Specific Ventilation Imaging, or SVI, is a qualitative lung imaging technique that will help us further our fundamental understanding of physiology and the management of respiratory diseases. SVI is radiation-free and requires only commonly available equipment, an MRI scanner and medical oxygen. This makes it safe for children and ideal for repeat studies in adults.
The translation of SVI to clinical practice may improve the management of asthma because it will allow us to identify areas that are most affected by the disease. The ability to map changes within the lung in response to therapy may help optimize inhale therapy and inform individualized therapeutic choices. Helping demonstrate this procedure is Vincent Tedjasaputra, a post-doctoral fellow in the Department of Medicine.
Begin beginning the procedure, obtain written informed consent from the subject and describe the potential risks presented by the exposure to rapidly changing magnetic fields and the potential discomfort of using a facial mask and breathing dry gas. Confirm that the subject can safely undergo magnetic resonance scanning utilizing the locally approved Magnetic Resonance Imaging or MRI safety screening questionnaire. Train the subject to breathe in time with the MR scan sequence and measure the subject's nose to chin dimensions to determine the size of the face mask that will best fit the subject.
Then verify that the subject's pockets and clothing are free from magnetic-based credit cards and iron-containing metal pieces. To prepare the MR scanner, connect a torso coil to the appropriate connector in the scanner table to configure the scanner for use and place sheets, pads, and pillows on the scanner table so that the subject will be comfortable for at least 30 minutes of imaging. To assemble the oxygen delivery system, place a two-way switch valve within reach of the scanner operator and use a piece of eight meter length one-quarter inch diameter plastic tubing to connect the oxygen supply to one inlet of the switch valve.
Connect the outlet of the switch valve in the control room to the one-quarter plastic tubing and feed the tubing through the pass through of the control room to the scanner room taking care that the tubing reaches the middle of the scanner bore. Secure the flow bypass attachment to the face mask for the subject. Connect the half-inch brass end of the tubing to the flow bypass mask attachment.
Set the pressure on the oxygen supply outlet regulator to a value that produces a flow of oxygen greater than the expected peak inspiratory flow according to the nature of the study and the overall resistance of the gas delivery system. Then activate the flow of oxygen to test the switch valve, making sure an adequate flow is present at the outlet of the flow bypass attachment and that no leaks are present in the plastic tubing. To prepare the subject for the imaging, have the subject lie on the scanner table making sure that the top of the lower coil element is higher than the subject's shoulders to ensure that the top of the lower coil element provides an adequate coverage of the lung apices.
Have the subject insert ear plugs and verify that sound is being blocked. Tape a safety mechanism to the subject's wrist so that it can be easily accessed and attach the mask and flow bypass system to the subject's face. Briefly occlude the expiratory side of the flow bypass attachment and ask the subject to attempt a normal inspiration and expiration to check for leaks.
Now load the subject into the scanner using the light centering tool to make sure that the torso coil occupies the center of the bore and guide the flow bypass line to assure subject comfort while moving the subject towards the center of the scanner. After selecting the lung location for imaging slices, acquire a localizer sequence to obtain an anatomical map that will be used to prescribe the rest of the exam. Use the scanner graphical user interface to click and drag the imaging slice centered in the lung field targeting the region of interest to the desired location to select up to four sagittal lung slices to be studied.
Then make a note of the location of the imaging slices with respect to the location of the spinal column so that the same volume can be re-imaged for longitudinal studies. For specific ventilation imaging, set the inversion time in the MR computer for the most medial slice to 1100 milliseconds to maximum the air oxygen contrast, the number of repetitions to 220, and the repetition time to five seconds. Monitor the consistency of the subject's lung volume and expiration during subsequent acquisitions and provide feedback to improve the quality if necessary.
Switch the subject's inspired gas mixture every 20 breaths during the acquisition breath hold for the subject's comfort, alternating between the room air and medical oxygen. Check the pulse oximeter regularly to verify the heart rate and oxygen saturation and check in with the subject frequently to give regular updates of the time remaining. After breath 220, the imaging is complete.
Return the subject to room air and remove them from the scanner. To create a specific ventilation map, import the images for registration into the image analysis software and visually inspect the entire 220 image stack to select the image for each slice that best represents the functional residual capacity. Using the mode image as a reference, use projective or a fine registration to register all of the images to the functional residual capacity reference and quantify the specific ventilation in the lung from the registered stack using an appropriate algorithm.
A map of the specific ventilation will be created. SVI produces quantitative maps of specific ventilation, for example as shown in this single slice image in the right lung of a 39-year-old healthy female. Note the presence of the expected vertical gradient in the specific ventilation with the dependent portion of the lung presenting a higher specific ventilation than the non-dependent portion of the lung.
A histogram of the map specific ventilation values with a best fit log normal probability distribution function allows the width of the best fit distribution to be used as a metric of specific ventilation heterogeneity. Here, a multiple breath washout acquired in the same subject in the same posture is shown. The temporal recording of nitrogen concentration was measured at the mouth following a shift from inspired air to inspired 100%oxygen.
In this distribution of specific ventilation as estimated from the washout for both specific ventilation imaging and multiple breath washout, the width of the distribution was found to be within healthy normal range. Training the subjects to breath in time with the scanner and giving them feedback during acquisition can help to ensure good quality data. Combined with measurements of density and perfusion, SVI can be used to map the local ratio of ventilation to perfusion and measure of gas exchange efficiency.
This technique has been used to understand distribution of ventilation after strenuous exercise and a special panel of repeated bronchial restriction in asthma. The MRI environment requires attention to safety as a strong magnetic field is always on and ferromagnetic objects can become projectile resulting in injury.
