Clinical benefits of First Pass Imaging
First Pass Imaging is an imaging modality that facilitates evaluation of left ventricular function both at rest and at peak exercise. Right ventricular function can also be assessed. This information is acquired in a period of less than one minute. Interpretable images are obtained from a high (>99%) proportion of patients.
First Pass Imaging is used primarily in conjunction with nuclear stress test protocols. It is now the standard of practice to assess left ventricular contractile function in patients undergoing myocardial perfusion imaging. The most common imaging modality used in this situation is gated SPECT imaging. This type of image acquisition evaluates resting left ventricular function. While resting left ventricular function alone does provide some additional clinical and prognostic data, the ability to compare resting left ventricular function to peak exercise left ventricular function is of immense clinical value. First Pass Imaging allows the nuclear cardiologist access to that additional benefit.
False positive and false negative perfusion scans:
Up to one-third of myocardial perfusion scans are of suboptimal technical quality and therefore difficult to interpret. Soft tissue attenuation, patient movement during image acquisition and extra-cardiac uptake of the perfusion tracer all degrade the quality of perfusion scans. These technically suboptimal studies are seldom interpreted as normal but instead are cautiously reported with the suggestion that coronary artery disease cannot be excluded (a conclusion which is both clinically and medico legally appropriate). These patients, then, are frequently referred for coronary arteriography to exclude coronary artery disease. However, if First Pass Imaging is added to the nuclear stress protocol, in place of gated SPECT, the interpreter can look at both perfusion and function at rest and at peak exercise. Myocardial territories that have questionable perfusion abnormalities but normal augmentation of function at peak exercise can then be interpreted as normal.
Conversely, patients with diffuse coronary artery disease may appear to have normal perfusion if they have so-called balanced ischemia. In that situation resting left ventricular function may be normal. If first pass imaging is substituted for gated SPECT however, the fall in left ventricular ejection fraction at peak exercise as a result of widespread hypoperfusion with a mandate further investigation and uncover the true diagnosis. First Pass Imaging therefore will avoid both false negative and false positive perfusion scans interpretations.
Timing of surgery in patients with severe asymptomatic aortic regurgitation:
It is widely acknowledged that patients with severe asymptomatic aortic regurgitation should undergo a replacement of the aortic valve if they develop left ventricular dilation or a fall in left ventricular systolic function. However, it has also been shown that earlier surgery is appropriate in patients who lose the ability to augment left ventricular ejection fraction with exercise. Identification of this subgroup, using first pass imaging, may lead to better short-term and long-term outcomes in this patient population.
“Stand-alone” assessment of resting left ventricular function:
Although the distinguishing feature of First Pass Imaging is its ability to measure ventricular function at true peak exercise, First Pass Imaging is a robust and reliable tool that can be used in any situation where resting left ventricular function needs to be measured.
Echocardiography, which is commonly used in this situation, provides reliable data (provided transthoracic imaging windows are adequate) in patients whose ventricles are of normal shape and size. In order to calculate ejection fraction echocardiography assumes that the shape of the left ventricle is a prolate ellipse. However patients with dilated cardiomyopathy have spherical ventricles that cannot the expected to fit to the calculations used in echocardiography. Thus, echocardiography has limited ability to detect small changes in ventricular systolic function in patients with diseased hearts. First Pass Imaging his a "blood pool" technique and as such it makes no assumptions regarding left ventricular shape or size. It therefore has the ability to detect small changes in function in healthy and diseased hearts alike.
MUGA is also a “blood pool” technique and does provide accurate data regarding left ventricular function. However MUGA is cumbersome to perform in that the blood pool has to be labeled in vivo or in vitro. Furthermore, it is extremely difficult to obtain interpretable data in patients with atrial fibrillation. First Pass Imaging does not require blood pool labeling and can be acquired in patients with atrial fibrillation. Parenthetically, First Pass Imaging may also be the modality of choice for assessment of right ventricular function. The left and right ventricles are temporally separated in a First Pass acquisition in that the tracer has cleared from the right ventricle before counts are evident in the left ventricle. When MUGA is used to evaluate right ventricular function both ventricles are simultaneously labeled and not infrequently it is difficult to separate the individual counts.
Gated SPECT has been widely used in combination with perfusion imaging for assessment of resting left ventricular function. This technique is known to underestimate left ventricular ejection fraction and is also unsuitable for patients with irregular heart rhythms.
Magnetic resonance imaging provides very accurate assessment of left ventricular function. However, the technology and interpretation expertise is not widely available and the imaging modality is not applicable in patients who have either pacemakers or implantable defibrillators (an increasing proportion of patients with cardiomyopathy). The requirements of magnetic resonance imaging (supine posture, breath holding and the confined space) also reduce its applicability particularly in patients with diseased hearts. As with gated SPECT and MUGA irregular heart rhythm is required.
When compared to other imaging modalities First Pass Imaging provides a reliable, reproducible and widely applicable assessment of resting left ventricular function. Images are rapidly acquired and can be read at any remote site where Internet access is available. First Pass Imaging is arguably the technique of choice for measurement of right ventricular function and the software can also be modified to evaluate left ventricular diastolic function. Most importantly however, First Pass Imaging is the only modality that facilitates evaluation of left ventricular function at peak exercise. As such, it may be used in combination with conventional perfusion imaging protocols to improve the accuracy of interpretation of perfusion scans.
Thomas P. Power, MD