Nuclear - Physics Flashcards
X-direction (horizontal) patient motion
- in the mid-portion of the sinogram diplayed there is abrupt discontinuity of the septal and lateral borders, indicative of lateral cardiac/patient motion
Describe how a signogram works
- sinogram is a stack of y-axis compressed (usually approximately 64) images obtained throughout the 180-degree SPECT acquisition
- the sinogram thereby tracks the motion of the heart throughout the 180-degree SPECT acquisition
- left border of the sinogram –> tracks the epicardial border of the septum
- right border of the sinogram –> tracks the epicardial boder of the lateral wall
What are the findings in a sinogram with:
- arrhythmic beat rejection
horizontal lines of decreased count density due to decreased count density in those individual frames
What can a linogram be used to detect
Y-direction motion
How should flood field non-uniformity be evaluated?
daily flood fields
For a Tc-99m sestamibi tetrofosmin myocardial perfsuion scan, setting the pulse height analyzer to 160 keV with a +/- 10% energy window:
- What will be the most apparent result?
Decreased image count density
- if the photopeak is set above the primary Tc-99m gamma ray energy (140 keV) most photons will be rejected, and image count density will markedly decrease
- 160 +/- 10% window accepts photons 144-176 keV
What is the standard energy window and peak for Tc-99m?
Why?
- Symmetric energy window: 15-20% centered around the
- Energy peak: 140keV
- this window width provides an appropriate tradeoff between
- accepting unscattered-photons
- and
- rejecting the scattered photons
- accepting unscattered-photons
What is the standard energy window and peak for most modern detectors:
- Tc99m
- Tl201
- Tc99m
- 9-10% for 140keV photons from Tc99m
- Tl201
- 15-17% for 72keV photons from Tl201
What is the standard energy window and peak for Tc-99m utilizing a CDZ detector?
energy resolution of 5-6%
For an ECG-gated post-stress myocardial perfusion SPECT scan acquired on a two-headed scintillation camera, rejection of 30% of cardiac cycles due to arrhythmic beat rejection will result in approaximately:
- what reduction in count density?
30% decrease in overall image count density
- when arrhytmic beats are rejected the camera stops acquiring data from the entire field of view (FOV), not just the myocardium
- percentage of rejected beats is roughly equivalent to the percent decrease in overall image count density, irrespective of the number of camera heads or stops
What defines definite infective endocarditis according to the Modified Duke Criteria?
- Pathological Critiera
- Microorganisms demonstrated by culture or histological examination of a vegetation, a vegetation that has embolized, or an intracardiac abscess specimen
- Pathological lesions; vegetation or intracardiac abscess confirmed by histological examination showing active endocarditis
- Clinical Criteria
- 2 major criteria; or
- 1 major and 3 minor criteria or
- 5 minor criteria
What are the minor criteria used in the Modified Duke Criteria for IE?
What are the major criteria used in the Modified Duke Criteria for IE?
Explain the difference between cameras:
- CZT-based SPECT cameras
- conventional sodium-iodide-based SPECT cameras
CZT-based cameras have a lower uniformity defect contrast trade-off
- CZT detector modules are smaller than conventional photon multiplier tubes (PMT) –> smaller pixel size
- smaller pixel size = improved spatial resolution
- Energy resolution is two-fold higher in CZT-based cameras
Describe the difference in crosstalk between:
- CZT-based SPECT cameras
- conventional sodium-iodide-based SPECT cameras
- Sodium-iodide-based SPECT cameras
- utilize photon multiplier tubes (PMT)
- PMT’s share photon signals because of the spreading of light between the PMT’s (crosstalk)
- crosstalk –> degrades effective pixel size
- CZT-based SPECT cameras
- utilize CZT detector modules (instead of PMT’s)
- have negligible crosstalk (due to direct conversion into an electric charge –> improved spatial resolution
Tc-99m sestamibi myocardial perfusion SPECT was performed in a 65 year old man. The VLA post-exercise tomograms below were processed without (A) and with (B) low-dose XR attenuation correction.
The decrease in anterior wall count density in the AC images (B) is attributable to?
