Sir.—The Radiological case of the month in a recent issue of the Journal (132:311-312, 1978) is an excellent example of the use of pulmonary perfusion imaging in congenital lobar emphysema. The legend for Fig 2, however, states that the authors are uncertain as to the reason for the activity that was noted within both kidneys. I suggest the following explanation.
Most radiopharmaceuticals enter the systemic circulation after intravenous injection, with subsequent localization within a specific organ in concentrations greater than that of adjacent organs. Lung scanning is unique, however, in that it utilizes particles in the size range of 10 to 20 μ that "embolize" the pulmonary precapillary arterioles. Thus, for particles to be seen in the arterial circulation, there must be a right-to-left cardiac shunt or deficiencies in the preparation of the lung imaging agent.
When smaller particles are present in lung imaging preparation, they traverse the