CLINICAL TECHNIQUES - CT & MRI and OCT and Ultrasound and Biometry Flashcards
Which imaging sequence of MRI is most likely coorelate with active orbital inflammation ie thyroid?
STIR-2 sequence MRI - bellies of muscles show enlargement and inflammation, but tendons are spared –> technique that suppresses signal from fat in order to enhance visibility form abnormalities in tissues
What is CT?
Multiple X ray images taken by tube rotating around a patient
What are the advantages and disadvantages of CT?
Pros:
1. Useful in showing bony lesions and calcifications and fractures
2. Useful for detecting metallic foreign body vs MRI
Cons:
1. Cannot differentiate well between different parts of the same organ / limited contrast as cannot differentiate between tissues of similar density
- Can miss very small lesions
- High ionising dose
Which MRI sequence is useful to highlight demyelination?
FLAIR.
Whats the difference in DWI and Functional MRI imaging?
DWI: Assess movement of water to characterise lesions
Functional: Assess difference in metabolic rate of tissues by detecting different oxygen concentrations
What are the advantages of MRI over CT?
- No ionising radiation - pregnant/children
- High soft tissue contrast
- No artefact from bone
- Vasculature imaged without contrast
What are the limitations to OCT? (3)
- Moderate dilation
- Transparent media
- Suseptible to motion artefact
- Patient co-operation
What is OCT?
Non-invasive noncontact imaging making use of near infrared light.
What is the mechanism of OCT?
- Low coherent light source is sent to a coupler and splits the beam
- One beam is sent to sample arm (the eye) and one beam is sent to reference arm
- Reference signal is reflected back by reference mirror and enters coupler which sends signals to detector.
- Same process occurs in sample arm except beam is backscattered when materials of different indices of refraction are encountered.
- these signals have inherent coherence and both together have an interference pattern which is picked up from detector and is shown on display.
What are the types of OCT (generations)?
Time Domain
Fourier Domain (Spectral domain and swept source OCT)
What is time domain OCT?
Uses monochromatic light source and uses the physical movement of the mirror (which changes optical length) to scan various depth of layers of retina –> TIME BASED MOVEMENT.
How many A scans can TD OCT do per second?
What is the axial resolution?
400 A Scans per second
Axial resolution: 8-10um
What is the problem with time-domain OCT? (4)
- Doesnt permit finer retinal structure visualisations - such as outer photoreceptor layers, RPE or choroid.
- Motion artefacts
- Less sharp and slow
- Low interscan repeatability.
What is the fourier domain OCT? (Spectral domain)
Uses a FIXED reference arm rather than a mobile reference arm.
Uses a broadband light source (840-850nm) to produce spectrum of wavelengths of backscattered light - sends multiple waves all at once, measured by high-speed spectrometer.
Then undergo fourier transformation to provide multiple A-scans with single exposure.
What is fourier’s transformation?
Decomposes a signal into the individual frequencies it is made up of.
What is the number of A scans per second? What is the axial resolution?
A scan: 18,000-70,000 A scans/s
Axial resolution: 5-7um.
What is the difference between swept source OCT vs spectral domain OCT?
- Uses tunable frequency swept LASER light source (1050nm) instead of reference mirror to sweep through a band of frequencies
- Undergoes inverse fournier transformation
What is the A scans / second of swept source OCT vs spectral OCT vs time domain OCT.
See table.
What are ultrasound waves? How are they generated?
Acoustic waves that have frequencies greater than 20 kilohertz
Acoustic waves are generated by vibration of piezoelectric crystal in the probe.
What is the relationship between frequency and wavelength of ultrasound?
Higher frequency –> shorter wavelength –> lesser depth of perception –> better resolution
Lower frequency –> longer wavelength –> greater depth of penetration –> lesser resolution
Which frequencies are used for anterior eye segment?
Which frequencies are used for posterior eye segment?
Which frequencies are used for orbit?
Anterior segment: 50MHz
Posterior segment: 20MHz
Orbit: 10MHz
What does acoustic impedance mean?
What does echo mean?
Acoustic impedance - VELOCITY - denser the medium, the slower the velocity of sound through it as most is reflected.
Echo: REFLECTION - Greater the acoustic impedance difference between two media, the greater the strength of reflection –> stronger echo.
What are the components of the B scan?
What is the difference between A scan and B scan?
A-scan: one dimensional scan which plots intensity of echo vs time delay
5 peaks:
- Corneal surface
- Anterior lens
- Posterior lens
- ILM of retina
- Sclera.
B-scan: 2-D images created from normal A scans.
What is the difference between these OCT scans:
- 3D scan
- Raster scan
- Radial scan.
- 3D scan - horizontal lines scan composed of rectangular box. 20 degrees.
- Raster scan - series of parallel lines orientated at different angles, high res.
- Radial scan - 6-12 lines arranged in equal angles with common axis - good for picking up pathology like macular hole.
What does the retinal thickness measure?
RPE and ILM distance in microns.
What is the centre point of OCT scan?
Where is the central subfield?
