Multi-Modality Imaging: The Use of Single Photon Emission Computed Tomography and Magnetic Resonance Imaging to Assess Regional Intra-Pulmonary Deposition of Nebulised Aerosols

Single Photon Emission Computed Tomography (SPECT) is now accepted as one method of providing 3-D images of an inhaled radioaerosol. Using SPECT, we have reconstructed 3-D images incorporating an attenuation correction technique utilising aligned Magnetic Resonance Imaging (MRI) of the thorax. This permits three-dimensional quantitative analysis of intra-pulmonary deposition by a variety of techniques performed on a RISC workstation. Deposition of nebulised radio-aerosols (Tc-99m labelled human serum albumin) of 1.5 mu m MMAD (Optimist, Medic-Aid) and 6.5 mu m MMAD (Inspiron Bard) has been now been assessed in 12 subjects. All subjects were non-smokers and with no evidence of small airways disease. The aerosols were inhaled at tidal volume in a controlled manner in the erect, seated position. Using a shell analysis (Conway and Holgate 1994) the pattern of deposition for the total mass per shell and for surface density per shell was assessed. SPECT and MRI were found to be more sensitive than planar scintigraphy in determining the greater central deposition for the larger aerosol. Individual patterns of deposition could also be described. SPECT and MRI further allowed the deposition within the three lobes of the right lung to be assessed. A striking preference for the posterior base of the lung was seen on all scans. The implications of this will be discussed.