CT and PET Scans Flashcards
What are the principle features of a CT scanner?
Typically use 120kVp x-ray tube, collimated into a fan beam, ~2-3 mm slice
There is an associated scan dose
Aperture (60cm diameter)
Patient positioning table moves through the scanning field at a constant speed
The x-ray tube rotates around the patient at the same time 1 revolution (360°) = 250ms
Interacts with fat, muscle, bone, tissue, etc. attenuating at different levels
X-ray detector detects the x-rays that pass through to the other side
What is the nature of the images produced by CT scan?
Measure the x-ray attenuation through object
Acquire attenuation along projections over 360 degrees for each slice
Reconstruct attenuation of each volume element (voxel = 3D view of attenuation broken down into squares = a volume pixel!) by filtered back projection
Values for each volume voxel are attenuation values relative to water, named ‘Hounsfield Units’
Whiter areas show denser tissues
How is CT used in oncology?
Electron density is used to calculate real-time dose distribution
CT number is calibrated by scanning a range of phantoms with known electron densities, and creating a CT – density calibration curve
In oncology the same can be applied in order to determine dose calculations
What are the applications of CT?
Cardiac CT
- Acquisitions in less than heart beat
- Coronary CT: Assessment of calcifications, calcification score
based on density bins
- Assessment of occlusion of coronary arteries and assessment of
blood flow
- Diagnosis of coronary artery disease
Angiograph
- Visualise blood flow in the body – iodine based contrast
- Stenosis (narrowing) of arteries
- Aneurysms in major blood vessels (including brain blood
vessels)
Orthopedics
- Complex joints
- High spatial resolution
- 3D reconstructions (compare with planar x-ray)
- Can see prolapse of vertebral discs
- Complex joints – hip
- Reconstructive surgery planning
Oncology
- Diagnostic, planning, treatment response
- Visualisation of tumors - delineation
- Staging and treatment decisions
- Difficulty in tumor definition in soft tissues in some disease sites, but
can see neighboring normal tissues
How does 4D CT work?
Collect data into ‘bins’ corresponding to the same point in the respiratory cycle
Reconstruct multiple images representing the different breathing phases (typically 10 bins used so 10 CT scans)
Requires multiple rotations at each slice to gather enough projection data in all bins
Therefore increased time and dose
What are the advantages of using CT for virtual colonoscopies?
More comfortable
No anaesthetic required
Quicker
BUT
Cannot take tissue samples
What are the principles of PET?
Functional imaging technique
- Don’t see anatomical information but metabolic activity
Use a tracer labelled with a radioactive substance, most common is 18F-FDG (Fludeoxyglucose – analogue of glucose), where an Oxygen has been replaced with fluorine-18
Tracer gets “stuck” in cell
Shows metabolic activity
Where is PET activity highest?
PET shows physiological function at a metabolic level
- Brain always “hot” as high metabolism
- Activity collects in bladder
- Brown fat
How does a PET scanner produce an image?
Tracer uptake at sights of metabolic activity
Decays with Positron emission
Positron annihilation with electron (within 1mm), causing 2x 511keV photons to be emitted in opposite directions to conserve momentum
Detectors pick-up simultaneous photons and reconstruct where they were emitted
What is PET-CT?
PET is functional - most machines are PET-CT
Combine the functional information of PET with the anatomical information of CT
Images inherently fused and therefore identify locations of activity
What are the applications of PET-CT?
Brain imagine
- Alzheimer’s disease differentiation (lower brain activity)
- Various neuroreceptors (schizophrenia, substance abuse, mood
disorders etc.)
Cardiology
- Diagnostic imaging of heart disease, (with stress testing)
Pharmacokinetics
- Used in drug development (animal studies), uptake and elimination of
drug
Oncology
- ~90% of scans: imaging of primary and metastatic disease
- Lung cancer patients
- Planning with PET changes tumour volume in large proportion of
case
- Increases nodes in CTV
- Helps differentiate tumour from associated atelectasis
- Improved intra-clinician variability
What other tracers are used aside from 18F-FDG?
Number of tracers under investigation increasing
- Many still research tools
Tracers to investigate an aspect of the patients biology
Need to replace or add a radioactive tracer
Radioisotope limited by half life
- 18F 110 minutes
- 11C 20 minutes
- 15O 2 minutes
What are the common tracers used in oncology?
Cell proliferation – 18FLT, 11C-Ch
Blood flow – H215O, 62/64Cu-PTSM
Hypoxia – 18F-MISO, 62/64Cu-ATSM
Neuroendocrine Tumours (NET) – 68Ga-DOTA-TOC, 18F-DOPA
CNS – 11C-MET
Prostate membrane activity – 68Ga-PSMA
What are the disadvantages of PET-CT?
Scan dose
PET has a low spatial resolution
- ~4mm
Motion during scan – long scan times 10-30min
- Image blurring
- PET/CT misalignment
Both have an associated imaging dose, therefore (small) risk of secondary cancers
Both are actively being developed for new applications and improved technology
