What is an HU?
Hounsfield unit. It represents 1/1000 of the difference between the linear attenuation coefficient of water and air. (That’s why techs do air calibrations.)
One can think of it as a scale based on two physical constants/conditions, just like the Celsius temperature scale is based on the freezing and boiling points of water. For this reason stating that -1000 is air, 0 is water, and +1000 is bone is incorrect. The original unit of measurement was the EMI unit, -500 for air and 0 for water.
Regarding window and level, window can be thought as contrast and level can be thought of as brightness.
A reference:
Sir Godfrey Hounsfield won the Nobel Prize in 1979 (along with Allan Cormack) "for the development of computer assisted tomography"
http://www.nobelprize.org/nobel_prizes/medicine/laureates/1979/
Dr. Hounsfield may not have had funding for his research if it were not for the Beatles making lots of money for EMI
http://www.nobelprize.org/nobel_prizes/medicine/laureates/1979/perspectives.html
EMI has seen better days
http://www.economist.com/node/21559370
Residents should understand some basics regarding image formation.
Step one
the CT scanner acquires "raw data", this is typically stored on the CT scanner for a few days. From the raw data of an abdomen and pelvis, one can reconstruct a lumbar spine CT study.
Raw data is processed with a "field of view" (FOV) and "algorithm" to determine image appearance.
e.g. a temporal bone CT uses a small FOV in order to maximize the spatial resolution.
e.g. we commonly reconstruct spine CTs with a "soft tissue"and "bone""soft tissue" algorithm to optimize diagnosis. Algorithms vary from smooth with blurring of edges to edge enhancement, good for bone.
see Core Exam practice question 110
Images can either go to PACS or a workstation for further processing (Vitrea)
from vitrea source images can be manipulated (source images on Vitrea are eventually discarded as well, only source images transferred to PACS are archived)
"2-D" reformatted images
"3-D" e.g. MIP CTA, shaded surface display images
CT perfusion
As you know images in PACS can be displayed with a variety of windows/levels including "bone" and "soft tissue" this can potentially lead to confusion.
Some PACS stations allow you to apply filters to images. e.g. a sharp filter applied to a brain image will approximate a "bone window"
Important Compliance Information
When we bill for a CTA or MRA, we must document that 3-D images were created. Our macros must have a phrase such as:
"3-D Maximum Intensity Pixel (MIP) images were created and reviewed."
We must also document the amount and type of contrast given, e.g.:
"100 ml of Isovue 300 was given IV." rate information may be useful, but is not required for billing purposes.