Imaging: Midterm to Final Flashcards
What are some common decay modes of radionuclides?
Common decay modes of radionuclides are:
- Beta minus decay
- Beta plus decay
- electron capture
- Internal conversion
- Alpha decay
What does the mass number A stand for?
A is the number of protons and neutrons.
What does the atomic number Z stand for?
Z is the number of protons.
Describe Isomeric transition.
An isomeric transition is a radioactive decay process that involves emission of a gamma ray from an atom where the nucleus is in an excited metastable state, referred to in its excited state, as a nuclear isomer.
The emission of a gamma ray from an excited nuclear state allows the nucleus to lose energy and reach a lower energy state, sometimes its ground state. In certain cases, the excited nuclear state following a nuclear reaction or other type of radioactive decay, has a half life that is more than 100 to 1000 times longer than the average 10−12 seconds, and this excited state is referred to as a metastable nuclear excited state. Some nuclei are able to stay in this metastable excited state for minutes, hours, days, or occasionally far longer, before undergoing gamma decay, in which they emit a gamma ray.
The process of isomeric transition (that is, the gamma decay of nuclear isomers), is therefore similar to any gamma emission from any excited nuclear state, but differs in that it involves excited metastable states of nuclei with longer half lives. These states are created, as in all nuclei that undergo gamma radioactive decay, following the emission of an alpha particle, beta particle, or occasionally other types of particles that leave the nucleus in an excited state.
The gamma ray may transfer its energy directly to one of the most tightly bound electrons causing that electron to be ejected from the atom, a process termed the photoelectric effect. This should not be confused with the internal conversion process, in which no gamma ray photon is produced as an intermediate particle.
What energy does 99mTc emit?
99mTc emits gamma rays of 140.5 keV.
What is the half life of 99mTc?
The half life of 99mTc is 6 hours.
What parent of 99mTc is used for its production for nuclear medicine?
99Mo is manufactured to, which decays to 99mTc inside the generator at the clinics.
After how many half-lives of 99mTc should one “milk the generator” for the best efficiency?
4 half-lives = 24 hrs for 99mTc.
What does SPECT stand for?
Single photon emission computed tomography.
What is used in conjuction with SPECT detectors to discriminate photons traveling in different directions?
Why is the emission energy of 99mTc a nice energy to work with?
140keV photons are nice because the photoelectric effect dominates at this energy, so the Compton continuum is not an issue. The energy is also nicely matched with the NaI(Ti) scintillator efficiency and spatial resolutions.
What is an acceptance window used for in nuclear imaging?
An acceptance window is the allowable energy range outside of the expected energy of the isomer that is accepted by the detector. It is used to discriminate signal from background.
How do you combine biological elimination and physical half-life to get an effective half life?
You add the activities:
lambda_e = lambda_b + lambda_p
where lambda_p=ln(2)/half-life
How is SPECT data reconstructed?
SPECT is reconstructed using filtered back projection or iterative reconstruction.
How big is the matrix for SPECT?
The matrix for SPECT is either 642 or 1282.
What type of filter is used for the FBP image reconstruction of SPECT?
What does the attenuation correction corecct for in SPECT? What does the image look like before correction?
The attenuation correction accounts for the fact that you will get more counts from near the surface of the object (that contains the isomer) than from the centre because the stuff in the centre had to travel more and got attenuated. Before the correction, the centre looks lighter.
How is spatial resolution measured for SPECT?
How many slices are typically used for PET?
PET uses ~109 slices.
What is FDG?
Fludeoxyglucose (18F) (INN), or fludeoxyglucose F 18 (USAN and USP), also commonly called fluorodeoxyglucose and abbreviated [18F]FDG, 18F-FDG or FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). The uptake of 18F-FDG by tissues is a marker for the tissue uptake of glucose, which in turn is closely correlated with certain types of tissue metabolism.
What is the energy of photons from PET?
