Introduction to Medical Imaging

Question 1

The greater the attenuation or absorption of the photons by tissue, the _____ the image will appear

Answer 1

Whiter

Question 2

AP vs PA views of heart

Answer 2

AP- shows magnification of the heart (transverse cardiac diameter not correct) and mediastinum, making the heart look larger than it really is
PA view is preferred over AP in this context, unless patient has difficulty standing erect

Question 3

Fluoroscopy

Answer 3

Uses continuous stream of X-rays to view the movement of structures in real time
Used for barium contrast studies of the GI tract, angiographic studies, catheter/tube placement, fracture repair and many others
X-ray images taken at 2-3 FPS for peripheral vascular studies and 15-30 FPS for coronary artery studies

Question 4

Digital subtraction angiography (DSA)

Answer 4

Angiography of the peripheral vasculature, this technique has largely replaced taking an x-ray after using contrast
X-ray is taken before contrast injection to digitally “subtract” bones and other tissues from view
Downside of this is it is an invasive procedure requiring an artery to be entered percutaneously.
In contrast, CTA and MRA are non-invasive procedures that only require IV catheter for contrast injection

Question 5

CT scan

Answer 5

Produced using x-rays passing through the body in a helical fashion as patient moves through a gantry
Algorithms produce axial (transverse plane) images from the data, which can be used to produce sagittal and coronal planes

Question 6

Hounsfield scale

Answer 6

CT density numbers are attenuation units measure by this scale
Computer monitors show 256 levels of gray, so only a portion of the Hounsfield scale can be displaced, and this “window” can be adjusted on the computer

Question 7

Density of water, air, bones, soft tissue in Hounsfield scale

Answer 7

Water: 0
Air: -1000
Compact bone: +3095
Soft tissue: b/w -100 and +100

Question 8

Window widths

Answer 8

The range of gray scale mapped on the Hounsfield scale

Wide window width is good for bone, narrow window is better for soft tissue

Question 9

Pros/cons of CT imaging

Answer 9

Pros: Very quick, motion is not as much of a problem, gray scale can be manipulated, resolution excellent for many areas, widely available and cheaper than MRI
Cons: Uses ionizing radiation, renal function must be evaluated for contrast, some patients are allergic to contrast

Question 10

MRI

Answer 10

No ionizing radiation used
Images created using energy emitted by hydrogen protons and strong magnetic field manipulation
Measures energy emitted by protons when they are returning to equilibrium
Altering magnetic field gradients can allow you to view a wide range of tissues

Question 11

T1 weighted MRI

Answer 11

Longitudinal relaxation time= measure of the time taken for spinning protons to realign with the external magnetic field
Fluid is dark in T1 weighted images

Question 12

T2 weighted MRI

Answer 12

Transverse relaxation time= measure of time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field
Fluid appears light in T2 weighted images

Question 13

Pros/cons of MRI

Answer 13

Pros: No radiation, better soft tissue contrast than CT, extremely versatile
Cons: Longer time, more expensive, images cannot be manipulated after the MRI is taken, worse for claustrophobic patients, no metal in body, noisy, cannot use Gadolinium contrast in pregnant women, patients with renal dysfunction have increased risk of NSF

Question 14

Ultrasound

Answer 14

High frequency transducers are used to visualize structures near the surface such as neck vessels and thyroid gland
Low frequency sound waves have greater penetrating power but less resolution and are used for deeper structures in abdomen and pelvis
Tissues deep to bone and air are difficult to visualize
Doppler US can visualize and measure blood flow