A Novel Approach to CT, MR, and PET Examination of Patients with Infections Requiring Stringent Airborne Precautions.

PURPOSE: To investigate the feasibility of using a modified portable isolation chamber, which conforms to Centers for Disease Control and Prevention (CDC) isolation requirements, in the imaging of infectious patients. MATERIALS AND METHODS: This study was approved by the ethics committee, and all participants gave written informed consent. In this prospective study, the isolation chamber was assessed for computed tomographic (CT), magnetic resonance (MR), and positron emission tomographic (PET) image uniformity and noise by using uniform phantoms. For each modality, equivalent phantom examinations were performed without the isolation chamber. Paired analyses of the differences from these baseline values were conducted by finding the mean difference in the matched sections for each image quality parameter. A potential increase in CT patient dose was assessed, and MR radiofrequency (RF) interference was monitored. Eight participants with active pulmonary tuberculosis (mean age, 48.1 years; age range, 26-88 years; five men, three women) were then examined within a hybrid PET/MR imager. The 95% confidence intervals for the difference in the two matched population means were determined by using the two-sided t distribution for each of the phantom study imaging modalities. RESULTS: Phantom images were evaluated for image uniformity and noise. Increased image noise can affect low contrast resolution, which has the potential to mimic or mask abnormalities when the differences between healthy and diseased tissues are small; clinically, CT image noise is maintained at a constant level with dose modulation. Increased attenuation of annihilation photons, when not corrected for, could lead to photopenic areas on the PET image; PET image nonuniformity complied with guidelines. Artifacts on the MR image due to RF noise spikes could mask abnormalities; paired analysis of variations in MR imaging mean signal-to-noise ratio and uniformity from baseline were within 5% for both gradient-echo and spin-echo sequences. In the eight participants who underwent imaging, the increased radiation dose for the attenuation of the isolation chamber would have resulted in a mean increase in patient size-specific dose estimate of 0.32 mGy ± 0.04 (standard deviation). The RF noise assessment revealed no prominent increase at any frequency band. The eight participants were examined within the isolation chamber without incident. CONCLUSION A modified portable isolation chamber, which conforms to CDC infection control guidelines, was found to be feasible within the confines of CT, MR imaging, and PET environments.

Cardiac motion and spillover correction for quantitative PET imaging using dynamic MRI.

PURPOSE: Cardiac positron emission tomography/magnetic resonance imaging (PET/MRI) acquisition presents novel clinical applications thanks to the combination of viability and metabolic imaging (PET) and functional and structural imaging (MRI). However, the resolution of PET, as well as cardiac and respiratory motion in nongated cardiac imaging acquisition protocols, leads to a reduction in image quality and severe quantitative bias. Respiratory or cardiac motion is customarily addressed with gated reconstruction which results in higher noise. METHODS: Inspired by a method that has been used in brain PET, a practical correction approach, designed to overcome these existing limitations for quantitative PET imaging, was developed and applied in the context of cardiac PET/MRI. The correction approach for PET data consists of computing the mean density map of each underlying moving region, as obtained with MRI, and translating them to the PET space taking into account the PET spatial and temporal resolution. Using these tissue density maps, the method then constructs a system of linear equations that models the activity recovery and cross-contamination coefficients, which can be solved for the true activity values. Physical and numerical cardiac phantoms were employed in order to quantify the proposed correction. The full correction pipeline was then used to assess differences in metabolic function between scar and healthy myocardium in eight patients with recent acute myocardial infarction using [11 C]-acetate. Data from ten additional patients, injected with [18 F]-FDG, were used to compare the method to the standard electrocardiography (ECG)-gated approach. RESULTS: The proposed method resulted in better recovery (from 32% to 95% on the simulated phantom model) and less residual activity than the standard approach. Higher signal-to-noise and contrast-to-noise ratios than ECG-gating were also witnessed (Signal-to-noise ratio (SNR) increased from 2.92 to 5.24, contrast-to-noise ratio (CNR) increased from 62.9 to 145.9 when compared to a four-gate reconstruction). Finally, the relevance of this correction using [11 C]-acetate PET patient data, for which erroneous physiological conclusions could have been made based on the uncorrected data, was established as the correction led to the expected clinical results. CONCLUSIONS: An efficient and simple method to correct for the quantitative biases in PET measurements caused by cardiac motion has been developed. Validation experiments using phantom and patient data showed improved accuracy and reliability with this approach when compared to simpler strategies such as gated acquisition or optimal regions of interest (ROI).