In vivo magnetic resonance imaging of the effects of photodynamic therapy

Nuclear magnetic resonance (NMR) proton imaging and measurements of the parameters T1 and T2, have been carried out in vivo on the murine mammary tumour T50/80. Tumours had been treated 24 h previously by photodynamic therapy (PDT, using haematoporphyrin derivative and 630 nm laser light). Proton images clearly demarcated a high signal-intensity region on the side of the tumour closest to the incident light beam, while the parts of the tumour more remote from the beam resembled the images from untreated controls. Both T1 and T2 values were raised in the high-intensity region. This high-intensity region was shown to correspond to PDT-induced histological necrosis, the low-intensity region to histologically intact tumour. Linear regression analysis of the relationship of depth of necrosis measured histologically and 'depth of necrosis' measured from the NMR images, yielded a slope of 0.93 (r2 = 0.95).