Scatter from subdiaphragmatic radiotracer concentration
- by eliminating attenuation of the overlying abdominal wall, attenuation correction often accentuates subdiaphragmatic activity
- such increased activity may scatter into the inferior wall of the LV
- When myocardial tomograms are normalized to a resulting “hot” inferior wall, the contralateral anterior wall appears to have relatively decreased activity
- this may simulate an anterior perfusion defect in AC-corrected images
***In this case, non-AC images are normal for a male
Metal artifact reduction in the CT component of a PET/CT is advantageous in the assessment of PET myocardial perfusion imaging because such artifacts may:
impair PET relative uptake and quantitation
- the map of attenuation coefficients for PET attenuation correction scales is a monotonically increasing function of the CT Hounsfeld units (usually a bilinear function)
- higher HU’s –> higher attenuation coefficients
- PET image reconstruction compensates for the loss of signal (absorbed photons) in its AC algorithm
- Consequently, regions of artifactually high or low HU’s typically can translate to regions of artifactually high or low radiotracer uptake in AC PET images
The AC quality control figure shown, displays a fused image: the CT used for the attenuation map overlaid with a red contour showing the outlines of the myocardium on SPECT. What AC-induced artifact is most likely created if the MPI SPECT/CT misregistration shown?
Apparent uptake decrease in the lateral wall
- higher CT numbers (in units of Hounsfeld units) give areas in the attenuation map that are assigned higher attenuation coefficients
- misalignment is associated with an artifactual decrease in the apparaent uptake in the lateral wall of the ventricle –> amount of AC from this region will be underestimated during AC SPECT reconstruction –> artifactually photon deficient region
What will MPI SPECT/CT misregistration show?
- Septum
- Apex
- Septum
- artifactual increase in radiotracer uptake
- Apex (misregistration with AC map or motion with respect to AC map)
- causes apparent apical thinning
When using a CT scan for PET attenuation correction, voxel A has a CT number of zero Hounsfield units (HU) and voxel B has a CT number of 400 HU. What best describes the linear attenuation coefficients, μ(A) and μ(B)?
μ(B) > μ(A)
- HU are obtained from a linear transformation of the measured attenuation coefficients
- materials denser than water have positive pixel values
- Pixel with a higher positive value in CT consists of more dense materials with higher linear attenuation coefficient
- higher pixels = denser material –> higher HU’s = higher linear attenuation coefficient
How are pixel values in CT images expressed?
usually expressed in Hounsfield units (HU) on a normalized scale with respect to the values for water
What effect does attenuation correction in SPECT MPI have:
- Specificity
- Sensitivity
- Specificity –> improved
- Sensitivity –> not affected
Define attenuation coefficient
characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles or other energy or matter
What affects does AC SPECT MPI have in obese patients?
- Increases specificity in all patients
- Greater effect in obese patients
How can AC SPECT MPI be obtained in absence of a CT?
- applying attenuation correction requires a transmission scan
- this scan contains the tissue attenuation and is also known as the attenuation map
- such a scan can be obtained with a line-source using the same SPECT detectors
- but can also be obtained - in a better quality - with a low-dose CT scan
In which isotope is the radiative particle ejected from the nucleus a beta-particle when used in cardiac imaging?
O-15
- beta-positive or positron emitter used as O-15 water to measure myocardial blood flow in cardiac PET
In which isotope is the radiative particle ejected from the nucleus a beta-particle when used inc ardiac imaging?
O-15
- O-15 is a beta-positive or positron emitter used as O-15-water to measure myocardial blood flow in cardiac PET
- not FDA approved, used in research in Europe
What is Ra-223 (radium) and what is it used for?
- alpha emitter
- the Ra-223 decay chain will also emit beta-negative particles, but these are irrelevant for cardiac imaging
- used in treatment of castration resistant prostate cancer with bone metastasis
What is one property of Rb-82 that makes it impractical for offsite delivery for MPI?
The physical half-life of Rb-82 is too short for transport from offsite
- 75 second half-life of Rb-82 means that in under 12.5 minutes (10 half-lives), there will be less than one-thousandth ( < 1/1000) of the original activity left
- Rb-82 is generated onsite from a Sr-82-/Rb-82 generator
Myocardial sympathetic innervation can be imaged with MIBG-I-123 SPECT. Successful imaging protocols can be established for simultaneous imaging of I-123 and Tc-99m radiotracers. What physical priniciple obviates the establishment of similar simultaneous dual-isotope studies in cardiac PET?
