A pilot study of respiratory rate derived from a wearable biosensor compared with capnography in emergency department patients

Purpose: Respiratory rate is assessed less frequently than other vital signs, and documented respiratory rates are often erroneous. This pilot study compared respiratory rates derived from a wearable biosensor to those derived from capnography. Methods: Emergency department patients with respiratory complaints were enrolled and had capnography via nasal cannula and a wireless, wearable biosensor from Philips applied for approximately one hour. Respiratory rates were obtained from both of these methods. We determined the difference between median respiratory rates obtained from the biosensor and capnography and the proportion of biosensor-derived respiratory rates that were within three breaths/minute of the capnography-derived respiratory rates for each patient. A Spearman correlation coefficient was calculated to assess the strength of the correlation between mean respiratory rates derived from both methods. Plots of minute-by-minute respiratory rates, per patient, for each monitoring method were shown to two physicians. The physicians identified time periods in which the respiratory rates appeared invalid. The proportion of time with invalid respiratory rates for each patient, for each method, was calculated and averaged. Results: We analyzed data for 17 patients. Median biosensor-derived respiratory rate was 20 breaths/minute (range: 7-40 breaths/minute) and median capnography-derived respiratory rate was 25 breaths/minute (range: 0-58 breaths/minute). Overall, 72.8% of biosensor-derived respiratory rates were within three breaths per minute of the capnography-derived respiratory rates. Overall mean difference was 3.5 breaths/minute (±5.2 breaths/minute). Respiratory rates appeared invalid 0.7% of the time for the biosensor and 5.0% of the time for capnography. Conclusion: Our pilot study suggests that the Philips wearable biosensor can continuously obtain respiratory rates that are comparable to capnography-derived respiratory rates among emergency department patients with respiratory complaints.

The magnetic susceptibility effect of gadolinium-based contrast agents on PRFS-based MR thermometry during thermal interventions

Background

Proton resonance frequency shift (PRFS) magnetic resonance (MR) thermometry exploits the local magnetic field changes induced by the temperature dependence of the electron screening constant of water protons. Any other local magnetic field changes will therefore translate into incorrect temperature readings and need to be considered accordingly. Here, we investigated the susceptibility changes induced by the inflow and presence of a paramagnetic MR contrast agent and their implications on PRFS thermometry.

Methods

Phantom measurements were performed to demonstrate the effect of sudden gadopentetate dimeglumine (Gd-DTPA) inflow on the phase shift measured using a PRFS thermometry sequence on a clinical 3 T magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) system. By proton nuclear magnetic resonance spectroscopy, the temperature dependence of the Gd-DTPA susceptibility was measured, as well as the effect of liposomal encapsulation and release on the bulk magnetic susceptibility of Gd-DTPA. In vivo studies were carried out to measure the temperature error induced in a rat hind leg muscle upon intravenous Gd-DTPA injection.

Results

The phantom study showed a significant phase shift inside the phantom of 0.6 ± 0.2 radians (mean ± standard deviation) upon Gd-DTPA injection (1.0 mM, clinically relevant amount). A Gd-DTPA-induced magnetic susceptibility shift of Δχ_Gd-DTPA = 0.109 ppm/mM was measured in a cylinder parallel to the main magnetic field at 37°C. The temperature dependence of the susceptibility shift showed dΔχ_Gd-DTPA/dT = -0.00038 ± 0.00008 ppm/mM/°C. No additional susceptibility effect was measured upon Gd release from paramagnetic liposomes. In vivo, intravenous Gd-DTPA injection resulted in a perceived temperature change of 2.0°C ± 0.1°C at the center of the hind leg muscle.

Conclusions

The use of a paramagnetic MR contrast agent prior to MR-HIFU treatment may influence the accuracy of the PRFS MR thermometry. Depending on the treatment workflow, Gd-induced temperature errors ranging between -4°C and +3°C can be expected. Longer waiting time between contrast agent injection and treatment, as well as shortening the ablation duration by increasing the sonication power, will minimize the Gd influence. Compensation for the phase changes induced by the changing Gd presence is difficult as the magnetic field changes are arising nonlocally in the surroundings of the susceptibility change.

