Michel Lalonde

Development and optimization of SPECT gated blood pool cluster analysis for the prediction of CRT outcome

PURPOSE Phase analysis of single photon emission computed tomography (SPECT) radionuclide angiography (RNA) has been investigated for its potential to predict the outcome of cardiac resynchronization therapy (CRT). However, phase analysis may be limited in its potential at predicting CRT outcome as valuable information may be lost by assuming that time-activity curves (TAC) follow a simple sinusoidal shape. A new method, cluster analysis, is proposed which directly evaluates the TACs and may lead to a better understanding of dyssynchrony patterns and CRT outcome. Cluster analysis algorithms were developed and optimized to maximize their ability to predict CRT response. METHODS About 49 patients (N = 27 ischemic etiology) received a SPECT RNA scan as well as positron emission tomography (PET) perfusion and viability scans prior to undergoing CRT. A semiautomated algorithm sampled the left ventricle wall to produce 568 TACs from SPECT RNA data. The TACs were then subjected to two different cluster analysis techniques, K-means, and normal average, where several input metrics were also varied to determine the optimal settings for the prediction of CRT outcome. Each TAC was assigned to a cluster group based on the comparison criteria and global and segmental cluster size and scores were used as measures of dyssynchrony and used to predict response to CRT. A repeated random twofold cross-validation technique was used to train and validate the cluster algorithm. Receiver operating characteristic (ROC) analysis was used to calculate the area under the curve (AUC) and compare results to those obtained for SPECT RNA phase analysis and PET scar size analysis methods. RESULTS Using the normal average cluster analysis approach, the septal wall produced statistically significant results for predicting CRT results in the ischemic population (ROC AUC = 0.73;p < 0.05 vs. equal chance ROC AUC = 0.50) with an optimal operating point of 71% sensitivity and 60% specificity. Cluster analysis results were similar to SPECT RNA phase analysis (ROC AUC = 0.78, p = 0.73 vs cluster AUC; sensitivity/specificity = 59%/89%) and PET scar size analysis (ROC AUC = 0.73, p = 1.0 vs cluster AUC; sensitivity/specificity = 76%/67%). CONCLUSIONS A SPECT RNA cluster analysis algorithm was developed for the prediction of CRT outcome. Cluster analysis results produced results equivalent to those obtained from Fourier and scar analysis.

SU‐FF‐T‐244: Investigation of Anomalous Recombination Behaviour in Cylindrical Ionization Chambers

Purpose: Measurements using air‐filled ionization chambers must be corrected for the effect of ion recombination. In recent years a number of authors have presented data for cylindrical and parallel chambers that do not agree with the standard Boag theory. An experimental investigation was carried out to look at the possible mechanisms that have been proposed for these deviations. Method and Materials: Four different Farmer‐type chambers ‐ PTW30001, NE2571, NE2581, Exradin A12 — were used to look at how the recombination behaviour depended on: i) chamber type; ii) continuous or pulsed radiation; iii) dose per pulse; iv) modality (photons or electrons). Three radiation sources were used: i) Sr‐90 check source; ii) Co‐60 beam; iii) Elekta Preciselinac.Results: It was found that all four chamber types showed similar non‐linear behaviour at polarizing voltages > 250 V using a low‐doserate Sr‐90 check source. This was a little surprising considering the significant differences in chamber design for these types. Also, all chamber types showed a difference in the shape of the 1/I vs 1/V plot for opposite polarities — the chamber signal was larger when collecting negative charge ‐ indicating that the effect may be due to free‐electron collection and/or charge multiplication. In the linac measurements no difference in behaviour was seen between photon and electron beams (as expected) but the non‐linear curvature and difference between polarities was only seen at low dose per pulse values (< 0.02 cGy). Stem and cable irradiation were also investigated and found to be insignificant. Conclusion: Deviations from classical Boag recombination theory have been observed for “well‐designed” Farmer chambers in a range of photon and electron beams. Measurements to date cannot single out one mechanism as the primary effect but indicate that insulator/stem irradiation and free‐electron collection are not significant.