Week 3: X-ray radiography + generation of X-rays Flashcards
1
Q
X-ray radiography
A
- The basis of X-ray radiography is the differential absorption of X-rays by various tissues ie bone + small calcifications absorb X-rays more effectively than soft tissue
- X-rays are a form of ionising radiation
2
Q
Mechanisms of X-ray production
A
1) If an electron passes close to the nucelus of an atom in th emetal forming the anode, the alternative forces between the positively charged nucelus + negative electron deflect it from its original trajectory, this deflection results in loss of kinetic energy which is emitted as an X-ray, but not all deflected electron energy become X-rays
2) Characteristic radiation of electron rejection from atom
3
Q
Interactions of X-rays with the body
A
- Photoeletric attenuation
- Compton scatterin
4
Q
Photoelectric attenuation
A
- This mechanism provides the contrast in X-ray images
- The energy from an incident X-ray causes an e- to be ejected from an inner shell of the atom, which will have energy equal to the difference between the energy of the incident X-ray + the binding energy of the electron
- An electron then fills the position of the removed electron, the difference in binding energies being transferred to a characteristic X-ray which is emitted
5
Q
Compton scattering
A
- In this mechanism, a small fraction of its energy can be transferred to a loosely bound outer electron from a tissue atom
- The electron is ejected from the atom + the X-ray is scattered (deflected) at angle delta with respect to its initial trajectory
6
Q
X-ray contrast agents
A
- X-ray contrast agents are designed to be very efficient at absorbing X-rays ie to have a strong contribution from photoelectric interactions
- Barium sulphate is used to investigate abnormalities of the GI tract that is administered orally for upper GI and rectally for lower GI
- Iodine-based contrast agents can be injected into the blood stream to visualise the vasculature of the brain, hear, peripheral arteries + veins along with very small blood vessels
7
Q
Clinical applications of planar X-ray imaging
A
- Widely used for:
+ Assessment of degree of bone fracture in acute injuries
+ The presence of masses in lung cancer/emphysema + other airway pathologies
+ The presence of kidney stones
+ GI tract disease
+ Vascular imahing in the brain, heart + peripheral venous/arterial systems
8
Q
Digital Subtraction Angiography (DSA)
A
- Produces very high resolution images of the vasculature in th ebody, able to reolve small blood vessels less than 100 micrometers in diameter
- The procedure involves acquiring a regular image, then acquiring a second image after injecting a bolus of iodinated contrat agent into the blood stream
- These 2 images are then subtracted to produce the angiogram
- DSA is used to investigate conditions ie stenoses + clotting of ateries + veins + irregularities in systemic blood flow
9
Q
Digital mammography
A
- Requires a low radiation dosage to examine breast tissue + for breast tumours
10
Q
Digital fluoroscopy
A
- Uses a continuous X-ray imaging to monitorinterventional surgery ie placement of catherters, guide wires, stents + pacemakers
- Also used for dynamic studies of GI tract + cardiovascular system using contrast agents
- Runs up to 30fps
- X-ray dose per frame can be as low as 1/1000 that used during serial image acquisition
11
Q
Computed Tomography (CT)
A
- Operates by synchronous rotation of X-ray tube + multiple detectors that records a series of projections that together with further 2D imaging of the patient can produce a 3D image
- Backprojections produced during CT can be merged together to help reduce the number of artifacts, that being the more projections there are the less artifacts there are in the image
12
Q
Clinical Applications of CT
A
- Cerebral scans
- Pulmonary disease
- Liver imaging
- Cardiac imaging
13
Q
Single Photon Emission Computed Tomography (SPECT)
A
- Produced multiple contiguous 2D axial slices of the distribution of radiotracer
- Most commonly used for myocardial perfusion
- Uses 2/3 gamma cameras which rotate around the patient to reconstruct multiple 2D axial slices from the acquired projections
- It can acquire fully qualitative images in which image intensity can be related to absolute concentrations of radiotracer, however this requires accurate correction of images for scattering + attenuation within the body
14
Q
Clinical Applications of planar scintography
A
- Has major applications in whole body bone scanning
- Specialised agents are used for SPECT scans of the thyroid, GI tract, liver + kidneys
15
Q
SPECT/CT in one
A
- Pros
+ Improved attenuation coefficient correction for SPECT reconstruction using the high resolution anatomical information from the CT scanner
+ Fusion of high resolution anatomical information with functional information which allows anatomical location of hot + cold radioactive spors to be defined much better compared to SPECT alone - Cons
+ Scans are acquired serially rather than simultaneously which may elad to misregistration between CT + SPECT images if the patient moves
