Positron Emission Tomography. Charles Laymon

Transcription

1 Positron Emission Tomography Charles Laymon

2 Outline PET Examples Imaging Goal Reconstruction/Data Requirements Method of Data Acquisition in PET Positron Decay/Annihilation Detectors/Scanner PET Tracers Attenuation Degrading Effects Combined Modalities: PET, CT, MR

3 Starting Point / Overview Patient administered radioactive drug or tracer that follows a metabolic pathway of interest. --Example: F-18 labeled Fluoro DeoxyGlucose (FDG) is a glucose analog that traced the first steps of glycolysis Emitted gamma rays detected using PET scanner. Acquired data reconstructed into 3D image of tracer distribution (perhaps as function of time). à Thus PET = Tracer + Scan

4 PET Scan Examples

5 PET/CT FDG Breast Cancer (Extent and location) Tracer: [F-18]fluordeoxyglucose_ ( [F-18] FDG) A glucose analog, Goes to regions with high glycolitic rate. CT PET

6 Colon Cancer (Extent and location) 10.9 mci FDG 6 X ( 4 min Emission min Transmission) = 39 min

7 Lymphoma Pre/Post treatment FDG-PET MIP Images Initial Exam Follow up (3 month)

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9 Example of Different Tracers MRI, T1+C FDG PET FLT PET (different patient) FDG Glucose metabolism. Normal gray matter tissue has high glucose metabolism. FLT (fluorothymidine) DNA synthesis/cellular proliferation. Normal brain has low signal.

10 Example of Different Tracers F-18- labeled PET apoptosis (programmed cell death tracer PET images (color) overlaid on MR (gray)

11 Example of Different Tracers Raclopride binds to dopamine receptors (D 2/3 antagonist) Narendran et al. Synapse 63: (2009)

12 Imaging Goal Main point: All nuclear medicine imaging studies involve administration of a molecule tagged with a radioactive atom (radiopharmaceutical or radio tracer). Purpose: As opposed to some other modalities, the purpose of nuclear medicine is to provide functional information. Contrast this with, for example, xray and CT procedures, in which we are mainly looking at structure. The particular function that we examine in a nuclear medicine mainly depends on the radiopharmeceutical used.

13 Imaging Goal Example: CT image of chest shows structure. Nuclear Medicine (PET) image shows metabolic activity. Tracer: [F-18] FDG Overlaid PET / CT image

14 Overview of Image Reconstruction

15 We treat as a 2-dimensional problem

16 We treat as a 2-dimensional problem Axial direction Transaxial plane

17 We treat as a 2-dimensional problem Transaxial plane

18 2-dimensional slice

19 Goal: Obtain image or map of some property (for example radioactivity distribution) of this patient. Constraint: Have to work from outside (no slicing allowed).

20 Definition: Line of Response (LOR): A line transecting the object.

21 With a complete set of LOR s, every point in the object is intersected by lines in all directions.

22 With a complete set of LOR s, every point in the object is intersected by lines in all directions.

23 Summary Input: integral of desired quantity for all LOR s in object Nuclear Medicine In: Line integrals of radioactivity concentration. Out: Image of radioacitity concentration Output: map of quantity for entire object

24 Reconstruction The point of this is The data we need require that we know: 1. where an emitted gamma ray hits the detector; 2. the direction from which the gamma ray came. In SPECT we use collimators. PET uses a different technique to get the same information.

25 Method of Data Acquisition in PET

26 Positron Decay Closeup Beta Decay: β + ν e + p This decay is not allowed for a free proton (energy conservation) n Initial State Final State

27 PET : Positron Emission Tomography Some neutron deficient nuclei decay by positron emission (β + ) decay. Example: F-18 O-18 + e + + ν Half life: 110 minutes

28 PET Positron - Electron annihilation Positron comes to rest (total distance traveled ~ 1mm) and interacts with ambient electron

29 PET Positron - Electron annihilation Result: Two back-to-back 511 kev photons traveling along a line that contains the point at which the annihilation took place (energy and momentum conservation)

30 PET In PET, the LOR upon which an annihilation took place is defined by the coincident observation of two 511 kev photons Gamma detectors Coincidence: Look for events within time τ of each other. (typical τ: few ns)

31 The PET Scanner

32 PET

33 PET Detectors The PET scanner consists of a cylindrical grid of blocks, each containing a number individual detectors 15 cm (typical)

34 Block Detector Photomultiplier(s) Scintillation Crystals Head on view Gamma ray hits crystal It may interact producing scintillation light Scintillation light is detected by photomultiplier tubes (PMTs) Struck crystal determined by light distribution in PMTs

35 Example Block Detectors 6.4 mm x 6.4 mm 8x8 crystals/block 4.0 mm x 4.0 mm 13x13 crystals/block 6.3 mm x 6.3 mm 6x6 crystals/block 4.7 mm x 6.3 mm 8x6 crystals/block Most Common PET Scintillators: Bismuth germanate (BGO) Lutetium oxy-orthosilicate (LSO)

