Method and apparatus for providing pulses inhalation of
Prior approaches have delivered 17 O 2 to a subject by inhalation, but the relationship between local signal changes and metabolism has been complicated by H 2 17 O created in non-cerebral tissues. During a brief pulse of 17 O 2 inhalation, this arterial input function for H 2 17 O is negligible due to convective transport delays. Additional delays in the arterial input function due to restricted diffusion of water makes pulsed inhalation of 17 O 2 even more effective. Accordingly, ventilator system are provided to deliver 17 O 2 as a brief pulse to a subject. Subsequent MR imaging demonstrates delayed appearance of H 2 17 O in the cerebral ventricles, suggesting that the arterial input function of H 2 17 O is delayed by restricted water diffusion in addition to convective transit delays. Delivery as a brief pulse therefore offers significant advantages in relating MR signal changes directly to metabolism.
1. A method of administering 17 O 2 to a patient for magnetic resonance imaging a region of interest remote from the patient's lungs, comprising:
providing 16 O 2 to the patient by inhalation;
a ventilator applying a pulse of 17 O 2 by inhalation comprising an approximately step increase in 17 O 2 , a duration of said pulse selected so as to be shorter than a diffusion time lag of metabolized H 2 17 O in the patient from the patient's lungs to the region of interest; and
imaging the region of interest during the duration of said pulse.
2. The method of claim 1 , wherein the pulse of 17 O 2 has a duration in a range of from about 30 seconds to about 60 seconds.
3. The method of claim 1 , wherein the region of interest is the brain, comprising the further step of calculating, using a processor, cerebral metabolic rate of oxygen consumption (CMRO 2 ) from the amount of 17 O 2 measured in an image taken during the imaging step by calculating:
CMRO 2 =k 1 ( dC b ( t )/ dt )/ C a ( t )
where C a (t) and C b (t) are the 17 O 2 concentrations in arterial blood as a function of time at times a and b, and k 1 is a conversion constant.
4. The method of claim 1 , further comprising recovering unused 17 O 2 exhaled by the patient.
5. The method of claim 1 , comprising the further step of assisting exhalation by the patient using a pump.
6. The method of claim 5 , comprising the further step of monitoring the pressure of exhaled gases and adjusting an exhalation speed of said pump so as to adjust the exhalation pressure to a target exhalation pressure.
7. A ventilation apparatus, comprising:
means for selecting either a source of 16 O 2 or 17 O 2 ;
an inhalation pump for pumping the selected source to a patient at a first pressure;
an exhalation pump for pumping exhaled gases at a second pressure; and
means for controlling said selecting means so as to apply a pulse of 17 O 2 to the patient,
said pulse comprising an approximately step increase in 17 O 2 for a duration selected by the means for controlling said selecting means so as to be shorter than a diffusion time lag of metabolized H 2 17 O in the patient from the patient's lungs to a region of interest in the patient's body, said region of interest being remote from the patient's lungs.
8. The apparatus of claim 7 , wherein the pulse of 17 O 2 has a duration in a range of from about 30 seconds to about 60 seconds.
9. The apparatus of claim 7 , wherein the selecting means comprises a valve.
10. The apparatus of claim 7 , further comprising means for recovering unused 17 O 2 exhaled by the patient.
11. The apparatus of claim 10 , wherein said recovering means comprises a sampling bag that receives exhaled gases output by said exhalation pump and a CO 2 absorber that removes CO 2 from the exhaled gases before the exhaled gases reach said sampling bag.
12. The apparatus of claim 7 , further comprising a pressure transducer that monitors the second pressure and adjusts an exhalation speed of said exhalation pump so as to adjust the second pressure to a target exhalation pressure.