With small children, the pressure change in pneumatic devices is often small, resulting in suboptimal synchronization. Though these devices are undergoing continued refinement [4
], often, obtaining an adequate respiratory waveform with these devices on small children requires a time-consuming trial-and-error approach of repositioning the device, sometimes with no success. This leads to prolonged examinations, prolonged anesthesia, decreased MRI schedule predictability, decreased MR scanner throughput, and suboptimal image quality.
Alternative approaches to detecting respiratory motion have been described. Methods based on optical sensors with electronics to generate a logic signal have been developed [5
]. Recently, the group at Cincinnati Children’s used a combination of a nasal mask, flow sensor and pressure transducer to record respiration on a laptop computer and retrospectively synchronize it with time-stamped respiratory images [6
Direct respiratory synchronization with the MRI scanner has also been described with the use of a dedicated pneumotachograph applied to a well-fitted face mask [7
To address this issue, we have developed a novel method of sensing the respiratory cycle in patients who have a supportive airway (e.g. an endotracheal tube, laryngeal mask airway, nasal canula with etCO2 sampling port, or face mask with etCO2 sampling port). The method consists of sensing pressure changes from the etCO2 port of the airway. To realize the technique, a three-way stopcock is placed on the etCO2 port. The etCO2 was monitored with one port of the stopcock attached to the patient’s airway device, the second port attached to the etCO2 tubing, and the third port attached to the respiratory trigger tubing. A filter (Invivo Filter Hydro 0.8 um) that permits air pressure to be transduced, but not respiratory droplets, was connected to the third port of the three-way stopcock, followed by tubing from the filter to the MR scanner (GE 1.5T Signa or GE 3T 750). This is shown in .
Method for sensing respiratory cycle for MRI data synchronization from endtidal carbon dioxide port.
A photograph of the waveform using conventional pneumatic belt on a seven month old male is shown in . In this case the waveform was only obtained after a lengthy trial-and-error repositioning of the belt, yet was inadequate for reliable triggering of data acquisition. Finally, waveform using etCO2 port in the same patient is shown in , proving feasibility of this method. Furthermore, obtaining the final waveform required minimal time, avoiding the time-consuming process of positioning the pneumatic belt. shows resulting T2 weighted images obtained with respiratory triggering enabled by exploiting the etCO2 port.
Waveform for synchronization using conventional pneumatic belt in a 7 month old male.
Figure 3 Waveform from end-tidal port on same patient as in . (A) Ventilator set to a rate of 24, which is accurately reflected in the waveform. (B) Ventilator rate adjusted to 34, which is still accurately reflected in the waveform. Vertical white lines (more ...)
Figure 4 Images of same patient discussed in and . (a) Axial respiratory-triggered fat suppressed T2 weighted images with minimal respiratory ghosting artifacts. (b) Coronal volumetric T2 with fat suppression shows a sharp diaphragm (thin (more ...)