Principles of Radiology for the Comprehensive Ophthalmologist

Question 1

When is CT a good imaging modality choice

Answer 1

acute haemorrhage, trauma, calcification, and diseases of the bone and orbit and in patients for whom magnetic resonance imaging (MRI) is contraindicated

Question 2

When is MRI useful

Answer 2

the modality of choice for assessing the central nervous system

Question 3

What is the relationship between ultrasound frequency and resolution

Answer 3

The frequency of ultrasound is directly proportional to its resolution and inversely proportional to its depth of penetration

Question 4

What is SPECT

Answer 4

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

Question 5

When is SPECT useful

Answer 5

evaulation of myocardial perfusion and brain function

Question 6

When is PET CT useful

Answer 6

diagnose and stage brain tumours as well as diagnose degenerative diseases of the brain

Question 7

What is the advantage of MRA/V, CTA/V

Answer 7

Less invasive than catheter angiography

Question 8

Limitations of CT

Answer 8

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

Question 9

What is the unit to determine the amount of tissue damage expected from absorbed dose of ionizing radiation

Answer 9

Millisievert (mSv)

Question 10

What is the unit for total dose of ionizing radiation delivered to a tissue during a particular scan sequence

Answer 10

Milligray (mGy)

Question 11

What is the risk of exposure to 10mSv of radiation

Answer 11

Cancer risk 1/1000 patients

Question 12

How does MRI work

Answer 12

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.

Question 13

What is echo time

Answer 13

The time it takes for the signal to reach the MRI machine following the applied RF (or gradient) pulse

Question 14

What is repetition time

Answer 14

The time between RF pulses

Question 15

What is T1W image

Answer 15

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)

Question 16

What is T2W image

Answer 16

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

Question 17

What is FLAIR

Answer 17

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)

Question 18

What is Diffusion weighted imaging

Answer 18

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

Question 19

What frequency is ultrasound energy

Answer 19

Ultrasound refers to sound waves with frequencies above the audible range. Electrical energy is converted into sound waves by means of a piezoelectric crystal

Question 20

What structures can be evaluated with Ultrasound

Answer 20

• intraocular structures with media opacities
• posterior sclera
• extraocular muscles and the surrounding orbit
• intraocular tumors

Question 21

Quantification of reflectivity on A-scan

Answer 21

Low 0–33%
Medium 34–66%
High 67–100%

Question 22

A-scan frequencies

Answer 22

Used for measuring axial length
uses frequencies between 8 and 12 MHz

Question 23

What is the relationship between reflectivity and homogeneity

Answer 23

The reflectivity within a tissue is inversely proportional to its homogeneity. The reflectivity within a tissue is inversely proportional to its homogeneity

Question 24

B-scan frequency

Answer 24

10Mhz commonly used with axial resolution of 100 micrometres

Question 25

What are the 3 types of B-scan

Answer 25

axial, transverse, and longitudinal

Question 26

How is an axial B-scan done

Answer 26

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

Question 27

How is a transverse B-scan done

Answer 27

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

Question 28

What are the 4 transverse scans

Answer 28

superior, inferior, nasal, and temporal. nasal and temporal scans are known as the lateral transverse scans

Question 29

How is a longitudinal B-scan done

Answer 29

performed with the probe placed on the sclera, with the marker oriented perpendicular to the limbus

Question 30

Frequency of UBM (ultrasound biomicroscopy)

Answer 30

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

Question 31

How are UBM scans generated

Answer 31

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

Question 32

Ultrasound findings in Retinoblastoma

Answer 32

Widening of the ocular walls, extremely high acoustic reflectivity, shadowing effect, atypical findings possible

Question 33

Ultrasound findings in Congenital cataract

Answer 33

Increased reflectivity from the posterior lens surface, vitreous space empty, ocular walls normal