02 Imaging

Fluoroscopy air-kerma rate

K̇ at IRP from displayed K̇ and inverse-square, plus cumulative KAP → time.

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Simulation

Fluoroscopy air-kerma rate — Change the numbers; the scene follows.

Where it works

Brachytherapy suite

C-arm imaging

At the C-arm image receptor during fluoroscopy — air-kerma rate at the skin.

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Formula

\dot K_{\mathrm{IRP}}=\dot K_{\mathrm{ref}}\left(\frac{d_{\mathrm{ref}}}{d_{\mathrm{IRP}}}\right)^2,\quad t=K/\dot K

Variables

K̇_ref · Displayed ratemGy/mind_ref · Display distancecmd_IRP · IRP distancecmK · Cumulative air kermamGy

Results

K̇_IRP
Rate at IRP
40mGy/min
t
Time for this kerma
12.5min
t_2Gy
Time to 2 Gy
50min

Explanation

\dot K_{\mathrm{IRP}}=\dot K_{\mathrm{ref}}\left(\frac{d_{\mathrm{ref}}}{d_{\mathrm{IRP}}}\right)^2,\quad t=K/\dot K

What it means

Interventional reference point (IRP) is 15 cm toward the tube from isocentre on a C-arm. Displayed air-kerma rate is legally capped (typical US: 88 mGy/min in normal fluoro, 176 mGy/min in high-level). Skin dose is this number × backscatter × f-factor × a table-attenuation correction, and it is what triggers the 2, 5, 10 Gy substantial-radiation-dose-level alerts. This is a working relation in Diagnostic imaging.

Where it is used

Clinically it sits on the Brachytherapy suite — C-arm imaging. At the C-arm image receptor during fluoroscopy — air-kerma rate at the skin. 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.

Brachytherapy suite · Open this machine

How to use it

Enter the displayed rate, the distances, and optionally cumulative air kerma. Read IRP rate and the minutes to a 2 Gy skin-dose alert (crude, no BSF). Change one input and watch the curve and the simulation follow.

Symbols

K̇_refDisplayed rate40 mGy/min
d_refDisplay distance60 cm
d_IRPIRP distance60 cm
KCumulative air kerma500 mGy

Worked example

A typical case from the default values: K̇_ref = 40 mGy/min (Displayed rate); d_ref = 60 cm (Display distance); d_IRP = 60 cm (IRP distance); K = 500 mGy (Cumulative air kerma). Substituting into the relation gives K̇_IRP = 40 mGy/min; t = 12.5 min; t_2Gy = 50 min. These are teaching numbers — align them with your machine.

Typical values give

K̇_IRP = 40mGy/min
t = 12.5min
t_2Gy = 50min

Where it comes from

The displayed formula is the working relation. K̇ at IRP from displayed K̇ and inverse-square, plus cumulative KAP → time. Usual reference: IEC 60601-2-54 / NCRP 168. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: IEC 60601-2-54 / NCRP 168

Assumptions & limits

Does not include backscatter (typically ×1.3 at diagnostic quality), table attenuation, or f-factor to skin (≈ 1.06). A real peak-skin-dose map needs the DICOM RDSR and a geometric model (e.g. em.dose, OpenSkin).

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. Does not include backscatter (typically ×1.3 at diagnostic quality), table attenuation, or f-factor to skin (≈ 1.06). A real peak-skin-dose map needs the DICOM RDSR and a geometric model (e.g. em.dose, OpenSkin).

Keep this

Technique is physics: kV sets contrast, mAs sets noise, filtration sets the spectrum. Does not include backscatter (typically ×1.3 at diagnostic quality), table attenuation, or f-factor to skin (≈ 1.06). A real peak-skin-dose map needs the DICOM RDSR and a geometric model (e.g. em.dose, OpenSkin).

In this specialty

Diagnostic imaging