13. X-Ray CT PET Flashcards
What is ionising radiation? and why use it?
Radiation that causes ionisation when it interacts with matter
Types used for medical imaging are:
- Gamma rays
- X-rays
=> Because they penetrate through the body (through the different mediums/tissues) and can be captured on our imaging detectors
Describe the two ways in which ionising radiation acts.
INDIRECT:
- the radiation interacts with the water in the body
- It will split the water molecules into free radicals
- these free radicals have an impact on the cell
DIRECT:
- the gamma radiation/proton can directly imapct the cell DNA strand and cause breaks, and therefore impact and have an effect on the cell
- Both these methods may or may not have a biological effects, such as:
- cell death (necrosis)
- cell loses ability to regulate cell growth
- causes genetic mutations
What are the two types of radiation damage and risk?
DIRECT EFFECT:
- only at high radiation dose, not noticed at usual diagnostic doses
- threshold effect
- e.g. Erythema and hair loss
INDIRECT EFFECT:
- risk of cancer induction
- risk of genetic change in subsequent population
- effect is proportional to radiation dose, no threshold -> all radiation has risk
What are the three types of ionising radiation?
- POSITRONS: Positive electrons interact with matter to create gamma rays
- PET scanning
- GAMMA RAYS: Penetrating radiation
- Gamma camera imaging
e. g. SPECT - X-RAYS: Spectrum of electromagnetic radiation
- X-Ray imaging e.g. radiographs, CT
- Positrons and gamma rays are emitted following the radioactive decay of an unstable nucleus
- X-rays are artificially produced in an X‑ray tube
How is attenuation important in x-rays?
- Attenuation means to stop
- X-Rays are essentially an attenuation map
- Attenuation increases with
- Higher atomic number
- Higher density
Describe emission and transmission imaging.
TRANSMISSION IMAGING: (X-rays)
- Radiation is directed through the patient
- A transmission map collected is essentially an attenuation map
- Good at showing structure, especially between tissues of different densities or atomic number
EMISSION IMAGING:
- The radiation is administered to a patient in the form of a tracer (patient is injected)
- Emitted radiation is detected outside the patient
How does an x-ray tube work?
There are some key components to an x-ray: the evacuated (vacuumed) tube, the target (anode) , the filament and the heating circuit. The target and the filament are separated by a potential difference. The heating elements produces electrons, which then hit the target, causing it to emit x-rays.
- X-rays only produced when tube is in action i.e. can be switched on/off
- We have control over the amount and energy of x-rays produced
List some detectors for planar x-rays
- FILM HARDCOPY:
- Film processor with tanks of chemicals
- High resolution
- COMPUTED RADIOGRAPHY COMPUTER COPY:
- Phosphor plate
- Special laser scanner or CR reader that reads and digitizes the image
- Digital enhancement and archiving
- DIGITAL RADIOLOGY (DR):
- Flat panel detector, fully digitised system
In mammography x-ray screening, why do we need a compression plate on the breast?
- Compression plate used to reduce breast thickness
- Improves resolution
- Lowers radiation dose (used as a screening tool)
Describe fluoroscopy as an x-ray imaging technique.
- Real-time imaging
- A catheter is fed inside an artery and radio opaque dye is injected
- Show blood flow inside vessels and can be used to assist with interventions
Describe coronary angiography as an x-ray imaging technique
- Real-time imaging using an image intensifier called fluoroscopy
- A cardiac catheter is fed inside the aorta
- Radio-opaque contrast agent used to identify areas of occlusion
- Treatment may be either balloon angioplasty or insertion of a stent
What are the limitations with planar x-rays?
- Cannot distinguish between overlying tissues
- Tissues other than those being observed reduce contrast in the image
- Historically partially solved by moving the film cassette and X-ray relative to the patient to blur out overlying tissues, called “tomography” (from Greek “part/slice” - “write”)
- Superseded by Computed Axial Tomography, now abbreviated to CT
What is helical scanning, and how has it improved?
- There was continuous rotation of the x-ray and continuous table feed.
- Now, we have helical MSCTs. These are multi-slice rotating round, making it more efficient (takes about 30 seconds for a full-body scan) and gives it more coverage each rotation.
