Key points
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Improvements in reconstruction software and dedicated cardiac camera design now allow for efficient ultrafast MPI acquisition. Some of the acquisition time reduction may be traded off for a reduction in the injected radiopharmaceutical dose without loss of image quality.
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The tradeoff between acquisition time and dose reduction is approximately linear. Thus, an ultrafast MPI protocol using a 30-mCi dose and a 2-minute acquisition could be replaced with a 15-mCi dose and a 4-minute acquisition and receive very similar image quality and diagnostic accuracy with either protocol.
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New reconstruction algorithms use RRNR techniques that yield MPI images acquired in half the standard imaging times with superior image contrast and resolution and similar perfusion and function diagnostic performance, compared to standard SPECT.
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These RRNR reconstruction algorithms have been shown to provide LV volumes that correlate to standard SPECT, but are significantly smaller, and LVEFs that also correlate, but are lower due to smaller end-diastolic volumes.
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Innovative design of dedicated cardiac-centric cameras have in common that many detectors are constrained to simultaneously imaging the heart and use either conventional NaI crystals and PMT or the more beneficial solid-state detectors, such as CZT or CSI.
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Compared to standard SPECT devices, solid-state detectors are more efficient in how they convert the energy and location of the radiation absorbed by the detector to an electronic pulse, thus giving rise to significant improvements in energy and spatial resolution.
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Compared to standard SPECT, these new ultrafast cameras have potential for a 5- to 10-fold count sensitivity gain at no loss or even a gain in contrast and spatial resolution.
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These new ultrafast cameras with solid-state detectors and tungsten collimators are optimized for imaging not only Tc-99m but also photons with energies from Tl-201 to I-123. This higher energy resolution has also been shown to allow for simultaneous dual isotope imaging.
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Clinical trials have demonstrated that using these new ultrafast cameras and a conventional 1-day Tc-99m tetrofosmin rest/stress MPI protocol of 4- and 2-minute acquisitions, respectively, yielded studies that diagnostically agreed 90% of the time with 14- and 12-minute rest-stress conventional SPECT acquisitions.
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Some of these ultrafast camera systems allow for AC using a CT transmission scan. In addition, attenuation artifacts may be reduced because not all views are through the attenuator—some may view the heart from above or below, thus reducing breast attenuation artifacts.
Background
Despite the clinical success of MPI, present clinical, scientific, and financial needs require further improvements in hardware and software to allow myocardial perfusion SPECT imaging to answer these challenges of modern health care. It is difficult to realize these improvements with the imaging hardware and software used in most nuclear cardiology laboratories today.
This chapter demonstrates, through patient examples, how MPI studies from conventional two-detector rotating SPECT systems compare to studies acquired with the new design cardiac-dedicated ultrafast SPECT cameras and with the new RRNR algorithms. These examples make evident that these advances are poised to meet present health care challenges by improving image quality while reducing study time, radiation dose to the patient, and ultimately reducing study cost.
Advances in image reconstruction
Recent software improvements in iterative image reconstruction take into account the loss of resolution with distance inherent in parallel-hole collimators. Using this knowledge in conjunction with the imaging properties of the system allows for a mathematical correction of this resolution degradation known as resolution recovery. At the same time, noise is suppressed because additional counts are now correctly accounted for instead of being treated as noise. Moreover, noise regularization techniques have been implemented that go beyond simple smoothing by considering the expected noise for the regional count density. Because resolution recovery actually reduces noise while improving spatial resolution compared to FBP, resolution recovery can yield reconstructed images from studies acquired in less time with the same signal/noise as FBP images reconstructed from studies acquired over a longer time.
New ultrafast camera designs
Several manufacturers have begun to break away from the conventional SPECT imaging approach to create innovative designs for dedicated cardiac cameras. These cameras’ designs have in common that all available detectors are constrained to imaging just the cardiac field of view. These new designs vary in the number and type of scanning or stationary detectors, and whether NaI, CSI, or CZT solid-state detectors are used. They all have in common the potential for a 5- to 10-fold increase in count sensitivity at no loss or even a gain in resolution, resulting in the potential for acquiring a stress myocardial perfusion scan in 2 minutes or less if injected with a standard dose. Some of this gain in sensitivity can be traded for a linear reduction in the injected dose to reduce the patient’s exposure to radiation. Thus, in an ultrafast camera with a 10-fold increase in sensitivity using conventional radiopharmaceutical doses, the dose could be reduced by half and still maintain a 5-fold increase in sensitivity.
Reduced dose versus increased efficiency
It is clear that these more efficient hardware/software imaging systems also allow for high-quality images obtained using a lower injected radiopharmaceutical dose and thus a decrease in the radiation dose absorbed by the patient and staff. This reduction in dose comes at an increase in acquisition time, even if the total time is less than what has been traditionally used in conventional systems. Since currently there are no financial incentives for using a lower radiopharmaceutical dose, a laboratory interested in just reducing costs would tend to opt for the most efficient protocol possible.
Recently, the American Society of Nuclear Cardiology published an information statement recommending that laboratories use imaging protocols that achieve, on average, a radiation exposure of 9 mSv or less in 50% of studies by 2014. Although there are many different protocols that may be implemented to accomplish this exposure goal, use of the more efficient hardware/software described herein would greatly facilitate this goal and allow for increases in efficiency over the imaging protocols used today.
Case 18-1
Half-Time Resolution Recovery Normal Study ( Figure 18-1 )
A 52-year-old, 5-foot 8-inch, 181-lb male smoker with hypertension, hypercholesterolemia, and family history of premature CAD underwent exercise and rest imaging on a standard dual-detector gamma camera for evaluation of an LAD stenosis that had been previously diagnosed with angiography in 2000. Twenty seconds per stop rest and gated stress acquisitions with 60 projections (10 minutes + rotation time) were reconstructed with OSEM. Ten seconds per stop rest and gated stress acquisitions with 60 projections (5 minutes + rotation time) were reconstructed with OSEM, incorporating correction for detector and collimator response.
Comments
This is an example of a patient with normal myocardial perfusion in which half-time imaging reconstructed with resolution recovery algorithms is diagnostically equivalent to full-time imaging reconstructed without resolution recovery. Note the appearance of sharper myocardial boundaries and the higher myocardium/chamber contrast resulting from the increased resolution. Functional measurements are also similar for both studies; although the LVEF is somewhat reduced because of increased end-systolic volume, which is seen with some resolution recovery algorithms. Note that the decreased count rates resulting from the half-time imaging do not appear to adversely affect gated image quality; however, the increased resolution of the detector response corrected images may cause different volumetric measurements so that serial studies reconstructed differently may not be comparable.
Case 18-2
Half-Time Resolution Recovery With Improved Defect Contrast ( Figure 18-2 )
A 75-year-old, 6-foot 1-inch, 157-lb man with hypertension, hypercholesterolemia, and a history of CAD diagnosed with angiography 1 year previously underwent dipyridamole ECG gated stress imaging and static rest imaging using a standard dual-detector rotating gamma camera. Twenty seconds per stop rest and gated stress acquisitions with 60 projections (10 minutes +) were reconstructed with OSEM. Ten seconds per stop rest and gated stress acquisitions with 60 projections (5 minutes +) were reconstructed with OSEM, incorporating correction for detector and collimator response.
