Thursday, May 9, 2013

Beam Me Up

[Written on May 28 from diary notes written May 9]

The CT scan was this afternoon. They used an intravenous contrast agent. The scan itself was pretty quick; I think I was in the machine for maybe 5 minutes.

This was the first time I've ever had a CT scan. I was fascinated by the machine. I learned about CT machines back in the 90s when I worked for a scientific visualization software company and did a lot of demos and consulting with CT data. I think in earlier generations of machines the x-ray sources and detectors were distributed around a stationary ring. But in this machine (Damn! I didn't note the make and model!) the ring rotated very rapidly during the imaging phase.

The radiology nurse told me results would be sent to my doctor within 48 hours.

Teck Talk:

Hey, it's our first opportunity for a Tech Talk! This is a regular feature of Cancer Fun Time! in which I'll explain how some piece of medical technology or procedure works.

A CT scan and CAT scan are the same thing. CAT stands for Computed Axial Tomography, but the "axial" part is not important so it is often referred to as just "CT".

To vastly oversimplify it, what CT does is take a large number of x-rays from many different angles all at once. The geometry of the radiation sources and the radiation detectors is known, so geometry and trigonometry (plus some rather complicated physics and math) can be used to process all those "images" and map the results to a regular 3-dimensional grid. Since x-rays basically measure density, you end up with a 3-dimensional grid of density measurements. Bone is about as dense as it gets for human tissues. Air is as undense as it gets. Soft tissues have various levels of density in between. When you assign a color scale to density, with bone=white and air=black and shades of gray in between, you get images that look like traditional x-ray films.

Given that you have that 3-dimensional grid of densities, you can use algorithms to "slice" the volume on any plane you wish. So instead of having just a flat 2D image like a conventional x-ray, the radiologist can move through the volume of the patient's body looking at any "cutting plane" he or she desires. In practice they tend to use standard cutting planes, called sagittal (the vertical plane formed by your flat hand held up edge-n to your nose), coronal (the vertical plane that would pass though both your ears) and transverse (the horizontal plane formed by your flat hand held edge-on in front of both your eyes). But then imagine being able to move any of those planes in the direction perpendicular to the plane.

CT scans are often performed with a contrast agent. A contrast is a fluid injected intravenously just before the scan. It contains a substance, such as iodine, that will diffuse differently into different tissue types and make it easier for the radiologist to see boundaries between different tissue types or structures. It does just its name implies: it enhances image contrast. For my scan I received a contrast called Optiray 350.

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