Nuclear Med Flashcards
Regions of chamber response graph
(V vs current)
Recombination
Ion saturation
Proportional
Limited proportional
Geiger Muller
Use ion saturation so small variations in voltage don’t affect current
4 types of nuclear medicine acquisition and examples
Static / whole body - bone
Dynamic - renogram
Gated imaging - MUGA or gated MPS
Tomographic imaging - DaTSCAN
Parameters used in gamma camera protocol
Image matrix - higher, increased resolution but lower, more counts and reduced noise
Collimator - higher res, better resolution but higher sens, more counts
Imaging time - longer, reduced noise and more counts but patient might move
Energy window - narrower, less noise but less counts too
Examples of quantitative analysis in NM
Regions of interest: relative renal function DMSA, striatal uptake DaTSCAN
Time activity curve: uptake and drainage MAG3, LV ejection fraction MUGA
What tests are perfumed on eluate post elution
Radionuclide purity - molybdenum breakthrough test
Chemical purity - aluminium breakthrough test
Energy of PE event and compton scatter for Tc99m
140keV PE
~40keV is upper energy event
Why use NaI
Efficient absorber of gamma rays
Signal proportional to energy deposited in crystal
High output of photons per keV - good energy res
Lifetime of excited state is short so high count rate
Transparent to own light
Equation for energy of photoelectron
E = (hv-w)eV
(v is frequency and w is work function)
What is effective half life
Takes into account physics half life and biological half life
What is CoV
Opposite of SNR - 1/root n
What three things can collimator geometry effect
Sensitivity, resolution, magnification
What is the result of low radiochemical purity of Tc99m
Free pertechtenate - taken up by thyroid and stomach. Could mask something else or cause confusion - also irradiates organs that wouldn’t have been otherwise.
Advantages of SPECT
Depth information
Increases contrast - removes effect of overlying tissues
Can do hybrid imaging
Compensation with attenuation and scatter possible
What compensations can be applied to SPECT
Main compensations are for:
Attenuation (use map of change due to attenuation - counts in centre are boosted)
Scatter (photons scattered more than absorbed, some in photopeak is scatter. Can use multiple energy windows to estimate amount, best done using CT information)
Collimator resolution (model effect of collimator parameters and take this into account)
Two brain scans
DaTSCAN - function of neurons in striata
HMPAO - brain perfusion scan
Advantages and disadvantages of FBP and OSEM
FBP: + computationally quick and okay for visual interpretation. - streaking/noise, difficult to apply image compensators - not qualitative technique, only view images
OSEM + physics can be modelled into iterative process, use of subsets speeds up process - can be slow and relies on accurate models, optimal number of iterations varies
HMPAO brain scan
99mTc-HMPAO.
Crosses BBB and binds in proportion to regional cerebral perfusion
Doesn’t measure absolute flow: reference to unaffected area
Used in neurodegredation and epilepsy
DaTSCAN
123I - cocaine analogue
Binds to dopamine transporters in striata
Doperminergic neurons reduced in parkinsons and dementia with lewy-bodies - low signal on scan
Can use automated software to compare to norm - ROIs
Geometric mean equation
C1 = root (CA1 x CP1)
Takes into account posterior and anterior counts so depth shouldn’t matter as much
How is Mo99 generate?
Fission of U235 (which produces other products which could contaminate)
What is copper filter used for
Filter out low energy x-rays that calibrator is sensitive to. 0.5mm copper. Need new calibration factor
Can we measure beta emitters in radionuclide calibrators?
Yes - measure bremsstrahlung radiation produced rom wall of vial/syringe and chamber and in source itself.
Kidney DMSA scan
Injection of DMSA accumulates in renal cortex
Delay of 2-4 hours
Assessment of renal function or scarring
Static
MAG3 renogram
80MBq MAG3
Images acquired immediately
Assessment of obstruction, relative renal function
Dynamic
3 phases of kidney processing
Perfusion 1-30s
Filtration/uptake 1-5 minutes
Excretion 5 minutes onwards
What do we need to consider in dynamic imaging?
