Imaging modalities and physics: line focus principle: practice question

A Radiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Radiology · Imaging modalities and physics: line focus principle

Sample item

The electron beam strikes a rectangular track several millimetres wide on the anode, yet the focal spot projected towards the patient is much smaller because the target face is steeply inclined. Which principle is this?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Line focus principle

A steeply angled target lets a large actual focal area (good heat loading) project as a small effective focal spot (sharp image). The smaller the anode angle, the smaller the effective spot, but the field coverage also falls.

Option by option

  • A. Space charge effect

    Space charge effect is the limit on tube current set by the electron cloud around the filament.

  • B. Heel effect

    The heel effect is the lower beam intensity on the anode side because of self-absorption, not a change in focal spot size.

  • C. Off-focus radiation

    Off-focus radiation is X-rays made away from the focal spot by stray electrons and adds image blur.

  • D. Line focus principle (correct)

    Correct. Target angulation shrinks the effective focal spot while the heat is spread over the larger actual area.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Radiology question

What is the answer to this Radiology question?

The best answer is D, Line focus principle. A steeply angled target lets a large actual focal area (good heat loading) project as a small effective focal spot (sharp image).

Why is "Space charge effect" not the answer?

Space charge effect is the limit on tube current set by the electron cloud around the filament.

Why is "Heel effect" not the answer?

The heel effect is the lower beam intensity on the anode side because of self-absorption, not a change in focal spot size.

Why is "Off-focus radiation" not the answer?

Off-focus radiation is X-rays made away from the focal spot by stray electrons and adds image blur.

What concept does this question test?

It tests Imaging modalities and physics: line focus principle in Radiology. The smaller the anode angle, the smaller the effective spot, but the field coverage also falls.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Imaging modalities and physics: line focus principle?

Learn signs with the image, and the disease they point to. Then use the flashcards and the other Radiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.