Annihilation photons in PET cannot be discriminated by energy windowing
- the signal in PET is carried by the annihilation photons which all have an energy of 511 keV, regardless of the radiotracer
- radiotracer discrimination, based on discrimination of separate energy windows for the detected photons, cannot be used
- Two data sets can be constructed based on energy discrimination
- Because of the need for photon energy discrimination, the higher energy resolution provided by solid state CZTdedicated cardiac cameras has the potential of facilitating sumultaneous dual isotope imaging
What is a positron?
positively charged electron
- a nucleus with too many protons is unstable
- a proton in the nuclus is transformed into a neutron
- and a positively charged electron and a neutrino are ejected from the nucleus
Describe a positron-electron annihilation reaction?
- positron combines with the negative electron –> annihilation reaction
- massess are converted to energy
- mass-energy equivalent = 511 keV (two annihilation photons)
- photons leave the site of the event in nearly exact opposite directions (180 degrees apart)
In regards to SPECT MPI, what is the best way to:
- increase capacity
- maintain image quality
- decrease scan time
- utilize the lowest possible Tc-99m Tetrofosmin activity
Use CZT technology with iterative reconstruction techniques
- can improve image quality through noise reduction and therefore allow a reduction in acquisition time and/or administered activity
What mainly determines the scan quality in MPI SPECT?
What terms are interchangeable in this regard?
- amount of photon counts (mainly determines scan quality)
- scan time and activity are interchangeable
What is the effect of a more sensitive collimator on SPECT MPI?
- increase amount of photon counts
But
- decrease the intrinsic resolution and quality
No, the down-scatter of photons from Tc-99m into the lower TI-201 energy window will degrade the image
- without sophisticated algorithms to remove the high degree of cross talk, the TI-201 scan should be acquired on another visit
What is the half-life:
- Tc-99m
- TI-201
- Tc-99m –> 6.01 h
- TI-201 –> 73.1 h
The greater the scattering angle, the less energy the scattered photon has
- scattered photons degrade image quality
- the lower energy limit of the imaging peak (typically 126 keV for the 140 keV Tc-99m peak) is set to reject many of these scatered photons:
- ideally this would reject photons scattered through angles greater than 53 degrees
A narrow energy window setting
- it is possible to discriminate against scattered photons by using pulse-height analyzer window (PHA) or energy window
Describe the biologic elimination of setimibi
- Renal - 27%
- Fecal - 33%
***at 48 hours after injection
Increased attenuation
- photon attenuation is much higher for tissue than air, more photons are attenuated in heavier patients
- this effect (up to a certain range) can be compensated by a higher activity –> number of detected photons remains the same
What can be used to decrease compton scattering of photons?
Pulse Height Analyzer (PHA)
or
Energy Window
B - annihilation event
- annihilation event in which two photons are emitted in opposing directions
- however, one of them is scattered and its direction is slightly changed
- both photons are still detected within the set window time and recognized as a photon pair
- However, the line of response is not correct: a scatter coincidence
Describe the effects on PET scanner
- A
- shows a true event in which the line of coincidence (blue line) passes through the point of annihilation
Describe the effects on PET scanner
- B
- annihilation event in which two photons are emitted in opposing dirrections
- scatter coincidence
Describe the effects on PET scanner
- C
- two events but as they are not in opposite site of the scanner, they are not recognized as an event
Describe the effects on PET scanner
- D
- Random effect in which two separate events are recognized as one event
Describe the effects on PET scanner
- E
- Annihilation but one photon does not reach the detector due to attenuation and this event is therefore not registered
15 mm
- the original shielding contains 3 half value layers (HVL)
- 50% of the radiation is attenuated per HVL
- to obtain total transmission of 12.5% (or shielding of 87.5%) will need 3 HVLs
- 3 HVL’s (to produced 12.5% total transmission) x 5 mm =
- 15 mm
- 3 HVL’s (to produced 12.5% total transmission) x 5 mm =
The scan quality will be lower due to increased scatter
- increase in scatter cannot be compensated by either an increase in activity or scan time, or by altering the energy acceptance window and will therefore negatively impact the image quality
Collimator hole geometry
- resolution recovery models the collimator hole length and width to correct for:
- distance dependent blurring
- loss of resolution
What is a common setting for a PHA (or energy window) in Tc-99m clinical studies?