Collimator (want sensitivity over resolution)
Frame rate (needs to be appropriate for speed of physiological process)
Matrix size (appropriate to spatial resolution of collimator)
Matrix size
Pixel size should be =< 1/3 FWHM
(if FWHM is 9mm, pixel size should be 3mm, if FOV is 400mm^2 then matrix size is 400/3, 128x128)
Frame rate throughout
Perfusion: 1s for 60s
Filtration/excretion: 20/30s for 30-40 minutes
Radiopharmaceuticals used in renal imaging
99mTc-DTPA
99mTc-MAG3
99mTc-DMSA - STATIC only, not filtered so can image later on
DTPA vs MAG3
DTPA:
Excreted by glomerular filtration (20% extraction efficiency)
200MBq dose
Effective dose 1.3mSv
Low cost
MAG3:
Excreted by active filtration / tubular secretion (80% extraction efficiency)
High extraction efficiency
Higher specificity
100MBq dose
Effective dose 0.7mSv
Higher cost
Prep for renogram
Drink 0.5L water
Void before start of test
Lay supine over camera
IV injection at start of imaging
Take image post voiding afterwards
Background correction for renogram
Needs to be done to remove non-kidney sources. Planar - counts above and below
Kidney ROI = K
counts in kidney ROI = K_counts
Background area = B
counts in BG = B_counts
K_corrected = Kcounts - (B_counts x K/B)
Quantitative assessment of renograms
Relative renal function (% in each kidney)
Time to peak
True obstruction vs non-obstructive dilation
If a diuretic is administered and there is a drop-peak/2 in less than ten minutes there is no permanent obstruction, if there is no drop in over ten minutes there is a true obstruction.
What is gated imaging
Recording images against time. Putting images into bins.
MUGA scan and scan parameters
MUltiple-Gated Acquisition scan.
Main aim is to measure Left Ventricular Ejection Fraction - amount of blood ejected per heart beat.
LEGP collimator to maximise counts
128x128 matrix - maximise counts vs getting resolution
Pixel size 2-4mm
Gamma camera at 45 degrees so can separate ventricles.
800MBq, effective dose ~7mSv - high so can get adequate count rate in comfortable time
Used to see damage post chemo or monitor cardiac patients
MUGA prep
Inject PYP before injection, adds tin to red blood cells
Patient blood sample then taken and labelled with Tc-99m and reinjected so we can follow it
How does MUGA work and what problems can there be
Get ECG while we acquire count data. Divide ECG into 8 or 16 bins between R-R intervals. All counts in each interval get put in that bin.
Issues rise from variable R-R, some patients have irregular ECGS. Can do rate tracking, beat rejection (that are more than 10% either way) or list mode
Eq for LVEF
(EDV-ESV)/EDV x 100
EDV is end diastolic volume and ESV is end systolic volume
Myocardial perfusion imaging goal
Image blood perfusion to left ventricle for diagnosing coronary artery disease. Looking at uptake in myocardium
Can also get LVEF
SPECT
Get 3 planes
Bullseye plot allows comparison to norm, 3D –> 2D
Why stress the heart?
Good for demonstrating reversible ischemia. Less blood volume at rest so may not be noticeable but problems are clearer under stress
Why are there different buttons for different radionuclides?
Current measured by the calibrator is not the same for two different types of radioactive source of the same activity.
Current measured also depends on the type of radiation and the energy of the radiation. Therefore each type of radioactive source has a dial setting that must be applied so that the correct corresponding activity can be read-out based on the current measured.
Give examples of radionuclides used with medium and high energy collimators
Medium: Indium 111m (173, 245keV)
High: Iodine-131 (364 keV)
Why is light given off in scintillation crystal?
Energy given to electron, pushed into conduction band. Gives off light as it falls back into the valence band.
How is Mo99 generated?
Fission of U235
(I-131 also from fission)
Parts of a PMT
Glass window
Photocathode
Dynodes
Anode
Connectors
Why is hybrid imaging not the answer to registration problems
Still need to align the images - SPECT CT and PET CT are not simultaneous so could have voluntary or involuntary movement. PETMR is simultaneous but over time, in frame motion can affect the alignment
Why use registration
Complimentary information: anatomical and functional
For computer aided segmentation and computer aided diagnosis
To evaluate disease progression or treatment response
To compare images for info on common patterns in diseases
To correct for patient motion or positioning changes (RT)
4 steps to registration
Transformation mode (rigid or deformable)
Similarity metric or registration basis
Optimisation method
Validation
What is ischaemia
Restriction in blood supply to tissues
What causes most coronary artery disease
Narrowing of blood vessels caused by plaque
More blood flow - not stenosed