The photons originate from a positron anihilation, so there are two 511 keV photons.
How do the scintillators in PET help guide the photons in a straight path?
What is the formula for the random rate of PET detectors?
Rrandom = tau S1S2
Where tau=coincidence window
Si = count rate of detector i
What is the random/true ratio for PET?
Random/true ratio is the ratio of random events (incorrectly matched events) to properly matched events. It increases with activity and decreases with the time window.
What happens when you remove the septa in PET detectors?
The septa are the tungsten scatter grids. Removing them allow you detect in 3D instead of 2D, but it increases the random coincidence rate and the scatter fraction.
Which uses attenuation correction before reconstruction, PET or SPECT?
PET uses attenuation correction before reconstruction, SPECT uses attenuation correction during reconstruction. PET achieves this by using a CT scanner at the same time.
What radionuclide has been proposed to replace 99Tc?
82Rb has been proposed.
What is the half-life of FDG?
FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). Its half-life is 110 min.
What is the principle of projection data collection for SPECT and PET?
SPECT uses collimation, PET uses annihilation coincidence detection.
How does attenuation affect SPECT and PET?
SPECT: Attenuation is less severe. Radioactive attenuation correction sources or x-ray CT scan can correct for attenuation.
PET: Attenuation more severe. Radioactive attenuation correction sources or x-ray CT can correct for attenuation.
What is meant by a Standardized Uptake Value (SUV)?
The SUV is the normalized uptake of FDG to:
- Administered activity
- radioactive decay from time of injection
- patient body mass
SUV = (activity concentration in a voxel or group of vodels)/(activity administered/body mass)
In other words, the SUV represents the ratio of (1) the image derived radioactivity concentration found in a selected part of the body at a certain time point, and (2) as reference the radioactivity concentration in the hypothetical case of an even distribution of the injected radioactivity across the whole body. Here, the two radioactivity measures need to be from the same time point, e.g. the time of injection or the time of the image (or “image frame”). For example in the latter case, the injected activity is to be corrected for the physical decay between time of injection (t=0) and the time of the image frame (t).
What factors affect SUV?
Factors affecting Standardized Uptake Value:
- Accuracy of administered activity (material left in syringe)
- Leakage of activity during administration (??)
- PET/CT Calibration and attenuation correction
- Elapsed time before imaging (accuracy)
- Patient Physiological state (fasting, insulin etc)
- Body composition
- Size of tumor (partial volume effects)
- Motion (respiratory)
- Region of Interest Selection
What is the half life of FLT?
The half life of FLT is 109 minutes and emits positrons with Emax = 1.65 MeV. It accululates in cells that are undergoing cell division (tumors divite a lot). This way you can look directly for dividing cells instead of metabolic uptake.
What particle interaction dominates for PET?
Compton interactions dominate for PET, with 511 keV photons.
What type of particle interactions dominate for CT scans?
At 70 keV, photoelectric effect dominates for CT scans.
Explain how a 99Mo/99mTc generator works.
A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope 99mTc of technetium from a source of decaying molybdenum-99. 99Mo has a half-life of 66 hours and can be easily transported over long distances to hospitals where its decay product technetium-99m (with a half-life of only 6 hours) is extracted.
The column, usually glass, containing a bed of aluminium oxide (alumina) as a support for the parent radionuclide. 99Mo (molybdate) will bind strongly to this support media and is not washed off during the subsequent elution of the daughter radionuclide 99mTc (pertechnetate).
Pouring normal saline solution through the column of immobilized 99Mo elutes the soluble 99mTc, resulting in a saline solution containing the 99mTc as the pertechnetate, with sodium as the counterbalancing cation.
When the generator is left unused, 99Mo decays to 99mTc, which in turn decays to 99Tc. The half-life of 99Tc is far longer than its metastable isomer, so the ratio of 99Tc to 99mTc increases over time. Both isomers are carried out by the elution process and react equally well with the ligand, but the 99Tc is an impurity useless to imaging.
