Mr. S is a 52-year-old entrepreneur in the waste management industry. He weighs 119 kg, is 175 cm tall (BMI of 38.9 kg·m−2), and is being investigated for “dizzy spells.” His medical problem list includes obesity, untreated hypertension, and possible obstructive sleep apnea (OSA) (based on his wife's observation that “sometimes he just stops breathing” at night). A previous attempt at a magnetic resonance imaging (MRI) scan was unsuccessful because Mr. S, startled by the onset of the loud noises made by the MRI machine, panicked and tried to get out of the MRI scanner. Upon further questioning, he admits to extreme claustrophobia, possibly the result of a protracted period of time spent in a car trunk as a child.
On this occasion, the MRI team decides that Mr. S might be more cooperative with pharmacologic assistance and to this end has given him 5 mg of IV midazolam (Versed®). Unknown to the clinical team, just before arriving at the MRI suite, Mr. S had also taken 6 mg of his wife's lorazepam (Ativan®) to help reduce his considerable anxiety. For the scan, a pulse oximeter and nasal capnograph are used to monitor respiration. Oxygen is administered by nasal prongs at 3 L·min−1.
About 10 minutes into the MRI scan, the pulse oximeter alarm activates, drawing attention to an oxygen saturation reading of 81%. The pulse oximeter waveform quality appears to be good. However, no waveform can be obtained from the capnograph. Since Mr. S is deep inside the MRI machine, it is difficult to visually assess his respiratory status. You are urgently summoned to the MRI suite by the radiology team to help manage this patient.
Discuss the Physics of MRI
A basic understanding of how MRI works is important for both medical management and the safety of both patients and medical personnel in the MRI suite. In simple terms, MRI systems use high-strength magnetic fields and radio waves to generate images based on interactions between the generated magnetic fields and hydrogen molecules in the tissues being imaged. More specifically, static magnetic fields generated by an MRI scanner interact with small fields generated by atomic nuclei. Some nuclei develop a magnetic dipole moment when subject to this field, deflected at a slight angle to the static magnetic field. These nuclei also “wobble” about the direction of the magnetic field much like a spinning “top” at what is known as the precession frequency, which is proportional to the magnetic field strength. When a variable magnetic field is applied at the precessional frequency, more nuclei move from a lower to higher energy state. When the variable field is removed, they relax, emitting energy at the same precessional frequency which is then detected by a receiving coil in the scanner. Medical grade MRI scanners are typically tuned ...