15% (130 to 150 keV)
or
20% (126 to 154 keV)
Decrease the size and severity of stress perfusion defects
- as the cutoff frequency is lowered, more high frequency data are eliminated from the reconstructed SPECT image
- lowering the cutoff frequency –> high frequency noise and also the sharpness and severity of discrete perfusion defects are decreased
Define Compton scattering
- gamma ray photon interacts with electrons in body and loses a part of its energy to an electron
- with reduced energy, compton-scattered photon is deflected from its original path
- photons scattered by a large angle have lower energy
- can discriminate against them using PHA or energy window
What will the relationship between scatter and higher activity be in a heavier patient?
Increased tissue layer –> increased scatter
When is resolution recovery not independent of photon energy?
when high energy photons result in septal penetration and further loss of resolution
What are new software methods utilized to preserve myocardial SPECT image quality despite lower myocardial counts?
- resolution recovery
- iterative reconstruction
- noise reduction/modulation
Which cameras do not require monthly center-of-rotation (COR) quality control tests?
solid-state CZT dedicated cardiac SPECT devices
- current solid-state CZT dedicated cardiac SPECT devices use stationary gantries
- COR is irrelevant to gantries that do not rotate during acquisition
Why should COR tests be performed monthly on conventional SPECT cameras?
a wobble in a loose detector head having potentially fatal clinical consequences
- would precede most catastrophic mechanical failures (rare crush injuries)
- would contribute greatly to the misalignment detected by COR
Describe a COR test
- determines if the heads of the system are aligned to the center of the field-of-view being imaged
- point source in the center of the field-of-view (FOV) should remain in the center of the projection data given by each head
- in unaligned systems, the projection of the point will drift as the heads rotate
- Correction:
- hardware
- reconstruction software
Describe the relationship in SPECT imaging between:
- collimators
- photon energy
less effective for radionuclides with a high photon energy
- the attenuaton coefficient decreases for photons with higher energy
- collimators with higher septal thickness are then required –> limit the efficiency of the collimator
Define collimator efficiency in SPECT imaging
- fraction of photons (Y rays) passing through the collimator per emitted photon by the source–> that actually project the Y-ray image onto the detector
- decreases for a larger hole length –> as less photons pass through the collimator
- typically only a few percent or less
Describe the relationship in SPECT imaging between:
- collimator length
- efficiency
- increasing length of a collimator by a factor of 2 –>
- efficiency will decrease by a factor of more than 4
Describe the relationship in SPECT imaging between:
- collimator diameter
- collimator efficiency
decreases linearly with decreasing diameter
What is the effect of smaller collimator diameter on intrinsic resolution?
improves intrinsic resolution
What is the effect of a collimator on “noise”?
- filters noise
- contributes greatly to “signal-to-noise” ratio
Why do dedicated multiple-pinhole cardiac CZT-SPECT cameras have higher spatial resolution?
increased number of detectors
- increased number of individual detectors - 19 detector modules (all consisting of)
- 32x32 detectors
- small pixel size (2.46x2.46mm)
- negligible crosstalk
What factors contribute to spatial resolution of a PET scanner?
- positron range
- non-colinearity
- detector element size
- pixel size in reconstructed images
What does “dead time” affect in FDG PET imaging?
affects the performance of a scintillation camera at high count rates
What is one key advantage of solid-state detectors used as current clinical SPECT cameras?
solid-state detectors offer better energy resoltuion with high detection efficiency
- they use silicon diodes instead of PMT’s
What is the optimal view in FPRNA for assessing:
- right and left ventricles
- RV function assessement
straight upright anterior view
- has the advantage of stabilizing the patient’s chest against the detector and positioning the patient so that the right and left ventricles will be in the field of view
shallow RAO view
- enhances right atrial-RV separation
What can first-pass radionuclide angiography (FPRNA) be used for?
- measure LV and RV function
- quantify atrial and ventricular shunts
Approximately what fraction of 140 keV gamma rays (technetium-99m) pass through a typical low-energy high-resolution (LEHR) parallel-hole collimator?