How often should you milk the 99mTc generator for the best efficiency?
You should milk it every 4 half-lives, or 24 hrs for the best efficiency.
What does a scintillator do and how does it work?
A scintillator converts high energy EM radiation into low energy visible light.Photons release electrons through ionizations and these energetic free electrons prodce electron-hole pairs.
Inorganic ones, such as NaI, BGO:
The scintillation process in inorganic materials is due to the electronic band structure found in crystals.
- An incoming particle excites an electron from the valence band to the conduction band.
- This leaves a hole behind in the valence band
- Electrons from the conduction band can fall back into the hole, emitting a photon in the visible light range.
- Impurities are added so that there are more acvitation sites closer to the valence band
- The photons emitted by the transitions of electrons from upper to lower states will be lower in energy than in the pure crystal: The emission spectrum is shifted to longer wavelengths and will
not be influenced by the optical absorption band of the bulk crystal. - The photons are emitted in the visible range.
Where does the visible light emission spectrum peak for NaI?
The visible light emission spectrum peaks for NaI at 415nm.
What are the typical gains for a PMT?
Typical gains for a PMT are on the order of 107 to 1010.
What is SPECT used for?
A SPECT scan is primarily used to view how blood flows through arteries and veins in the brain. Tests have shown that it might be more sensitive to brain injury than either MRI or CT scanning because it can detect reduced blood flow to injured sites.
The main advantages of SPECT/CT are represented by better attenuation correction, increased specificity, and accurate depiction of the localization of disease and of possible involvement of adjacent tissues. Endocrine and neuroendocrine tumours are accurately localized and characterized by SPECT/CT, as also are solitary pulmonary nodules and lung cancers, brain tumours, lymphoma, prostate cancer, malignant and benign bone lesions, and infection. Furthermore, hybrid SPECT/CT imaging is especially suited to support the increasing applications of minimally invasive surgery, as well as to precisely define the diagnostic and prognostic profile of cardiovascular patients. Finally, the applications of SPECT/CT to other clinical disorders or malignant tumours is currently under extensive investigation, with encouraging results in terms of diagnostic accuracy.
What types of imaging modalities are used to verify patient positioning before and during treatment?
kV CBCT, MV CBCT, Ultrasound, 2D kV, MRI, PET, and MV portal imaging are all used to verify patient positioning. In almost all cases, these images are compared to the kV fan-bam CT that is used to create a patient’s treatment plan.
What is the mean dose given to a patient with an on-board cone-beam CT imaging system?
3-35 mGy as of 2009.
Who performs contouring?
The radiation oncologist performs contouring, with simple structures occasionally contoured by the radiation therapist.
What needs to be taken into account in planning the margins of contouring?
The full range of the tumour motion (due to breathing, etc) needs to be captured in the contour.
What is the MIP?
The Maximum Intensity Projection: when comparing a set of voxels with the same planar coordinates, the intensity of the voxel with the maximum value is projected into a single image. This is mostly used when images are taken over time to show movement. For some region, the maximum value during movement is assigned to the voxel. The tumor is assumed to be more dense than the surrounding region. This is often used for lungs, where this is definitely true.
What is a digitally reconstructed radiograph?
A digitally reconstructed radiograph is an image obtained from a CT that has been reconstructed to look like an x-ray film. The radiological thickness along each ray line are summed together to achieve this.
What is the typical dose from a single 2D image from a kV imaging apparatus?
About 0.02 to 0.1 mGy.
What is portal imaging?
Portal imaging uses the MV radiation source. The treatment field is loaded, the jaws and MLCs are put in treatment position, and an image is taken. The resultant image shows a port, which can be matched to the DDR (digitally reconstructed radiograph) from the kV CT simulation.
What is the dose given by a portal image.
A portal image uses the MV radiation source and the dose given is 10-30 mGy per field.
What is a GTV?
In contouring, the GTV is the Gross Target Volume, the tumour itself.
