Boyd Vomocil, MD, Marshfield Clinic Nuclear Medicine, on staff at Ministry Saint Joseph’s Hospital
Ministry Saint Joseph’s Hospital features Rubidium (Rb-82) myocardial perfusion imaging (MPI) as an adjunct procedure for evaluation of ischemia in patients with coronary artery disease (CAD).
Rb-82 is a potassium analog incorporated into the myocardium by the Na+/K+ ATPase pump. Rb-82 is a positron emitter (e+) with an extremely short half-life (75s). Because Rb-82 is a positron emitter it must be imaged using a dedicated positron emission tomography (PET) camera. This creates both drawbacks and benefits. The drawbacks center around the high cost of these cameras and their specialized imaging suites and their limited availability within the Ministry Health Care/Marshfield Clinic systems compared with the more common and widely available single photon emission computed tomography (SPECT) units.
However, the benefits are substantial within at least two patient subsets: the hospitalized and the obese.
Definition: Obese = patients weighing more than 250 pounds (113.6 kg) or with body mass indices of more than 35. As a positron emitter, Rb-82 penetrates body tissues more readily than Technicium 99m based SPECT MPI agents such as Sestamibi (Cardiolite), allowing superior counting statistics especially in overweight
patients. Imaging these patients, with the widely available SPECT cameras, results in substantial numbers of suboptimal or equivocal studies. Rb-82 MPI greatly improves the diagnostic accuracy in patients up to 450 pounds (the current maximum patient/table weight limit). The most common and troublesome areas in evaluating SPECT MPI are caused by photon absorption in the inferior wall from the adjacent liver and stomach and in the anterior wall from large or dense breasts.
Since Rb-82 MPI decays with an extremely short half-life (75s), the entire study can be completed in one imaging session in less than 1 hour. This compares to 4-6 hours (or 2 days in obese patients!) and 3 separate sessions. (1. Nuclear medicine rest imaging, 2. Cardiology department treadmill/pharmacologic stress, 3. Nuclear medicine stress imaging) for SPECT MPI.
Rb-82 MPI thus allows much less patient travel/motion, much better utilization of hospital nursing/transport staff, and tighter control/monitoring of tenuous inpatients. Additionally, results are available more quickly to assist in attending staff decision-making.
However, because of its short half-life, Rb-82 MPI must occur during actual stress. Therefore, adenosine is utilized as the pharmacologic stressor and all patient precautions and preparation apply identically to the adenosine SPECT MPI studies. Patient preparation should be in accordance with published guidelines. Patients should be NPO for 4-6 hours and all methylxanthine-containing products should be withheld for 12-24 hours as in ACC/AHA/ASNC guidelines.
Thus for obese, hospitalized/tenuous or patients with previous suboptimal or equivocal SPECT MPI, Rb-82 offers an excellent alternative to standard protocols for achieving improved image quality and superior diagnostic accuracy in these difficult cases.
Ministry Saint Joseph’s Hospital is the first center in Wisconsin (January 2008) to offer this service. The technologists and staff are highly trained and have performed more than 700 Rb-82 and countless SPECT MPI examinations.