Focal Spot, Anode, and Heel Effect

Focal spot

General term for the area on the anode where electrons hit

Actual focal spot

Real physical area on the anode struck by electrons

Effective focal spot

Apparent size of the focal spot as seen from the image receptor

Line-focus principle

Uses an angled anode so a large actual focal spot appears as a smaller effective focal spot

Small effective focal spot

Improves spatial resolution

Why small focal spot improves spatial resolution

X-rays originate from a smaller area, causing less geometric blur

Small anode angle

Produces a smaller effective focal spot

Large anode angle

Produces a larger effective focal spot

smaller, improves

Small anode angle: Creates a ___ effective focal spot, which ___ spatial resolution

Large anode angle and spatial resolution

Lower spatial resolution

Anode heel effect

Unequal x-ray beam intensity caused by absorption of x-rays within the angled anode

Cathode side intensity

Stronger

Anode side intensity

Weaker

Heel effect patient placement

Place thicker anatomy under the cathode side

cathode side

Thicker side goes under the

Short SID

greater visible heel effect because beam divergence is greater.

Long SID

What SID makes the heel effect less noticeable?

Large field size and heel effect

Increases the visible heel effect

Small FOV and heel effect

Decreases the visible heel effect

Small angle, anode, greater heel effect

_____ → more anode absorption → weaker _____ side → bigger cathode/anode difference → ______

Large anode angle

less heel effect because anode-side x-rays escape more easily, making the cathode and anode sides more equal in intensity.

Large filament

Produces a larger actual focal spot

Small filament

Produces a smaller actual focal spot

more, less, higher

Larger actual focal spot: Spreads electron impact over ____ of the anode, so heat is ____ concentrated and the tube can handle ____ loading

Small focal spot and heat

Concentrates heat over a smaller area and has less heat-handling ability

Anode rotation

Spreads heat around the focal track

Faster anode rotation

Improves heat-handling ability by spreading heat more evenly

Anode cooling curve

Shows how stored anode heat decreases over time

heat units (HU)

Cooling curve Y-axis stands for:

Cooling curve X-axis

Cooling time

Remaining cooling time

Time still required for the anode to cool from its current heat level

Repeated exposures

New heat is added to the heat still remaining from earlier exposures

HU = kVp × mAs × gen. factor

Heat units formula

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