What is the CTV?
In contouring, the Clinical Target Volume is the tumour plus any other sites you want to irradiate. This includes sites where micro cell cancer may have spread, such as lymph nodes. Often these spread sites are determined by autopsies on patients with the same cancer.
Name a use for PET/CT imaging.
PET/CT images are useful for diagnosis, staging, and monitoring the progression in many cancers, such as lung and lymphoma.
Name a common tracer for PET/CT.
A common tracer for PET/CT is Fluorine-18.
What difference might be observed between a kV and an MV CBCT image of a skull?
An MV beam can’t distinguish between bone and tissue very much, so the bone does not look as prominant in them.
What is a typical gradient for MRI magnets in the longitudinal direction?
A typical gradient is around 5 mT/m on top of the main field of ~1.5T.
What are shim coils used for in MRI?
Shim coils create small gradients in the field which act to level out the field, or make it even. They work similar to how physical shims work to make a structure level.
Name a contrast commonly used by MRI.
Gadolinium is a paramagnetic and is often used as an MRI contrast.
How many nuclei are required to produce a measureable signal in MRI?
1015 nuclei are needed to produce a measurable signal with MRI.
What has a higher energy: a proton with antiparallel spin or one with spin parallel to an external magnetic field?
A proton with spin anti-parallel to the magnetic field has a higher energy.
When an external magnetic field in an MRI is applied, what fraction of protons are aligned parallel to the field?
3 parts per million more are parallel than antiparallel. Since there are about 1021 protons per voxel, this works out to about 3x1015 more being parallel per voxel. This is handy, since an excess of about 1015 protons are needed to produce a measureable signal.
What is the gyromagnetic ratio for Hydrogen?
The gyromagnetic ratio for hydrogen is 42.58. This, when multiplied by the B field/2Pi gives the preccession frequency.
What happens when an RF energy synched with the precession frequency is applied to a precessing group of protons?
Two things occur at once. Consider this initial state:
Reduction in Mz: Some of the protons resonate and move to the anti-parallel state, resulting in a reduction in the longitudinal magnetization, as illustrated below:
Phase Coherence: the vectors align with each other in phase, as illustrated below:
The outcome of phase coherence is the establishment of a magnetic sum vector in the X-Y plane, called the Transverse Magnetization.
So, once longitudinal magnetization is established by placing a patient in the magnet of an MRI scanner and radio waves at the resonant frequency are generated, we in effect establish a transverse magnetization while reducing the longitudinal magnetization.
The magnitude of the Mxy magnetization compared to the Mz magnetization characterizes the “flip angle”:
An analogy of how this occurs is to think of pushing many people on a swing with a long board at the resonance frequency. Even if they aren’t initially swinging in phase with the board, they will eventually all be swinging together after many pushes.
How is T2 described mathematically?
T2 is the decay constant by which the transverse bulk magnetization Mxy signal decays after having been hit with an RF pulse that caused this bulk magnetization:
Mxy=M0e-t/T2
It is strongly affected by:
- Molecular structure of the magnetized sample
- Characteristics of the bound water.
Explain what happens after a 90o flip is applied to a net magnetization vector.
After a 90o flip, the magnetization vector is now lying on the xy-plane. It continues to process, or rotate, along the plane. The signal diminishes over time as the proton spins dephase (no longer precess in phase together). If a receiver coil is placed in the field, an electromotive force will be induced in it.
Below shows the projected magnetization onto the xy-plane, which diminishes over time, along with the signal induced in the coil. This rate of decay is T2.
What type of structures have short T2?
Large, non-moving structures like bone have a very short T2.
What is the difference between T2 and T2*?
T2* characterises dephasing due to both B0 inhomogeneity and transverse relaxation. T2 characterizes dephasing due only to the transverse relaxation.
How is T2 found?
