Читать книгу Interventional Cardiology - Группа авторов - Страница 106

Separation of the bifurcation of the LMS in the LAO caudal view shows the ostium of the LAD clearly, with some foreshortening of the proximal LAD, and a good separation of the origin of the left circumflex and the first diagonal: for these reasons the LAO caudal view is useful for wiring the proximal LAD, for stent positioning at the ostium of the LAD or across the LM bifurcation. If possible, it should be avoided as a working view because X‐ray attenuation caused by the highly angled projection through the spine results in higher X‐ray doses. Alternative working views for the LAD ostium include RAO or AP caudal views. After wiring the LAD in a caudal view, operators move to a cranial view which shows more clearly the mid LAD and separates diagonal and septal branches. In the RAO cranial, more than 30° of rightward angulation is sometimes required to move the circumflex off the region of interest. Although the RAO cranial view can clearly demonstrate lesions in the proximal and mid LAD, this is not the ideal working view because steep rightward angulation over 40° is required to eliminate overlap with diagonals and wide diaphragmatic excursions during breathing cause highly variable contrast inhomogeneities in the field of view. Simply moving the gantry from AP to a steep 35‐40° AP cranial elongates the proximal LAD, reduces the superimposition with the left circumflex and separates the diagonals to the right of the screen. A rightward tilt of less than 5° may be required to separate the proximal segment from the spine and avoid superimposition with the mid catheter or sheath in the aorta, in the rare instances a femoral approach is used. The AP cranial is an excellent standard working view for the proximal and mid LAD and is less affected by movement of the diaphragm. For diagnostic purposes, the ostia of the diagonals may be better seen in the LAO cranial view. However, LAO cranial is seldom used as a working view because a deep breath hold is required to reduce foreshortening and superimposition of the diaphragm over the proximal and mid LAD, a maneuver that can disengage the catheter during radial approach. The body habitus of some patients also requires steep leftward angulation to project the LAD off the spine. The left lateral is an alternative, though less frequently used, working view for lesions in the proximal, mid and particularly for the distal LAD. If the only vessel of interest is the LAD, as the anterior chest where the LAD runs is not covered, it is not necessary to ask patients to remove their arms from the field of view by keeping them above their heads, a movement impossible in case of a radial approach and uncomfortable, especially for elderly patients with arthritis old shoulder injuries.

Modern X‐ray system have sharp definition of the vessel contours also using a relatively wide field of view (23 cm/9 inch), imaging the entire heart except in case of severe enlargement and offering off‐line electronic magnification to study vessel details. These wide fields of view reduce radiation burden, especially if appropriate filtering is used, and allow fitting all the arteries without panning. For 18 cm/7 Fr or less panning is usually required in the RAO cranial and LAO cranial views to image the distal LAD. Smooth slow panning allows the X‐ray generator to adjust automatically to changes in X‐ray attenuation. The lateral view is a good alternative for demonstrating the distal LAD around the apex but can also require controlled movement of the table during the acquisition toward the floor and/or in the direction of the head. The RAO caudal view can include the distal LAD without the requirement for table movement or being affected by diaphragmatic movement.