02 Imaging

K-edge energy

K-edge ≈ 1.17 × Moseley Kα. Iodine 33.2 keV, barium 37.4, gadolinium 50.2, tungsten 69.5.

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Simulation

K-edge energy — Change the numbers; the scene follows.

Where it works

Radiography room

X-ray tube

In the beam filtration and contrast agent — K-edge energy versus kVp.

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Formula

E_K\approx\frac{13.6(Z-1)^2}{1000}\times\frac{4}{3}\ \mathrm{keV}

Typical values

Variables

Z · Atomic number

Results

E_K est.
Estimated K-edge
49.0325keV
E_K
Measured K-edge if known
33.17keV

Explanation

E_K\approx\frac{13.6(Z-1)^2}{1000}\times\frac{4}{3}\ \mathrm{keV}

What it means

The K-edge is the binding energy of the K-shell: photoelectric absorption jumps by a factor of 3–5 just above it. Iodine (33.2 keV) and barium (37.4 keV) are matched to the 60–80 kVp spectra of fluoroscopy and GI work. Gadolinium K-edge (50.2 keV) is why Gd can be a CT contrast when iodine is contraindicated. Tungsten K-edge (69.5 keV) is why a 70 kVp beam barely makes W characteristic x-rays. This is a working relation in Diagnostic imaging.

Where it is used

Clinically it sits on the Radiography room — X-ray tube. In the beam filtration and contrast agent — K-edge energy versus kVp. Diagnostic equations live on the tube, the detector, and the patient: magnification, air kerma, CTDI, and why bone lights up at 70 kV. They turn a technique chart into physics you can defend.

Radiography room · Open this machine

How to use it

Enter Z. The calculator uses a hydrogen-like estimate and also lists the textbook measured K-edge when Z matches a common radiology element. Change one input and watch the curve and the simulation follow.

Symbols

ZAtomic number53

Worked example

A typical case from the default values: Z = 53 (Atomic number). Substituting into the relation gives E_K est. = 49.0325 keV; E_K = 33.17 keV. These are teaching numbers — align them with your machine.

Typical values give

E_K est. = 49.0325keV
E_K = 33.17keV

Where it comes from

The displayed formula is the working relation. K-edge ≈ 1.17 × Moseley Kα. Iodine 33.2 keV, barium 37.4, gadolinium 50.2, tungsten 69.5. Usual reference: Bushberg / NIST. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Bushberg / NIST

Assumptions & limits

Moseley-based estimate, ~5–10% low for high Z. Use NIST XCOM for the real edge. L-edges matter in mammography (Mo L, Rh L) and are not computed here.

Pitfalls

kVp is not the same as effective energy. CTDI is not patient dose — SSDE and organ dose come after. Do not quote DLP as if it were effective dose without a k-factor. Moseley-based estimate, ~5–10% low for high Z. Use NIST XCOM for the real edge. L-edges matter in mammography (Mo L, Rh L) and are not computed here.

Keep this

Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Moseley-based estimate, ~5–10% low for high Z. Use NIST XCOM for the real edge. L-edges matter in mammography (Mo L, Rh L) and are not computed here.

In this specialty

Diagnostic imaging