A series of spin echoes are created using 108o pulses. The peak signal amplitude of each spin echo is reduced from its previous peak amplitude due to T2 dephasing which cannot be rephased by the 180 degree pulses. Figure 9 shows how the signal from a spin echo sequence decays over time. A line drawn through the peak amplitude of a large number of spin echoes describes the T2 decay, while individual spin echoes exhibit T2* decay.
What is T2 relaxation?
T2 relaxation is the decrease in the x-y component of magnetisation back to 0.
What is T1?
Which is larger, T1 or T2?
T1, which is related to the time it takes to restore the longitudinal magnetization, is longer than T2.
Are T1 values longer or shorter for higher B0 values?
T1 is longer for higher B0 values.
Are T2 values longer or shorter for higher B0 values?
T2 values are unaffected by B0.
What can better discriminate cerebrospinal fluid from gray matter, T1 or T2?
T1 can better discriminate. For 1.5T, the T1 values for CSF and GM are 900 and 1,800, while the T2 are 100 and 160 respectively.
What is the Time of Repetition?
What is the Time of Echo?
How does the spin echo work?
The 90°-pulse first tips these spins into the transverse plane. Because the local microscopic fields may differ slightly, some spin groups may precess faster (and gain phase) relative to others. This is represented by the spreading out of arrows in the transverse plane. The faster spins initially rotate toward the viewer and the slower spins rotate away.
The 180°-pulse now turns the entire system on its head. After the flip, the faster precessing spins now find themselves at the back of the pack. With continued evolution they eventually catch up with the slower spins. This occurs at time TE = 2 x t which is the center of the spin echo. Beyond the echo center the faster spins once again leave the slower ones behind and the system again dephases.
What is a spin echo pulse sequence?
What is the mathematical form of the overall signal intensity (S) of a SE sequence?
,
where K is the proton density.
What quantities can you weight the MRI signal towards?
You can weight the MRI towards proton density, T1, or T2.
What is free induction decay signal?
Free induction decay is the oscillating decaying MRI signal in the transverse plane.
What T term does a long TE accentuate?
A long TE accentuates T2:
Also want a long TR to help with T2 accentuation.
What does a short TR and short TE weight towards?
A short TR and short TE weight towards T1.
What does a long TE and a long TR weight towards?
A long TE and a long TR weight towards T2.
How should TE and TR be chosen to weight towards T1?
TE and TR should be short to weight towards T1:
- TE: 5-30ms
- TR: 400-600ms
How should TE and TR be chosen to weight towards T2?
TE and TR should be long to weight towards T2:
- TE: 60-150ms
- TR: 2,000-4,000ms
How should TE and TR be chosen for proton density weighting?
For proton density weighting:
- Long TR: 2,000-4,000ms
- Short TE: 5-30ms
What is an inversion recovery spin echo?
Inversion recovery pulse sequences are used to give heavy T1-weighting. The basic part of an inversion recovery sequence is a 180 degree RF pulse that inverts the magnetization followed by a 90 degree RF pulse that brings the residual longitudinal magnetization into the x-y or transverse plane where it can be detected by an RF coil. In imaging, the signal is usually refocused with a 180 degree pulse as in a spin echo sequence. The time between the initial 180 degree pulse and the 90 degree pulse is the inversion time (TI).
With a TI of about 140 ms on a 1.5 T MRI machine, the fat signal is nulled while the water proton signal is still present. This occurs because the T1 of fat is significantly smaller than the T1 of water. The diagram below shows the magnetization of water (black arrow) and fat (red arrow) during the STIR sequence used for fat suppression. One drawback of this sequence is the partial loss of proton signal during the TI time. Also the TR time must be longer than that of a spin echo sequence for recovery of longitudinal magnetization.
What are the magnetic coils in an MRI?
- Main B0 coil
- RF coil
- Shim coils
- Gradient coils (x, y, z)
What are the three steps for localizing an MRI signal?
