Defining the lung outline from a gamma camera transmission attenuation map

The Use of Single Photon Emission Computed Tomography in Aerosol Medicine

Imaging of radiolabeled aerosols provides useful in vivo data on both the initial site of deposition and its subsequent transport by mucociliary clearance and epithelial permeability. Single Photon Emission Computed Tomography (SPECT) uses a gamma camera with multiple rotating heads to produce three-dimensional (3D) images of inhaled radioaerosol labeled with technetium-99m. This enables total lung deposition and its 3D regional distribution to be quantified. Aligned 3D images of lung structure allow deposition data to be related to lung anatomy. Mucociliary clearance or epithelial permeability can be assessed from a time series of SPECT aerosol images. SPECT is slightly superior to planar imaging for measuring total lung deposition. However, it is more complex to use, and for studies where total lung deposition is the endpoint, planar imaging is recommended. However, SPECT has been shown to be clearly superior to planar imaging for assessing regional distribution of aerosol and is the method of choice for this purpose. It therefore has applications in studying the influence of regional deposition on clinical effectiveness and also in validating computer models of deposition. The inability to directly radiolabel drug molecules with ^99mTc is a clear disadvantage of SPECT and limits its potential use for pharmacokinetic studies. SPECT provides a wealth of data on aerosol deposition, which has been relatively underused at present. Optimal methods of analyzing and interpreting the data need to be developed. SPECT can also, in principle, provide detailed information of mucociliary clearance and has the potential to significantly improve knowledge of this process and hence clarify the role of clearance as a biomarker.

Comparison of methods of acquiring attenuation maps for cardiac SPECT in the presence of respiratory motion

BACKGROUND

We investigated the effect of the respiratory motion on attenuation-corrected (AC) SPECT images for three different SPECT systems, each using a different approach in obtaining attenuation maps: scanning-line sources (SLS) acquired simultaneous with emission; slow cone-beam CT (CBCT) acquired sequentially to emission; and fast helical CT (HCT) acquired sequentially to emission.

METHODS

A torso phantom filled with (99m)Tc was used to model a cardiac perfusion study. Stationary baseline acquisitions were followed by the acquisitions with the phantom moving axially using a computer-controlled motion platform to simulate breathing. In addition, HCT acquisitions were made simulating breath-hold at different extents of misalignment between CT and emission. HCT images were also used to simulate the Average-CT method. Acquisitions were repeated with added breast attachments, and the heart insert in two different orientations. Visual comparison was made of AC maps, AC emission slices and polar maps. Quantitative comparisons were made of global uniformity based on the percent fractional standard deviation (%FSD) of the polar map segment values, and the ratio of the segment values in the Anterior and Inferior walls divided by that of the Lateral and Septal walls (AI/LS ratio).

RESULTS

The AC maps for the SLS were inferior to the CT's, and most impacted by added large breast attachment. Motion artifacts seen on CBCT slices were minimized in the derived attenuation maps. AC maps obtained from HCT showed inconsistent organ sizes depending on the direction of respiration at the time of acquisition. Both visually and quantitatively CBCT resulted in the best uniformity (up to 3.4 % lower in %FSD) for all the stationary acquisitions, and for the motion acquisition of the female phantom with large breast attachment (up to 4.0 % lower). For the motion acquisition of the male phantoms, HCT resulted in slightly better uniformity (<0.5 % lower) than CBCT. Breath-hold at end-expiration slightly improved (up to 1.1 %) the uniformity over the HCT acquired during regular breathing. Further improvement was achieved with the Average-CT method. For all the systems, phantom respiratory motion reduced the AI/LS ratio compared to when the phantoms were stationary.

CONCLUSIONS

The CBCT approach resulted in the best uniformity of the AC emission images. For the female phantom with larger breast attachment, HCT and SLS were truncated at some projection angles introducing artifacts into the AC emission images. The emission image artifacts observed with HCT could be mitigated by performing breath-hold acquisition at end-expiration or Average-CT type acquisitions.