 MeV

41 Non-colinearity ~0.3 FWHM 100 cm Ø ~ 2.5 mm FWHM 15 cm Ø ~ 0.3 mm FWHM

42 Spatial Resolution The measured resolution (intrinsic resolution) of the system is a convolution of the various resolution components. If the different resolution components are assumed to be Gaussian in shape and are described by a FWHM then the combined resolution is the squared sum of the individual resolution components: FWHM = FWHM + FWHM + FWHM total det ector positron angulation FWHM = FWHM + FW M + FWHM total det ector 20% positron angulation Sánchez Crespo et. al. EJNM, 2004

43 Improved NEC Performance

44 Time-of-flight PET R R Det 2 Det 1 x R - x R + x s= vt R + x = vt R x = vt x= vt ( t) x= 2 1 c t 2

45 Time-of-flight PET For ideal detectors, TOF would eliminate the need for image reconstruction, since the measurement would allow each event to be accurately positioned in space. All detectors have a finite time resolution, or uncertainty in timing. This translates to an uncertainty in positioning. BGO ~ 5 ns NaI ~ 1.5 ns CsF, LaBr 3 ~ 0.45 ns BaF 2, LSO, LYSO ~ 0.3 ns 75 cm 22.5 cm 6.7 cm 4.5 cm

46 Time-of-flight PET Figure 1. Image elements contributing to a LOR, for conventional PET (left) and TOF PET (right).

47 Time-of-flight PET Even with a finite time resolution, using the TOF information an improvement in signal-to-noise ratio (S/N) can be achieved: 2 D D SNR SNR = SNR x c t TOF conv. conv.

48 Can we see TOF improvement? non TOF TOF 5 min 3 min 1 min 6-to-1 contrast; 35-cm phantom Noticeable improvement with TOF with large size phantom J. Karp, U of Penn

49 QUALITY CONTROL (QC)

50 Quality Control Ensure operational integrity of the system Maintain consistent and high image quality Minimize chance for artifacts Catch potential problems early Maintain quantitative accuracy

51 PET System, storage of data Each detector is in coincidence with a sector of opposite detectors. The angle of the section defines the Field of View (FOV)

52 PET System, storage of data All coincidence lines that are parallel at a given angle form a projection which is the accumulated radioactivity in that angle (1D) of the object (2D). In the computer we store the projection in a matrix x-axis radioactivity profile, y-axis the different angles

53 Projection & Sinogram Projection: All ray-sums in a direction P(θ,t) y t Sinogram: All projections θ π θ x f(x,y) X-rays Sinogram t

54 PET System θ r In most PET scanners, a large number of scintillation detectors are arranged in a circle. Each detector is in coincidence with a number of opposite detectors. The field-of-view (FOV) of the scanner is defined by the width or the angle of the fan. All coincidence lines that are parallel at a given angle form a projection in the sinogram.

55 PET System θ r All coincidence lines (or lines of response) for a given detector form a diagonal trace in the sinogram.

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57 Daily QC / Blank Scan

58 The sinogram as a help to identify errors Normal QC Detector Failure Detector Controller Failure Coincidence Processor Failure

59 Buchert et. al. JNM 40, 1999

60 Daily QC / Cylinder Scan Before Normalization After Normalization

61 Daily QC Procedure Daily Detector Check Sources & Phantoms needed Transmission/Rotating Rod Sources Uniform 68 Ge cylinder phantom

62 Daily / Weekly Quantitation Scan Scan uniform 20 cm Ø 68 Ge or 18 F Cylinder Reconstruct: All corrections applied Standard reconstruction parameters Visual inspection Compare image ROI activity to calibrated activity Always perform after: Service Re-tuning Re-normalization

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64 Quantitation 2D Cylinder ( µci/ml) FBP OSEM Calculated Atten. Corr % % Meas. Atten. Corr. CT % % Meas. Atten. Corr. Rods % %

65 Quarterly QC Procedures Detector setup (if needed) PMT tuning Detector setup Coincidence timing Normalization Gantry alignment (for PET/CT) Always after Service Software upgrades Other calibrations (well counter, etc)

66 Jaszczak Phantom

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68 PET/CT Gantry Alignment CT PET

69 Annual QC Procedures Perform a sub-set of the acceptance test: Uniformity Resolution Count Rate Test Dead Time correction

70 Test Phantoms