SPECT/CT imaging of temperature-sensitive liposomes for MR-image guided drug delivery with high intensity focused ultrasound

The goal of this study was to investigate the blood kinetics and biodistribution of temperature-sensitive liposomes (TSLs) for MR image-guided drug delivery. The co-encapsulated doxorubicin and [Gd(HPDO3A)(H₂O)] as well as the ¹¹¹In-labeled liposomal carrier were quantified in blood and organs of tumor bearing rats. After TSL injection, mild hyperthermia (T=42 °C) was induced in the tumor using high intensity focused ultrasound under MR image-guidance (MR-HIFU). The biodistribution of the radiolabeled TSLs was investigated using SPECT/CT imaging, where the highest uptake of ¹¹¹In-labeled TSLs was observed in the spleen and liver. The MR-HIFU-treated tumors showed 4.4 times higher liposome uptake after 48 h in comparison with controls, while the doxorubicin concentration was increased by a factor of 7.9. These effects of HIFU-treatment are promising for applications in liposomal drug delivery to tumors.

Dual-isotope 111In/177Lu SPECT imaging as a tool in molecular imaging tracer design

The synthesis, design and subsequent pre-clinical testing of new molecular imaging tracers are topic of extensive research in healthcare. Quantitative dual-isotope SPECT imaging is proposed here as a tool in the design and validation of such tracers, as it can be used to quantify and compare the biodistribution of a specific ligand and its nonspecific control ligand, labeled with two different radionuclides, in the same animal. Since the biodistribution results are not blurred by experimental or physiological inter-animal variations, this approach allows determination of the ligand's net targeting effect. However, dual-isotope quantification is complicated by crosstalk between the two radionuclides used and the radionuclides should not influence the biodistribution of the tracer. Here, we developed a quantitative dual-isotope SPECT protocol using combined (111)Indium and (177)Lutetium and tested this tool for a well-known angiogenesis-specific ligand (cRGD peptide) in comparison to a potential nonspecific control (cRAD peptide). Dual-isotope SPECT imaging of the peptides showed a similar organ and tumor uptake to single-isotope studies (cRGDfK-DOTA, 1.5 ± 0.8%ID cm(-3); cRADfK-DOTA, 0.2 ± 0.1%ID cm(-3)), but with higher statistical relevance (p-value 0.007, n = 8). This demonstrated that, for the same relevance, seven animals were required in case of a single-isotope test design as compared with only three animals when a dual-isotope test was used. Interchanging radionuclides did not influence the biodistribution of the peptides. Dual-isotope SPECT after simultaneous injection of (111)In and (177)Lu-labeled cRGD and cRAD was shown to be a valuable method for paired testing of the in vivo target specificity of ligands in molecular imaging tracer design.

Magnetic resonance imaging of high intensity focused ultrasound mediated drug delivery from temperature-sensitive liposomes: an in vivo proof-of-concept study

Temperature-sensitive liposomes (TSLs) co-encapsulating doxorubicin and 250 mM [Gd(HPDO3A)(H₂O)] were evaluated for HIFU-mediated drug delivery under MR image guidance. In vitro studies showed simultaneous and quantitative release of the drug and the MRI contrast agent from the lumen of the TSLs at 42°C, while no leakage was observed over 1 h at 37°C. In a proof-of-concept study, local hyperthermia has been applied for 30 min in 9L rat tumors using a clinical MR-HIFU system. The local temperature-triggered release of [Gd(HPDO3A)(H₂O)] was monitored with interleaved T₁ mapping of the tumor tissue. A good correlation between the ΔR₁, the uptake of doxorubicin and the gadolinium concentration in the tumor was found, implying that the in vivo release of doxorubicin from TSLs can be probed in situ with the longitudinal relaxation time of the co-released MRI contrast agent.