Principles of Radiology for the Comprehensive Ophthalmologist Flashcards
When is CT a good imaging modality choice
acute haemorrhage, trauma, calcification, and diseases of the bone and orbit and in patients for whom magnetic resonance imaging (MRI) is contraindicated
When is MRI useful
the modality of choice for assessing the central nervous system
What is the relationship between ultrasound frequency and resolution
The frequency of ultrasound is directly proportional to its resolution and inversely proportional to its depth of penetration
What is SPECT
CT scans can be combined with nuclear medicine imaging, as in single-photon emission computed tomography (SPECT) and positron emission tomography (PET-CT). These modalities use radiolabeled molecules to help evaluate metabolic activity in a wide range of diseases
When is SPECT useful
evaulation of myocardial perfusion and brain function
When is PET CT useful
diagnose and stage brain tumours as well as diagnose degenerative diseases of the brain
What is the advantage of MRA/V, CTA/V
Less invasive than catheter angiography
Limitations of CT
1) CT scans employ ionizing radiation, a potential concern especially in pediatric cases and pregnant patients
2) Poor visibility of posterior fossa due to streak artifact from skull base
3) Lower spatial resolution than MRI
What is the unit to determine the amount of tissue damage expected from absorbed dose of ionizing radiation
Millisievert (mSv)
What is the unit for total dose of ionizing radiation delivered to a tissue during a particular scan sequence
Milligray (mGy)
What is the risk of exposure to 10mSv of radiation
Cancer risk 1/1000 patients
How does MRI work
MRI uses a strong magnetic field that causes hydrogen atoms found in water and fat to align themselves with the field. Once the atoms are aligned, protons within a selected imaging section/volume are exposed to a series of radiofrequency (RF) and/or magnetic gradient pulses and become excited. As the protons relax again to a steady state, they emit radio waves, which are detected by a receiver coil in the MRI system.
What is echo time
The time it takes for the signal to reach the MRI machine following the applied RF (or gradient) pulse
What is repetition time
The time between RF pulses
What is T1W image
Images generated with shorter TEs and TRs, are typically used for contrast-enhanced studies. In a T1WI, water appears dark (hypointense) and fat appears bright (hyperintense)
What is T2W image
use a longer TE to depict differences in water content, thus revealing inflammatory, ischemic, and neoplastic-related edematous changes. On T2WIs, vitreous, cerebrospinal, and other fluids are bright
What is FLAIR
In fluid-attenuated inversion recovery (FLAIR) images, the fluid signal is suppressed on T2WIs, facilitating visualization of signal abnormalities associated with changes in the periventricular white matter (eg, as in multiple sclerosis)
What is Diffusion weighted imaging
this sequence is the most sensitive for the detection of acute ischemic changes (eg, cerebrovascular accident). DWI can detect changes within minutes compared with potentially hours with other MRI methods
What frequency is ultrasound energy
Ultrasound refers to sound waves with frequencies above the audible range. Electrical energy is converted into sound waves by means of a piezoelectric crystal
What structures can be evaluated with Ultrasound
- intraocular structures with media opacities
- posterior sclera
- extraocular muscles and the surrounding orbit
- intraocular tumors
Quantification of reflectivity on A-scan
Low 0–33%
Medium 34–66%
High 67–100%
A-scan frequencies
Used for measuring axial length
uses frequencies between 8 and 12 MHz
What is the relationship between reflectivity and homogeneity
The reflectivity within a tissue is inversely proportional to its homogeneity. The reflectivity within a tissue is inversely proportional to its homogeneity
B-scan frequency
10Mhz commonly used with axial resolution of 100 micrometres
What are the 3 types of B-scan
axial, transverse, and longitudinal
How is an axial B-scan done
performed by placing the probe directly on the cornea with the patient looking straight ahead and the probe marker oriented vertically at 12 o’clock or horizontally with the probe marker oriented nasally. This allows visualization of the posterior pole and the optic nerve
How is a transverse B-scan done
probe is placed on the sclera, avoiding image degradation from the anterior segment, and is oriented parallel to the limbus, providing a circumferential scan of the opposing retina ie, when imaging the nasal quadrant, the probe is placed on the temporal sclera with the patient adducting his or her eye
What are the 4 transverse scans
superior, inferior, nasal, and temporal. nasal and temporal scans are known as the lateral transverse scans
How is a longitudinal B-scan done
performed with the probe placed on the sclera, with the marker oriented perpendicular to the limbus
Frequency of UBM (ultrasound biomicroscopy)
utilizes the highest frequency available in ophthalmic ultrasonography, usually 50 MHz with axial resolution of 37 μm and is used to evaluate the anterior segment of the eye
How are UBM scans generated
by centering the probe at the limbus, with the marker oriented perpendicular to the limbus. The anterior chamber angle, iris, and ciliary body can be evaluated with this scan
Ultrasound findings in Retinoblastoma
Widening of the ocular walls, extremely high acoustic reflectivity, shadowing effect, atypical findings possible
Ultrasound findings in Congenital cataract
Increased reflectivity from the posterior lens surface, vitreous space empty, ocular walls normal
