Medical Imaging Flashcards
What are the three key parameters for medical imaging?
- Spatial resolution
- SNR
- CNR
What are the three measures used in ROC analysis?
Accuracy: No. correct diagnosis/total no. diagnosis
Sensitivity: No. true positives/no. condition positives
Specificity: No. true negatives/ no. condition negatives
What indicates test efficacy in ROC analysis?
Area under the curve gives an indication of test efficacy.
What is shown by the line spread function (LSF)?
Shows degree of blurring in an image. Smallest resolvable feature defined by full-width-at-half-maximum (FWHM).
How is FWHM related to standard deviation of LSF distribution.
FWHM approx. 2.35 * sd
What is the point spread function (PSF)?
3D equivalent of LSF. Relates object to image by convolution of object with PSF.
What is the modulation transfer function (MTF)?
Most common measure of spatial resolution. Measures response to high and low frequencies.
Ideal system has constant MTF for all spatial frequencies.
How is the CNR between two tissues related to SNR?
CNR_AB = SNR_A - SNR_B
What is the effect of a high-pass filter?
Accentuates high spatial frequency components. Decreased SNR due to accentuation of noise also.
What is the effect of a low pass filter?
Noise levels reduced. Increased SNR. High frequency components smoothed.
What are the basics of X-ray imaging?
Relies on differential absorption of X-rays by body tissues.
Heated cathode emits electrons, accelerated towards anode by voltage. At anode KE converted to X-rays.
Detected X-rays on detector other side of patient convert to light which is then digitised.
What three parameters can be varied to influence an X-ray image?
- Accelerating voltage.
- Tube current.
- Exposure time.
Briefly explain the two mechanisms by which x-rays are produced.
- Braking Radiation: Electrons passing close to nucleus in anode deflected. Loss of KE converted to x-ray. Produces wide spectrum of energies.
- Characteristic Radiation: Typical of cathode. Incident electron causes inner electron to be ejected. Electrons in high shells move down energy levels, emitting characteristic X-rays.
What three conditions are required for high SNR and CNR in x-ray images?
- Sufficient x-rays transmitted through body for high SNR.
- x-ray absorption different for different tissues for high CNR.
- Scattered x-rays removed.
What are the two ways in which x-rays are absorbed?
Photoelectric Absorption: Incident x-ray causes electron to be ejected. Other electron moves down energy level emitting characteristic x-ray (does not reach detector)
Compton Scattering: X-rays passing close to outer shell electron cause it to be ejected. X-ray scattered at angle.
What parameter is used to describe x-ray attenuation of tissue?
X-ray attenuation coefficient (mu/rho). Varies with x-ray energy.
How does attenuation change for increased x-ray energy?
Fewer compton interactions, much fewer photoelectric interactions. Increased transmission.
How does increased tissue atomic number effect x-ray attenuation?
No change in compton. Increase in photoelectric interactions. Reduced transmission.
How does tissue density effect x-ray attenuation?
Proportional increase in both compton & photoelectric. Decreased transmission.
What is a contrast x-ray?
Contrast medium used to image hollow or fluid filled structures such as intestines.
What is phase-contrast x-ray imaging (PCI)? How can it be used?
Uses changes in phase rather than attenuation between tissue to form image. Can be used to identify type I and II microcalcifications for early detection of pre-malignant and malignant lesions in breast cancer screening.
What does CT stand for?
Computed x-ray tomography.
What are the four historical configurations of CT scanner?
- Single parallel beam source, single detector. Synchronous translation.
- Hybrid. Fan beam + multiple detectors.
- Rotate-only. Wide fan pulsed beam covering object. Detector rotates with beam
- Rotate-only. Only source moves. 360 degree detector array.
How are 1D CT projections reconstructed to form a 2D image?
1D projections of x-ray attenuation acquired by beam passing through 2D slice at different angles.
Projection plots intensity as function of r.
Projections plotted as f(phi) in sinogram, with signal amplitude represented by sinogram brightness.
Image reconstructed by summing p(r, phi) over total no. projections.
How are sinograms used in CT scans?
Discontinuity can be used to identify movement in patient which may lead to artefacts in final image.
How can artefacts in a CT image be reduced? What is a side-effect of this?
Increase number of back projections to reduce artefacts. Also increases image blurring.
How is blurring of CT images reduced?