To localize an MRI signal:
- Slice selection (main gradient in Z)
- Phase encoding gradient (in y), quick gradient to cause phase shifts in each row
- Frequency encoding (readout gradiant in x). Gradient turns on to induce a shift in frequencies, those further along gradient spin faster.
What is the Phase Encoding Gradient?
The phase encoding gradient is used to create a gradient along a single slice (once that is selected). The gradient is turned on briefly to speed up rows along the x-direction, so that those further along go faster. It is only on briefly, which causes a phase shift, which is used to measure the distance along the x-coordinate of the slice.
What type of detectors read the signals for SPECT?
PMTs.
How many degrees does the SPECT detector rotate for a) Most studies b) cardiac?
Spect detectors rotate a) 360 degrees for must studies, but only b) 180 for cardiac. This is because the heart is too far away from the other side of the body.
What type of reconstruction is increasingly used for SPECT?
Iterative reconstruction is increasingly used for SPECT.
What can cause artifacts in SPECT?
Artifacts in SPECT can arise if the centre of rotation is misaligned. This causes a ring effect.
What is a typical coincidence timing window for BGO?
BGO is used as a scintillator for PET detectors and has a timing window ~12ns.
What is a typical timing window for LSO?
LSO is a scintillator used for PET with a typical coincidence timing window of ~4.5ns.
Why might BGO be preferred for PET detectors?
BGO has a high Z, this is helpful as 511 keV photons is relatively large.
What would you want to weight an MRI by to emphasize CSF?
T2 is most different for CSF vs everything else.
What is the purpose of the inversion recovery pulse sequence?
The inversion recovery pulse sequence is used to give T1 weighting.
What is the STIR (Short Tau Inversion Recovery) pulse sequence used for?
STIR is a fat suppression technique with an inversion time TI = T1 ln2 where the signal of fat is zero. This equates to approximately 140 ms at 1.5 T.
What are FLAIR pulse sequences used for?
FLAIR, fluid attenuation inversion recovery, is used to suppress water.
How is T1 measured?
What is T1 for CSF at 1.5T?
T1 for CSF at 1.5T is 2,400ms.
What is T1 for fat at 1.5T?
T1 for fat at 1.5T is 260ms.
What is T1 for Gray matter at 1.5T?
T1 for Gray matter at 1.5T is 900.
What is a common pixel size for MRI?
A common pixel size is 0.5 to 1mm.
What is a common slice width for MRI?
A common slice thickness is 5-10mm.
Does the spatial resolution increase or decrease with B0?
The spatial resolution can increase with an increase in B0 due to increased SNR and ability to collect thinner slices. It is proportional to roughly B01 to B01.5. Expect a 3 to 5 fold increase in SNR from 0.5 T to 1.5 T.
How does the RF bandwidth of the receiver affect the signal to noise ratio in MRI?
The SNR is proportional to 1/sqrt(Bandwidth).
How is the SNR affected by the number of repeats in the same voxel?
The signal to noise ratio is proportional to the square root of the number of repeats in the same voxel.
What is meant by the quality factor of the coil in MRI?
The quality factor of the coil is the sensitivity of the coil.
Why would you skip every other slice while scanning in MRI?
The badnwidth is not perfectly rectangular, so some tissue outside of the slice can be excited.
What type of features do artifacts produce in MRI?
Artifacts often appear as negative signal intensity, instead of bright spots like a CT has.
What are the three categories of artifacts in MRI?
The three categories of artifacts in MRI are:
- machine dependent
- patient dependent
- signal processing dependent
What is the most common usage for SPECT?
SPECT is most commonly used for myocardial perfusion imaging, which looks at the prognosis of coronary heart disease.
What radionuclide is often used for PET?
PET radionuclide:
- FDG - glucose uptake - dose: 7mSv + 4mSv for CT scan
- FLT - accumulates in cell division - 1.65 MeV positron, half-life 109 minutes.
Name some scintillators commonly used for PET.
BGO, LSO, GSO.
