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(b) The source-to-panel or source-to-tabletop distance for all healing arts fluoroscopic installations may not be less than 15 inches, except that an FDA-approved mini C-arm may not be less than 10 inches. (c) The half-value layer (HVL) of the useful beam may not be less than as shown in the following table: Measured Potential (kilovolts peak) Half-value Layer (millimeters of aluminum equivalent) 70 and below 1.5 71 2.1 80 2.3 90 2.5 100 2.7 110 3.0 120 3.2 (d) The equipment must be constructed in a manner that ensures that the entire cross section of the useful beam is attenuated by a primary barrier designed to automatically terminate exposure when the barrier is removed from the useful beam, which is usually the image intensification mechanism, and

(1) the required lead equivalent of the barrier may not be less than

(A) 1.5 millimeters for up to 100 kVp;

(B) 1.8 millimeters for greater than 100 and less than 125 kVp; and

(C) 2.0 millimeters for 125 kVp or greater;

(2) a collimator must restrict the cross-sectional dimensions of the useful beam to less than the corresponding dimensions of the barrier; the tube and collimating system must be linked with the image intensification assembly so that the useful beam at the image intensifier input phosphor is confined within the barrier regardless of the panel-screen distance;

(3) the tube mounting and the barrier (the viewing device) must be linked together in a