====== Prelab ======

We're going to start out by thinking about a somewhat analogous situation in order to think about the basics of tomography.

Imagine you have a setup with a laser (the <wrap text_lake>blue</wrap> rectangle), a detector (the <wrap text_ivy>green</wrap> rectangle), and a box that is covered from above.  

<div figure>
| {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:tomography_setup.png}} |
</div>

If we turn on the laser, we measure some initial high intensity of light.  As we sweep our laser and detector along, we find that the detector is blocked for some distance, and is then unblocked (see below)

<div figure wide>
| {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:simple_initial.png?250}} |  {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:simple_mid.png?250}}  | {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:simple_end.png?250}}  | 
</div>

We know that there's //something// in the shaded area that's blocking the laser, but we don't know anything about where it is on the $y$ axis.  To fix that, we'll rotate our detector $90^\circ$ and measure the laser intensity again, shown below.

<div figure wide>
|   {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:simple_side_initial.png?250}}  |  {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:simple_side_mid.png?250}}  |  {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:simple_side_end.png?250}}  |
</div>

At this point, we can say that there's something in the center of our box.  We can't quite tell if its a square, a circle, or something else.  We just know that it is bounded by a square outline so far.

<div figure wide>
| {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:shape_examples_2x.png}}|
| Examples of what could be lurking in our unknown box.  |
</div>


<div checkpoint>
==== Prelab exercises Pt 1  ====

  *  For the situations depicted below, indicate which regions may contain an object obstructing the laser.

<div figure wide>
| {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:question_1.png?350px}} | {{phylabs:lab_courses:phys-211-wiki-home:positron-emission-tomography:question_2.png?350px}} | 
</div>