Filter applied in spatial domain prior to back-projection using convolution.
What does PET stand for?
Positron emission tomography.
Which method of imaging is capable of providing a 3D map of metabolic activity?
PET.
How are carrier molecules used in PET?
Carrier molecule bound to unstable isotope. Usually traced 18-F with FDG.
Glucose analogous, taken up by glucose using cells.
No further reactions in cell. O2 for metabolism replaced by F-18.
Leads to radiolabelling of tissue.
What features can be highlighted by PET radiolabelling?
Brain, liver and most cancers.
How are PET signals detected?
Unstable isotope in radiolabelled molecules decays releasing positron and neutrino.
Positron annihilates with electron producing two gamma ray photons travelling in opposite directions.
Allows 3D location of positron emission.
What are the advantages of combining CT with PET in PET-CT?
PET shows metabolic activity by low resolution. Overcome by combining with CT.
What is SPECT?
Single-photon emission computed tomography. Tracer directly emits gamma radiation which is detected.
Reduced spatial resolution but able to use longer-lived more available isotopes - cheaper.
What is MRI?
Magnetic Resonance Imaging.
Imaging technique providing a spatial map of hydrogen nuclei in different tissue.
What are the advantages of MRI?
- Non ionising radiation
- 2D or 3D
- Excellent soft tissue contrast
- Good spatial resolution
What are the disadvantages of MRI?
- Slow image acquisition
- Unable to image bone
- Not suitable for some patients (metallic implants)
- Similar CNR for some tissues
- Expensive
Explain the MRI process.
- Proton spin gives rise to magnetic field.
- Protons align to applied magnetic field. Vibrate at frequency proportional to field strength.
- RF pulse of correct frequency applied causing protons to flip.
- When pulse removed, proton realigns releasing an energy pulse.
What is the significance of T1 and T2 relaxation times for MRI?
Times govern return of protons to equilibrium. T1 and T2 times differ between tissues.
What are the advance uses of MRI?
- Determine health of tissues.
- Insight into brain function.
How can MRI be used to determine cartilage health?
- Healthy cartilage contains proteoglycans (PG) consisting of GAG. Loss of GAG indicates osteoarthritis.
- Negatively charged contrast agent used to quantify GAG content.
- Compound injected. T1 relaxation times indicate GAG content. Compound negatively charged so inverse relationship with GAG content.
How is fMRI used to image brain activity?
- Increased synaptic transmission consumes energy -> increased blood flow.
- Increased oxygen consumption causes decrease in deoxyhaemoglobin.
- Deoxyhaemoglobin paramagnetic, changing T2 recovery time. Deoxyhaemoglobin decreases MRI signal.
- Increased brain activity reduces deoxyhaemoglobin so increase in MRI signal (BOLD signal).
What is a BOLD signal?
Blood oxygen-level dependant signal.
What is a B-mode ultrasound image?
Brightness modulated.
Cross-sectional view of tissues and organ boundaries constructed from echoes generated by reflections at interfaces.
What is the process for forming a B-mode ultrasound image?
- Formed using large no. of B-mode lines.
- As TDR transmits pulse, display spots travel on screen.
- Echoes close to TDR return first and increase brightness of spot.
- Distance down display indicates depth.
- Rate that spot moves down screen determines image scale.
What are the four main types of TDR array in ultrasound imaging?
- Linear - Useful for imaging both superficial + deeper areas simultaneously.
- Curvilinear/trapezoidal - Wide FoV near TDR increasing with depth.
- Sector Narrow FoV close to TDR increasing with depth. Heart echography between ribs.
- Radial TDR - Intravascular probes.
What is different about M-mode US images and where are they often used?
Beam aligned with moving target. Allows real-time tissue motion to be observed. Used in cardiology to measure rate and timing of movements such as valve closure.
What is acoustic impedance?
Acoustic impedance, z, is a measure of the response of particles to a wave of given pressure. Determined by density and stiffness.
z = local pressure/local particle velocity.
How are sound waves reflected at tissue boundaries in ultrasound?
Difference in impedance causes abrupt change in particle velocity.
For energy to be conserved, extra wave formed travelling in opposite direction.
Why is there a need to compromise regarding frequency of ultrasound?
High frequency = better detail. However, also increases attenuation. If large, returning echoes may be too weak to detect.
