Sample Lectures – How X-rays and CT Scans Work

 

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Next Step: Make Cards on the Automatic Key Concepts, and Vignettes

Remember, the more you automatically know what each sentence means on your test, the better you will do. There are 4 stages in making interpretation more automatic:

  • Stage 1: Unable to Make Pathophysiologic Chronologies in Either Timed or Untimed setting
  • Stage 2: Basic Pathophysiologic Chronologies, but with Significant Gaps
  • Stage 3: Detailed Pathophysiologic Chronology Without Time, but Unable to Consistently Generate PC During Timed Setting
  • Stage 4: Consistent Pathophysiologic Chronologies in Timed Setting

My goal with these vignettes is to help you reach Stage 4. How do you do so?

  • With the Automatic Key Concept cards, you can master the underlying information to move past Stages 1 + 2.
  • Then, with the Vignette/Pathophysiologic Chronology cards, you can teach yourself to make these connections on your exam.

Automatic Key Concepts:

Copy + paste these into your cards, to make these key concepts more automatic.

X-ray vs. CT: on a basic level, what determines whether something is brighter vs. darker?

Denser elements (lower on periodic table)

 

How would soft tissue appear on x-ray/CT?

Darker (less dense elements)

Recall that:

  1. Generally, elements lower on the periodic table have a higher nuclear density, and
  2. Nuclear density ↑ → chance of an x-ray passing through it to hit the detector on the other side ↓

Soft tissue is generally composed of hydrogen, carbon, nitrogen, and oxygen – elements with a low nuclear density – and thus would allow the x-rays to pass through to the other side

 

How would bone appear on x-ray/CT?

White (very electron dense)

Recall that:

  1. Generally, elements lower on the periodic table have a higher nuclear density, and
  2. Nuclear density ↑ → chance of an x-ray passing through it to hit the detector on the other side ↓

Bone has a high concentration of calcium and phosphorus – elements with higher nuclear density – which would prevent the x-rays from passing through → appear lighter on x-ray/CT

 

How would blood appear on x-ray/CT?

Lighter

Recall that:

  1. Generally, elements lower on the periodic table have a higher nuclear density, and
  2. Nuclear density ↑ → chance of an x-ray passing through it to hit the detector on the other side ↓

Blood has a high concentration of iron (within hemoglobin), which has a high nuclear density. Thus, x-rays would NOT pass through well → appear lighter on x-ray/CT

 

IV contrast – using the underlying basis, and assuming no contraindications (e.g., allergy, kidney failure) explain when you would NOT want to use it

IV contrast makes tissues brighter (more electron dense). You shouldn’t use IV contrast if the thing you are looking for is already electron-dense –

Bones (e.g. fractures),

(Intracranial) Bleeding (classic = stroke, to rule out hemorrhage stroke prior to giving thrombolytics), or

Kidney stones

 

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