Past paper Flashcards
(40 cards)
Why are bone scans better than X-rays
Uptake of Tc-99m into active areas of bone growth mkaes isotope bone scan a functional measurement.
X-rays only detect a difference in density, which may not be apparent until months after abnormal function of the tumour
HOw does a gamma camera detect gamma rays
Crystal is Sodium iodide doped with Thalium. Gamma ray undergoes a photoelectric interaction in the detector, emitted electron has energy KE = Y - BE.
This interacts with the lattace of outer electrons in the detector to produce a few thousand excited electrons which decay in 250ns to gove photons.
Photomultiplier tubes convert photons into electrons and amplify the signal to power of n.
Positioning electronics collect outputs from the photomultiplier tubes to determine the position of the event and sum outputs to determine if the energy is within the photo peak range
Describe the term quantum mottle
IMages have significant routeN noise which is Poisson distributed but Gaussian is a good approximation.
To nulify this, image for longer as noise reduces with time however introduces movement problems. Also can increase activity however this also increases the received dose by the patients.
Explain the term FWHM, how it is measured and how to find minimum matrix size
Full Width Half Maximum of the line or Spread Function, usually determined by imaging a narrow line source.
Nyquist implies a minimum sampling of around 2*FWHM.
Describe how an Ultrasound signal is generated and detected by the same transducer
Ultrasound systems use Piezoelectric transducers as both a source and detector. Dipolar molecules in random orientations are aligned by applying an electric field at temperatures above the curie temperature. Apply a voltage across the crystal and the crystals will distort (vibrate) creating an ultrasound pulse.
Similarly when the crystal experiances a mechanical force it will distort, creating a voltage difference.
What material is an Ultrasound Piezoelectric crystal
Quartz is a naturally occurring piezoelectric. Lead titanium Zirconate (PZT) is commonly used in ultrasound transducers.
What is the main difference between standard and harmonic imaging
In harmonic you detect the 2nd or 3rd harmonic instead of the fundamental. The fundamental harmonic is transmitted at a low broadband of frequencies and as it hits the tissues it resonates at higher harmonics. As echoes return to the receiver (which is tuned to the harmonic frequency) it filters out the noise from the fundamental frequency.
State the Advantages and Disadvantages to Harmonic Ultrasound over Standard
Advantages - Better visualization of interfaces - Reduces reflections and scattering from tissues close to the transducer - Improved lateral resolution as the majority of echoes are produced along the centre path of the beam.
Disadvantages - Can lose axial resolution & require higher acoustic power
How is a positron localised in a PET detector. advantages and disadvantages
e+ annihilate e- producing 2 511 KeV anti-collinear photons. 3D reconstruction from the line response between two detected photons.
Main advantage - no requirement for a collimeter, able to increase sensitivity of the system.
Disadvantage - Attenuation is more difficult due to the necessity of detecting 2 photons. Attenuation correction is essential for the image.
What is the main difference is the detector for PET and single photon emission events
PET - no need for collimator and must be able to detect 511 KeV whereas SPECT only requires 100 KeV.
higher counts rate requires segmented (block) detectors. Detector ring around the patient to increase detector efficiency. Fast detectors allow for coincidence detection
Explain why attenuation correction is essential and why it is easier in PET and SPECT
Essential in PET as you have to detect 2 photons, it becomes a major issue.
Easier in PET as it is independent of depth unlike SPECT. Therefore we do no need to know the position of the source but instead the thickness of the object.
In MRI explain the process of recovery of longitudinal magnetisation
Spin - lattice relaxation.
Following the absorption of energy from the RF pulse the excited protons transfer their energy to the surroundings (Lattice) instead of neighbouring protons.
The energy is transferred through molecular motion e.g. rotation & vibration. Energy transfer is possible as the molecular motion occurs in the vicinity of the excited proton & has an intrinsic frequency that = larmor frequency.
Resulting in the Net magnetisation vector gradually returning to its equilibrium position along the z-axis parallel to Bo.
What is the characterisitc time associated with Spin-lattace relaxation and define it
T1. Is the time required for longitudinal componant of the magnetisation to recover to 63% of its original pre-excitation value.
What is contrast diagnostic radiology
Contrast is the ability to distinguish between two tissues with different attenuation properties.
Therefore they depend on the energy of the X-rays amd the composition and electron density of the tissue. The two dominant interactions at X-ray energies are the photoelectric effect and Compton scattering.
Describe Axial CT image acquisition
Early CT scanners, which could only acquire axial slices: patient bed remains still while the tube completes a single rotation. The bed then moves and the process is repeated.
Reconstruction is straightforward, the image plane is the same as the scan plane and the data comes directly from the measured projection.
Describe Spiral CT image acquisition
Slip ring technology allowed continuous rotation of gantry without cables becoming tangled.
The patient moves slowly through the scanner as teh detectors rotate continuously.
Spiral reconstruction is more complex as each prjection is now from a different section of the body as the patient is moving during acquisition. Projection data points are for the selected slice location are obtained through interpolation.
Describe Multislice CT image acquisition
Volume scanning by using multiple rows (banks) of detectors which obtain several slices at once. Several images are obtained during each rotation allowing high resolution scans with short imaging times & reduced motion artefacts.
Describe how SPECT images are obtained
Planer gamma camera images are acquired at a series of different projection angles around the body (usually 64 to 128 projection). Image processing techniques are used to reconstruct these projections into thin axial slices through the body.
Spatial resolution deteriorates rapidly with distance to patient. If we do not minimise collimator to patient distance for each projection the resultant reconstructed image is composed of planer images with poor spatial resolution.
The overall affect is amplified in SPECT images.
Describe the technique of iterative reconstruction
- MAKE INITIAL ESTIMATE of 3D distribution
- FORWARD PROJECT: trace gamma rays from estimated distribution from point of origin through collimator into gamma camera. Can handle the physics of the emission/detection process, here to correct image degrading effects (attenuation/scatter).
- COMPARE estimated projections with measured projections.
- ADJUST ESTIMATIONS to optimise cost function which is the likelihood of getting the acquired projections from the estimated activity distribution.
- BACK PROJECT adjusted estimated projections to get new estimated 3D distribution.
- REPEAT for a given number of iterations
Tc-99m is a common isotope, What does the m mean and what does it decay to
metastable, Tc 99.
Physically identical, X-rays from the electrons and gamma rays from the nucleus
What are the 4 main resolution elements which affect on the image quality of ultrasound
Axial, Lateral, contrast and Temporal
Explain how Axial resolution affects the image quality of ultrasound
Affects the ability to resolve objects at different depths. Depends on spatial pulse length (structures less than pulse length will no be resolved) which in turn is frequency dependant. Higher frequency gives better axial resolution.
Explain how Lateral resolution affects the image quality of ultrasound
Affects the ability to resolve objects perpendicular to the beam. Dependant on beam focussing/width which is dependant on frequency. Higher frequency gives longer near field with a less divergent far field.
Explain how Temporal and Contrast resolution affects the image quality of ultrasound
Contrast: affects the ability to differentiate between different tissues, affected by transducer design and processing.
Temporal: Affects the ability to detect an object that has moved over time. Affected by transducer design and processing.