36 Open PET Scanner. Block detector housings are visible.

37 PET Nuclides and Tracers

38 Positron Decay Nuclide half-life C min N min O sec F min Rb sec e.g., 18 F 18 O + e + + ν

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41 Attenuation and Attenuation Correction

42 The Problem: Attenuation of radiation by the patient In a nuclear medicine study a gamma-ray emitted within the patient may be reabsorbed. Thus the quantities that we measure for each LOR are not just integrals of the radioactivity distribution. Instead they are a complicated function of both the activity distribution and the patient attenuation properties.

43 Attenuation Correction In PET, we can make an exact attenuation correction by dividing the counts recorded on each LOR by the coincidence attenuation probability (or attenuation factor [AF]) for that particular LOR. Corrected Counts= (Recorded Counts)/AF (This is not true in SPECT.) Notice that the correction is applied to the raw data before or as part of the reconstruction.

44 Attenuation Correction The required AF s can be determined by performing a transmission measurement using an external radiation source. In which the source orbits about the subject. Positron (511 kev photon) source Sources were an integral part of a PET scanner. Modern day PET/CT scanners accomplish this using the CT data.

45 Attenuation Effects Attenuation Corrected Not Attenuation Corrected x-ray CT

46 Degrading Effects

47 Degrading Effects Scatter Randoms Limited Spatial Resolution Limited Counts -> Image Noise

48 Scattered Coincidence In-Plane Event Out-of-Plane Scatter Fraction S/(S+T) ~30-80%

49 Scatter Control 1. Scattered events have energies less than 511 kev. Using a tight energy window eliminates some scatter events. However the energy resolution of scintillators used in PET (BGO, LSO, etc.) is not so great. Therefore if we make the windows too tight, we lose good events.

50 Scatter Control 2. There are several procedures for estimating the distribution of scatter in the PET raw data or images. The estimated scatter is then subtracted. Images for quantitative use must have a scatter subtraction performed.

51 Random Coincidence Event b ±τ a R R =2τR a R b

52 Random Compensation Very good estimates of randoms can be made. Method 1: monitor the rates in the detectors to deduce the randoms rates. Method 2 Delayed coincidence : For each detector hit, look for coincidences after a delay (i.e. look at the wrong time). There will be no true coincidences, only randoms.

53 Noise Due to counting statistics including the effects of scatter and random compensation More counts Fewer counts

54 Spatial Resolution Limits Detector Size Smaller crystal elements yield better resolution.

55 Spatial Resolution Limits Positron Range Positron moves before annihilation Size of effect depends on nuclide, typically on the order of a millimeter

56 Spatial Resolution Limits Opening Angle Gamma rays emerge with angles slightly different than 180 o due to center-of-mass motion of positron/electron pair. Angular blurring of few tenths of a degree. Effect on resolution proportional to ring diameter. Typical Resolution in a modern PET scanner 4-6 mm. (Not uniform throughout the field-of-view)

57 Combining Modalities PET and CT and MR

58 PET/CT

59 PET/CT Systems All new systems sold in the USA are now PET/CT (or PET/MR).

60 Hardware fusion: function + anatomy PET PET/CT CT FDG-PET

61 NHL-Better Localization Case: 53 y/o male with hx of NHL s/p chemotherapy with c/o " "weight loss and pain for follow-up PET/CT" Findings: Two foci of intense FDG uptake in soft tissue adjacent to bones" "consistent with malignancy."

62 Hardware fusion: function + anatomy A combined PET/CT scanner allows automatic correlation of functional image (PET) with anatomy (CT) The CT data can be used for producing the attenuation correction Note that for practical* and technical reasons PET and CT are basically separate units within a single gantry. Thus the PET and CT scans are acquired sequentially. *PET scans are generally much longer than CT scans

63 PET MR Emerging technology combining PET and MR Data processing/reconstruction issues: How do we get a 511 kev mu map (required for PET reconstruction) from MR? This issue is still being addressed à Improved pulse sequences allowing improved identification of tissue types à Post acquisition image processing methods to identify tissue types and assign appropriate µ-value

64 PET MR Despite challenges commercial systems are becoming available Philips Two separate back-to-back units Scans are acquired sequentially (Similar to PET/CT)

65 PET MR Despite challenges commercial systems are becoming available Siemens Integrated units PET and MR fields of view overlap True simultaneous acquisitions possible

66 PET MR Emerging technology combining PET and MR Technological difficulties: Distortion of magnetic field (B) by PET components Effect of high B field on PET (Example : photomultiplier tubes, a main component of traditional PET scanners are useless in a high B field) à Replace photomultipliers with solid state avalanche photo diodes