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What is the Half Value Layer (HVL)

• the thickness of material (like aluminum or tissue) needed to reduce the x-ray beam intensity by half.

• It shows beam penetrability or quality.

• Higher HVL = more penetrating beam.

The HVL for a diagnostic x-ray beam is typically

3–5 mm of aluminum (Al).

This means it takes 3 to 5 mm of aluminum to reduce the x-ray beam’s intensity by 50%, which reflects how penetrating (hard) the beam is.

The HVL in soft tissue is typically

3–6 mm.

This means it takes 3 to 6 mm of soft tissue to reduce the x-ray beam’s intensity by half, depending on the beam energy (kVp)

The total filtration in the x-ray tube must be at least

2.5 mm aluminum equivalent for any tube operating over 70 kVp

The 2.5 mm aluminum equivalent is made up of

• 0.5 mm inherent filtration (from the tube housing, glass envelope, oil, etc.)

• 2.0 mm added filtration (usually aluminum sheets placed in the beam path)

The added filtration is located between th

x-ray tube and the collimator box.

It usually consists of thin aluminum sheets placed in the path of the x-ray beam to help filter out low-energy, non-diagnostic x-rays before the beam reaches the patient.

The added filtration is typically made up of:

The added filtration is typically made up of:

• 1.0 mm from the aperture diaphragm

• 1.0 mm from collimator components (like mirrors or metal plates)

Together, they contribute to the 2.0 mm added filtration required as part of the total 2.5 mm Al equivalent in x-ray tubes operating over 70 kVp.

The aperture diaphragm:

• Cleans up low-energy x-rays that would increase patient dose without improving the image

• Reduces off-focus radiation, which can blur the image or expose unwanted areas