An ultrasonic imaging apparatus and method are described for monitoring the progress of therapy for pathology such as lesions, tumors, and metastases by means of contrast agent imaging. A sequence of images are acquired as a bolus of contrast agent infuses the tissue containing the pathology. Time-intensity curves of pathology and normal tissue are produced and used to measure perfusion in the pathology as affected by therapy. The motional effects of respiration can be taken into account during image acquisition by detecting the position of a landmark such as the diaphragm in each of the images of the sequence and discarding from processing those images which exhibit a change in the position of the landmark relative to the probe.