Radiation Imaging Flashcards
Give the definition of medical imaging
Medical imaging is the the science, collection of processes and methods that allow us to produce a visual representation of the interior of a patient from non-destructive measurements taken from the outside.
“Non-destructive measurements” means non-invasive, i.e. nothing is introduced in the patient’s body like we do in nuclear medicine.
What are x-rays?
How are they produced?
X-rays are a type of radiation in the energy range of 124 - 1.24e6 eV, wavelength 0.01-10 nm and frequency 310^16 - 310^19 Hz
They are produced by the radiation collision of charged particles with matter, when the incident particle interacts with the coulomb field of the target nucleus and the deceleration/deflection causes the emission of an x-ray photons.
Practically they can either be produced in an x-ray tube or in a linac. For imaging we use x-ray tube since we need less energetic beams.
Describe the working principle of an x-ray tube
- anode and cathode are filaments in an evacuated tube
- voltage is applied between the anode and the cathode
- current is applied to the filaments to heat them up
- the cathode emits electrons by thermoionic emission, these are accelerated by the electric field towards the anode, the kinetic energy transferred is converted into heat (99%) and x-ray energy (1%; can be either Bremsstrahlung or characteristic radiation)
How does the spectrum change if we increase:
-current
-potential
How does it change if we change the irradiated tissue?
Why is filtering important in the production of x-rays?
1) If we change the current in the cathode wire, there will be more electrons emitted; the energies (max, min, char) of the produced photons are going to stay the same.
2) If we change the potential we will increase both the number of produced photons and their mean/max energy.
3) If we irradiate different tissues, since they may contain different different elements, we expect to see shifts in the characteristic peaks. Moreover, due to the density difference and the difference in the attenuation coefficient, we also expect changes in the Bremsstrahlung part.
4) The filtering is important because it’s the high energy photons that produce the image, whereas low energy photons get absorbed.
What is the energy range for imaging?
Which photon interactions are relevant for imaging?
The energy range of the imaging beams is 10-150 keV, therefore the main interaction the photons undergo is photoelectric effect.
Eventually, for the high energy range we could also have Compton scattering events (from 100 keV to 10 MeV), but in much smaller proportion.
How are the x-rays detected?
The detection takes advantage of a so-called scintillator: this is usually made out of crystal material excited by photoelectric effect. This is able to emit visible light pulses in response to the stimulation by the x-ray photon. The light pulse is then transmitted via a fiber optic plate to a CCD camera, where the signal is converted to electrical signal read by the computer.
What are the advantages and disadvantages of tomography VS. radiography?
A) The tomographic image allows us to reconstruct the 3D structure and to retriever information about the depth / It also has a higher soft tissue contrast.
D) longer exposure time (plays a role in the choice of the x-ray tube material), higher costs, higher dose, more advanced filtration level
What are the main components of a CT scanner?
1) multiple rows of detector to acquire more than 1 axial image
2) the detectors rotate in order to acquire different projections at different angles.
3) Bowtie-shaped filters that increase the uniformity of the beam by differential attenuation.
Explain how the tomographic image reconstruction works
1) the working principle is that of reconstructing the imaged object starting from a full set of line integrals over the directions given by the acquisition angles. These projections are called called sinograms
The reconstruction can be done either by filtered back projection or iterative reconstruction.
FBP) Fourier slice theorem ensures the equivalence of two different techniques involving the 2D intensity profile obtained by the radon transform, the Fourier transform and the end projection.
IR) The raw image is compared with expected result image in a reconstruction loop. The comparison is made between the number of hounsfield units per voxel.
Initial image can either be empty or given by FBP. This is projected forward in the detector space and there it gets compared to the raw data. The residuals are calculated and the estimate is updated. This process is iterated to reach the final image.
What are the CT artifacts? How can we avoid them?
PA) patient related:
-motion –> can’t avoid but there is the 4DCT
-metal artifacts –> reduced by using a DECT, iterative reconstruction
PH) physics-based:
-noise –> Photon counting CT, filtering, iterative reconstruction
-beam hardening –> DECT, Photon counting CT
-to few projections
-photon starvation
-partial volume effects
D) detector related
What is the difference between nuclear medicine and CT scans for imaging?
Nuclear medicine images the distribution of the radiopharmaceutical injected into the patient, meaning that the radiation source is inside the patient’s body.
Nuclear medicine gives functional information, whereas CT scans provide density/anatomical information
The detector geometry is also different although the detector type is the same: gamma camera/scintillator. In CT scans we have multiple arrays of detector and these are arc-shaped, whereas in nuclear medicine we have 1/2 flat gamma cameras rotating around the area of interest.
How can we increase the quality of a CT scan?
Overall quality decreases if the spatial resolution is increased, because this also enhances the noise.
By reducing the noise we decrease the spatial resolution so we see less detail but the contrast resolution is high.
What is the difference between a single energy CT and dual energy CT?
The dual energy CT irradiates the body with two beams of different energy and shows the weighted average of the two channels. By doing so, we see more information than we would with a single energy CT because we can see both the bone structure and the soft tissues.
What are the roles of CT in radiotherapy? What are its roles in diagnostics?
D) used to diagnose multiple things, such as:
- fractures
-tumors
-heart diseases
-brain injuries
T) it is also used in the treatment planing process as it shows both anatomy and density information, so we can define the contours of the CTV and OAR, we use it in IGRT for ensuring the correct positioning of the patient. It is used in the dose calculation process and also for follow-up imaging.
