The anode heel affect 1. should be considered when the structure of interest is more than 43 inches long. 2. is a variation in photon intensity across the exposure field. 3. occurs because the heel of the anode absorbs more photons than the toe. 4. is best used with the thinner body toward the cathode.

Option 2. and 3.

Explanation:

Anode heel effect or simply heel effect is observed in X-ray tubes.

It is the change in the intensity of the emitted X-rays from the anode and this intensity is direction dependent and depends on the direction of the emitted X-rays along the axis of anode-cathode.

This effect results from the absorption of X-rays prior to leaving the anode, their production source.

The absorption of photons is more in the anode heel than the toe of the anode which results in the anode heel effect.

1 is a answer of a question