. The American journal of roentgenology, radium therapy and nuclear medicine . ror.—Of these the mostdifficult to eliminate is the question of theexact location of the focus points. With this localizer, when several localiza-tions are to be made before any of themare operated it is most essential that thevarious measurements and adjustments beaccurately recorded. HiRTZ Compass.—^Another graphic meth-od using a widely different application ofthe triangulation process is the compassof Hirtz 2^ In Fig. 5 let (R) be the sensitized its envelope is placed a lead marker(V) and upon this is
. The American journal of roentgenology, radium therapy and nuclear medicine . ror.—Of these the mostdifficult to eliminate is the question of theexact location of the focus points. With this localizer, when several localiza-tions are to be made before any of themare operated it is most essential that thevarious measurements and adjustments beaccurately recorded. HiRTZ Compass.—^Another graphic meth-od using a widely different application ofthe triangulation process is the compassof Hirtz 2^ In Fig. 5 let (R) be the sensitized its envelope is placed a lead marker(V) and upon this is accurately centeredthe direct ray from focus point (F). Theprojectile is (P). {A,Band C) are three metalmarkers at conveniently chosen points onthe skin. If the focus is displaced to (F) andto (F), (Fig. 6), and an exposure made ateach point, there will result upon the platetwo shadows each of (A, B, C and P). Knowing the distances (VV) and (FF)we are able, by measuring the displace- ment in each pair of shadows, to set forthall the data needed for the primary Fig. 5. Diagram of the Hirtz Compass. Making the Plate.—(i) The approximateposition of foreign body should first be de-termined by one of the fluoroscopic meth-ods. (2) Upon the center of a plate of propersize place a marker, say a lead triangle (F).Center the focus accurately above thispoint. (3) Being careful not to change positionof either tube or plate, place the patient sothat, as well as can be judged, the shadowof the projectile will fall a little to one sideof the center (F). The position should alsobe one desirable for operation. (4) Choose and mark with lead indica-tors three convenient skin points to form atriangle, of which the normal ray (VV)shall be the approximate center; not toonear together lest they embarrass the oper-ative incision; not too far apart lest theirshadows, one or both, be projected outsidethe plate. It is well to have these markersperforated in the center. (5) Fix the focus
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