MEDRADSC 3K03 Lecture 7 - Image Reconstruction and CT Numbers

DFOV is also called a

Zoom or target

What does DFOV determine?

How much raw data is used to reconstruct an image → diameter of reconstructed image

DFOV/SFOV cannot be larger than DFOV/SFOV

DFOV, SFOV

Small DFOV

Takes small portion of data and fills with matrix

Large DFOV

Condenses large amts of tissue into matrix, more data per pixel so decreases spatial resol

Image center

When reconstructing image, in addition to identifying DFOV diameter, must specify which part of SFOV we want

How can we specify which part of the SFOV we want?

• Cartesian coordinates

  • Mathematical X & Y coordinates

• RAS coordinates

  • Right/left, ant/post, sup/inf

  • Still cartesian but based on body locations

When is a narrow WW used?

Show tissues of similar densities, small difference in CT number

• Values spread out over available greyscale

• Small differences are going to be assigned separate shades of gray

Why do we want a narrow window to visualize the brain?

CSF, white and gray matter all have CT numbers close together

When do we use a wide WW?

To visualize tissues that vary greatly; can also decrease visualization of noise

Downside of a wide WW

Dec image contrast, difficult to see differences between gray and white matter

What are the different WW, WL needed for when visualizing a CT scan of the thorax?

1. Mediastinum 

2. Air filled lungs

3. Ribs and calcifications

For a lung window for a thorax scan, why do we want a wide WW?

To show more gray for smaller structures

When do we need a bone window for a thorax scan?

Requested if pt has suffered any trauma or if looking for calcifications

What can we see in a mediastinal window for a thorax scan that we can’t see in a lung scan?

Pulmonary artery