1 in 10,000 (0.01%)
- typical LEHR collimatory has a sensitivity of around 200 cpm/μCi for technetium-99m, which equals (3.3 cps)/3.7x104 Bq) = 0.9 x 10 -4
If a gamma camera with parallel-hole collimator is moved farther away from the patient, how are resolution and sensitivity affected?
Resolution worsens, sensitivity remains approximately constant
- collimator blur increases with distance –> decreased resolution
- number of counts passing through the collimator does not change appreciably as distance increases
For thallium-201 imaging, the photons detected in the 70-keV window are produced by which mechanism?
X-ray emission
- Tl-201 decays by electron capture to mercury-201
- mercury-201 is in an excited state and releases x-rays with energies near 70keV
What are reliable methods of AC?
- Transmission source AC
- CT-based AC
What are unreliable methods of AC?
calculated AC from external boundaries
- does not directly measure
- calculates based on assumptions of uniform attenuation
- used successfully for brain SPECT or PET since attenuation coefficient is approximately constant over the entire slice
- cardiac imaging has a large variance in attenuation coefficient (lungs vs. other tissues such as diaphragm and breast) –> making calculated method unreliable
How often should the energy peaking for a gamma camera be performed?
Daily
- should be checked each day before use
- to be sure the energy window has not drifted and
- energy spectrum is of the appropriate shape
- Energy specturm of a radionuclide source is acquired –> photopeak is checked (either automatically or visually) to confirm that it is centered in the energy window
What is the most likely cause of a photopeak shift, which causes progressive increase of the Tc-99m photopeak on a gamma scintillation camera?
High-voltage supply to the photomultiplier tube (PMT)
- PMT consists of a photocathode that is light sensitive and a series of metallic plates (dynodes)
- high voltage is applied to the plates –> when a light photon strikes the crystal –> the photoelectron is absorbed by the photocathode –> multiplied by each dynode –> pulse is generated, which is proportional to the amount of energy of the isotope interacting with the crystal
- voltage to these plates has to be very stable, and a slight variation will greatly affect the pulse generated
What test should be performed to check if all the PMT’s are working properly on gamma cameras?
Flood-field uniformity
- a uniform source/radioactive source (flood sheet) is placed on the camera detector to check if all the PMTs are operational
- if a PMT tube is malfunctioning, then a blank spot is observed on the flood scan
- Large white area shows the abnormality, the rest of the detector is uniform
Describe the frequency of quality control procedures:
- Energy peaking
- Uniformity test
- Linearity and resolution
- Sensitivity test
- Energy peaking - daily
- Uniformity test - daily
- Linearity and resolution - weekly
- Sensitivity test - weekly
What is energy peaking?
verifying that the pulse height analyser’s energy window is over the photopeak energy
What are two ways to perform energy peaking and Uniformity QC testing?
- 5 UFOV’s (universal field of view’s) from the uncollimated camera with a point source
- sheet source (Cobalt 57) with collimated camera
The interaction of a Tc-99m gamma photon with the scintillation camera is primarily by:
photoelectric effect
- gamma ray transfers all of its energy to an orbital electron of the absorbing material and a photoelectron is released
****Compton scatter –> occurs when the gamma photon transfers only part of its energy
What is the window setting for Tc-99m to be acquired within a 20% window?
126 keV - 154 keV
What is the best method to check for patient motion on a cardiac SPECT study?
check projection images in a cine format
What artifacts can be created by motion during acquisition in SPECT?
- misalignment of the apex
- matching 180-degree areas of decreased intensity in the anterior and inferior walls
- hotspots in the septum or lateral wall
- “blobs” or a ring of activity in the anterior wall or an “apical nipple”
In a cardiac SPECT study, the acquired planar projections are first reconstructed into which body plane?
Transaxial
In a multigated acquisition (MUGA) study, the cardiac cycle is divided in 20 equal time frames, and the patient heart rate is 60 beats per minute. What is the average time per frame?