Central subfield mean thickness takes into account how many thickness values?
Intersection of 6 radial scans of fast macular thickness protocol of OCT
Central subfield: Circular area of 1mm diameter centered around CP - correlates well with VA.
128 thickness values.
What principle is used in OCT-A?
Diffracti
ve particle movement detection
What are the limitations of OCT-A? (2)
- Presence of artifacts
- Detection of choroidal vessels does not equate to activity
What principle is the OCT based on?
Interferometry - splitting of two light beans
What layers are involved in the ganglion cell complex? (GCC)
RNFL + GCL + IPL.
What are the different types of A scan? (2) Its advantages and disadvantages?
- Applanation - can lead to falsely low axial length and overestimation of IOL power (the opposite in highly myopic eyes)
- Immersion - saline filled scleral shell bewteen probe and eye - it avoids compression of AC but is more time consuming
What is optical biometry (IOL master) and why is it better compared to ultrasound (A scan)?
Uses delay and intensity of infrared light reflected back from a diode to determine axial length
Pros:
1. Measures distance from anterior cornea to RPE vs ultrasound (US measures until ILM and affected by myopic status)
- More accurate in pseudophakia or silicon oil filled eyes
Compression of cornea of A scan is between…?
0.14mm and 0.28mm
If MRI is performed in optic neuritis, the increase in T2 signal enhancement can be seen within how many weeks of symptoms?
3 weeks.
What is the orbital protocol of MRI? (5)
- Precontrast axial - coronal T1 - assesses marrow/bony orbit walls, muscles, intracranial masses
- Axial T2 (after GAD): best for orbital apex and optic canal.
- Coronal T2 with fat suppression (after GAD): best for optic nerve, chiasm and tracts
- Post contrast fat suppression: disrupted if dental/craniofacial implants
- STIR: used as substitution for post contrast fat supression - for thyroid: Prone to motion artefacts / pulsation of vessels.
When should hard/soft contact lenses be removed before biometry?
- Soft contact lens - 1 week
- Rigid - 2-4 weeks
What are the indications for repeat biometry? (3)
- > 4 years old
- IOL exchange required
- Previous corneal surgery / progressive corneal disease
What are the indications for corneal topography? (3)
- Irregular cornea
- Significant astigmatism
- Previous corneal surgery
Which Axial length for which formula?
<22mm : Haigis/Hoffer Q
22-26mm: SRKT/Baret Universal
>26mm: Haigis, SRK/T
If previous refractive surgery: Haigis L
What is higher gain useful for in ultrasound? What is lower gain useful for in ultrasound?
Higher gain –> allows weak signals to be visualised –> small foreign body, PVD, vitreous opacity
Lower gain –> allows stronger signals to be visualised –> masses or tumours.
What is the annual background radiation?
2.7msV
What is the equivalent background radiation for the following
CXR, AXR, CT head, CT abdo
CXR - 3 days
AXR - 3 months
CT head - 9 months
CT abdo - 3 years
What is the axial resolution of ocular ultrasound?
What is the transverse resolution of ocular ultrasound?
Axial: 150um
Transverse: 450um
What is the ultrasound probe used in corneal pachymetry?
20MHz ultrasonic probe
Which imaging technique uses a longer-wavelength light, anterior segment OCT or posterior segment OCT?
Anterior segment OCT - 1300nm vs 800nm
Whats the difference in T1 vs T2 weight imaging sequences in MRI?
Air, Bone, Calcium, White Matter, Grey matter, blood, fat, high protein/low protein fluid
See table.
What’s the difference between T1, T2 and T-1 FLAIR and T2 FLAIR?
T1 FLAIR: general anatomy and post-contrast imaging
T2 FLAIR: visualising structures with high water content (CSF, oedema, white matter) –> useful in assessment of multiple sclerosis.
What is the relationship between reflectivity within a tissue and homogeneity?
internal reflectivity is inversely proportional to its homogeneity.
Tissues that are less organised (vascular such as choroidal hemangioma) will have high internal reflectivity compared with homogenous tissue (choroidal melanoma).
What is the unit used in radiodensity in CT?
How much is Air? How much is water?
Hounsfield unit.
Air = -1,000 HU
Water = 0 HU
Higher gain vs lower gain in ultrasound in terms of noise artefacts and resolution?
Higher gain: large noise artefacts, overall reduction in artefacts.
Lower gain: enhances resolution.
What is the mechanism of MRI?
- Magnetic field used to align hydrogen atoms.
- Radiofrequency wave then used to realign hydrogen atoms.
- Atoms release energy when they return to their original position when radiofrequency is switched off
What is the order of ultrasound spikes on A scan biometry?
Cornea
Anterior lens capsule
Posterior lens capsule
Retina
Sclera
Orbital Fat.
T1 vs T2 vs FLAIR vs STIR
T1 weighted: fat > white matter > gray matter > CSF
T2 weighted: CSF > gray matter > white matter > fat
FLAIR: fat > gray matter > white matter > CSF
STIR: CSF = gray matter > white matter > fat