How are x-rays used in A&E?
With the case of a suspected haemorrhage or blood clot, the opposite treatment can be fatal for the condition. For example, Clot busting drugs may increase bleeding, which would be fatal if it was actually a haemorrhage.
- So, x-rays are used for the urgent diagnosis required for treatment
How are x-rays used in disease progression?
- X-ray imaging is used for monitoring response to therapy
How is CT scanning used in treatment planning?
- External beam radiotherapy irradiates normal tissue as well as tumour
- Multiple beams are used to spare normal tissue
- CT is used to define area to be treated and the direction of the radiotherapy beams that are used to only affect the tumour area and not the surrounding tissue.
How does nuclear medicine: emission imaging work?
- You inject radioactive tracer, patient is emitting the gamma rays which are picked up by the detector
- Image depends on the metabolism of the tracer: Functional Imaging
Describe the difference between a gamma camera and a PET scanner.
GAMMA CAMERA:
- Uses single photon emitting radionuclides
- Can operate in 2D (planar) or 3D (SPECT)
PET (Positron Emission Tomography):
- Uses positron emitting radionuclides
- Always 3D
Describe nuclear medicine imaging.
- We are making an image of the distribution of a radioactive tracer
- Nuclear Medicine only shows function
- It may reflect anatomy but without metabolism, the tracer will not be taken up
- Nuclear Medicine is a functional modality
What does half-life mean?
Half-life is time taken for the radioactivity to reduce to 50%
Describe gamma camera imaging.
- Gamma cameras have imaging “heads”
- For radionuclides that decay with direct emission of gamma rays
- Most common radionuclide is Tc-99m (T1/2 = 6 hours)
Give examples of some tracers used in gamma camera imaging.
- Tc-99m MDP (bone scans)
- Tc-99m DTPA (kidneys)
- Tc-99m White Cells (infection/inflammation)
Describe a dynamic renal transplant scan.
- Tc-99m DTPA injected IV
- Camera positioned above the patient
- Gamma camera records gamma rays and collects image over time
- Functional Time –Activity curves are obtained
Detects how well the kidneys are functioning
What is a SPECT?
- Single Photon Emission Computed Tomography.
- Essentially the same as gamma camera imaging, but makes the image 3D as it reconstructs the transaxial slices it took.
- It can acquire up to 64 images from around the head.
How can SPECT be applied for a brain scan?
- We can perform a SPECT “Datscan”
- We use Ioflupane-123 FP-CIT, which binds to dopamine transporters (DAT) on the neurones
- Can be used in diagnosing Parkinson’s disease ~ reduced uptake in the Putamen differentiates from Essential Tremor
Describe β+ particle radiation
- Both the positron and electron are annihilated.
- This results in 2 gamma rays that are created at 180° to each other
How is the metabolism of FDG useful in radioimaging?
- FDG is a glucose analogue which enters cells in the same way as glucose
- Good reflection of the distribution of glucose uptake and phosphorylation by cells in the body
- ? FDG reflects metabolic activity. It can be used to help diagnose:
- Alzheimer’s disease (hypometabolism, mostly in temporal and parietal regions)
- Pick’s disease (fronto-temporal hypometabolism)?
Describe a PET scanner.
- A ring of scintillation detectors supported in a fixed gantry
- Operated in “coincidence mode” - only photons emitted from an annihilation event are recorded
Describe PET coincidence
- 2 gamma rays originate from one annihilation event
- Both are detected within a short time (a few ns)
- Defines ray path for subsequent reconstruction of image
What are some examples of hybrid imaging?
- PET-CT
- SPECT-CT (used to localise uptake)
- PET-MR (very expensive)
How is the hybridisation of PET-CT helpful?
- Fused PET and CT show the exact location of the “hot spot”
- Gamma rays originating from the centre of the patient will travel through more tissue which mean they are attenuated more
- The CT image is used as an attenuation map to correct the PET image
What is SPECT-CT used for?
To localise uptake
Expand on WB PET-MR
- First WB system
- Biograph mMR
- 3T magnetCrystal
- 4x4x20mm
- 64 rings -> axial length 26cm
- Very expensive