0.05 seconds
- 60 s / (HR x number of frames)
- 60 / (60 x 20) = 0.05s
Why are dual-head camera detectors mounted next to each other in a 90-degree orientation to the gantry for a cardiac study?
the time is reduced in half compared to full 180-degree rotation
- typically 64 projections are obtained over 180 degrees at 3-degree increments, and each projection usually takes about 20s –> 21 minutes total required
- If 2 detectors at 90-degree orientation are used, the time to acquire 64 projection will be half –> 11 minutes
Define linear transfer energy (LET)
amount of energy deposited per unit length of the absorber
- units are expressed as keV/μm
Describe the linear transfer energy (LET):
- electromagnetic particles
- beta particles
- alpha particles
- electromagnetic and beta particles –> low LET
- lose little energy per interaction
- alpha particles –> high LET
- heavy particles and lose energy very rapidly
What type of attenuation is more prevalent in:
- males
- females
- males –> diaphragmatic (inferior) attenuation
- females –> breast (anterior) attenuation
What effects will AC and resolution compensation have on SPECT imaging?
Reduces the number of false-positive SPECT studies
- AC significantly improved the normalcy rate compared to uncorrected perfusion data using either the corrected images (96% vs. 86%) or the corrected data and quantitative analysis (97% vs. 86%)
- No impact on sensitivity
What is one disadvantage of PET compared with SPECT?
PET agents (Rb-82 and N-13) have limited availability
What percentage of 140-keV gamma rays (Tc-99m) is attenuated by 10cm of water (attenuation coefficient of water at 140 keV = 0.15cm-1)?
Interaction of gamma rays with the medium is dominated by which process?
- 78%
- Compton scattering
What are collimation methods currently used in SPECT cameras?
- Parallel-hole collimation
- Converging collimation
- fan-beam, cone-beam, cardiofocal
- Pinhole collimation
*****Diverging collimation no longer used
What is the main advantage of “solid-state” cameras using photodiodes coupled to pixelated scintillation crystals - as compared to conventional gamma cameras with large-area scintillation crystals with PMT’s?
Compact size and lower weight
- PMTs are several inches long and much larger/heavier than solid-state photodiodes
What are the main challenges associated with PET/MR?
- Accuracy of PET AC
- MRI safety associated with cardiac implants
- MRI safety associated with radiopharmaceutical administration
Compare resolution and sensitivity of PET/CT vs. PET/MR
similar
What are the advantages of PET scanners list mode acquisition of raw data compared to sinograms?
- Retrospective EKG or respiratory gating
- Troubleshooting patient motion
- Smaller data files (gated and dynamic studies)
***higher count statistics - no effect on the sensitivity of the camera or number of counts
What are advantages of iterative reconstruction (over filtered backprojection algorithms) in regards to methods addressing the blur in the raw data asociated with the detector and collimator?
- better image quality
- potential for lower radiation dose
- ability to recover some spatial resolution
- by modeling the scanner’s inherent resolution (aka point spread function) –> spatial resolution is enhanced (to some degree)
***need to acquire only half the projections
When using iterative reconstruction, the number of iterations is specified for stopping the program.
Why is this done?
to control excessive image noise
- a sufficient number of iterations is necessary to ensure convergence and accuracy
- however, a large number of iterations –> amplifies noise
- typical clinical protocol
- specifies an optimal number of iterations to limit noise
- while ensuring accuracy
- along with a filter to smooth out fluctuations in the reconstructed images
For cardiac PET acquired with a PET/CT, why is it important to control the respiratory phase of the CT portion of the examination?
the respiratory phase affects PET attenuation correction
- important during CT-AC acquisition to ensure that the respiratory phase is near end expiration to best match the anatomy of the free-breathing PET scan
- if lungs are inflated in the CT images, the position of the heart shifts relative to the PET data
What method of cardiac PET attenuation correction is not susceptible to artifact from metallic implants?
Transmission attenuation correction (using external Ge68 or Cs137 sources)
- transmission imaging with external sources involves photons with the same or similar energy, and the measurement of attenuation is accurate for PET attenuation correction for both body tissue and metallic implants
- CT photons have much lower energy (40-100 keV range) and interact with metallic implants primarily by photoelectric absorption and are much more strongly absorbed
A cardiac imaging department (without access to N13 or Rb82 generators) wishes to perform a rest/stress Tc-99m SPECT study in addition to a viability F18-FDG PET study, on the same day.
In which order should these studies be scheduled?
First perform rest/stress Tc-99m SPECT –> F18-FDG PET study
- Tc-99m SPECT collimators are designated as “low-energy collimators” –> collimate 140keV photons
- F18-FDG PET requires “high-energy” photons (511 keV) that would easily penetrate the collimator and contribute excessive background counts
