The overarching goal of our ongoing prospective trial is to ultimately validate the use of ultralow-dose abdominal CT through the use of novel iterative reconstruction algorithms. This interim analysis provides the necessary data to determine whether our initially aggressive dose reduction goal should be modified. One important advantage of our prospective study design is the acquisition of the matching ultralow-dose series immediately after the standard-dose clinical series. This step allows not only direct side-by-side comparison of image noise and image quality but also assessment of diagnostic accuracy, for which it is critical to maintain an acceptable level of performance. Such comparison becomes less reliable when CT images obtained at different points in time are compared, as in most retrospective studies to date, or when simulated low-dose cases are used whereby noise is artificially introduced onto the images.
To our knowledge, this investigation is the first to use the FDA-approved commercial version of MBIR (Veo). Our preliminary findings show that MBIR is a substantial improvement over ASIR and FBP in terms of image noise, subjective image quality, and diagnostic performance. Interestingly, although ASIR had a modest incremental benefit over traditional FBP in terms of image noise and subjective image quality, the diagnostic performance of ASIR at the ultralow dose trended slightly poorer than that of low-dose FBP in terms of focal lesion detection and much poorer than that of MBIR. It should be noted, however, that ASIR is generally not intended for the aggressive dose reduction levels attained in this trial.
The interesting and important discordance between image quality measures and focal lesion detection seen for ASIR and FBP reconstructed images in this preliminary investigation was an unexpected finding that to our knowledge has not been previously reported and may be related to the aggressive levels of dose reduction in this study. This discordance also applies to the comparison between low-dose MBIR and the clinical reference standard of routine-dose FBP. The former had lower image noise but depicted fewer focal lesions overall. Clearly, image noise and subjective quality measurements alone are insufficient for validating novel ultralow-dose iterative reconstruction techniques. Additional objective quality metrics likely need to be developed and validated to better evaluate iterative reconstruction techniques. Beyond these quality evaluation metrics, it is critical to also assess lesion detection capability, which is a more direct assessment of diagnostic adequacy. As we accrue more patients in this ongoing prospective trial, we will eventually be able to assess closer-to-uniform discrete cohorts according to specific clinical indication and imaging technique.
As seen in the provided figures, the qualitative differences between MBIR and the other reconstruction techniques are readily apparent at very low doses. However, given that lesion detection was still compromised somewhat with ultralow-dose MBIR relative to standard-dose FBP, careful consideration is required in terms of defining the proper balance between dose reduction and diagnostic performance. Future investigations will focus more closely on diagnostic accuracy according to specific study indications, such as IV contrast-enhanced studies for oncologic follow-up, urolithiasis evaluation, and colorectal cancer screening. This interim analysis was neither powered nor intended to tackle these specific issues. Rather, by pooling the blinded detection of organ-based focal soft-tissue lesions in a more generic sense, we gain early insight into diagnostic performance. However, although clear trends were noted in terms of focal lesion detection, we must refrain from drawing firm conclusions with regard to diagnostic performance at this early point in the trial.
A number of recent studies have investigated the use of ASIR (ASiR, GE Healthcare), typically in a 30–40% blend with FBP [3
], for achieving more modest dose reduction in the range of 25–40% on average. Similar studies have investigated a variety of other vendor-specific iterative reconstruction methods, such as iDose (Philips Healthcare) [14
], iterative reconstruction in image space (IRIS, Siemens Healthcare) [15
], and adaptive iterative dose reduction (AIDR, Toshiba) [16
]. In general, the results of these studies all suggest or show a modest incremental benefit in terms of dose reduction, typically on the order of approximately 30%. However, most studies have largely been focused on noise reduction and subjective image quality, and diagnostic performance has not been directly assessed. At dose reduction levels approaching 90%, well beyond the usual indicated range for ASIR, our preliminary findings suggest that ASIR appears to be inadequate. This may also be the case for other vendor-specific algorithms that are not truly model based. Further investigation for these algorithms (ASiR, iDose, IRIS, AIDR) is needed to determine the radiation dose reduction levels at which diagnostic adequacy can be maintained relative to the current clinical reference standard in terms of lesion detection capability.
Despite the advantages of MBIR over ASIR and FBP found in our study, it is important to consider the current potential limitations. In addition to being vendor specific and requiring the raw projection data, the primary disadvantage of MBIR at this time lies in the demanding computational requirements, which lead to a prolonged reconstruction time. In the current study, the typical reconstruction time for a low-dose MBIR abdominal CT series was on the order of hours. As part of our prospective ultralow-dose clinical trial, we are assessing other novel iterative reconstruction techniques. One such method, prior image constrained compressed sensing (PICCS) [9
], may approach MBIR in terms of diagnostic quality but has the advantages of a reconstruction time that is approximately two orders of magnitude faster, can be used with DICOM image data, and is currently vendor neutral. As such, PICCS could service multiple scanners from different vendors within a department or group. At this time, however, use of the technique for dose reduction has not been fully clinically evaluated, nor is it commercially available.
The need for further lowering of dose levels for body CT is clear, regardless of whether this reaction is to a real or to a perceived health threat [1
]. In some ways, CT is a victim of its own clinical success; the sheer number of studies performed in the United States has increased from approximately 13 million in 1990 to approximately 46 and 67 million studies by 2000 and 2010 [20
]. The results of our preliminary work suggest that submillisievert abdominal CT is feasible for indications that call for unenhanced imaging. For standard IV contrast-enhanced abdominal CT, it appears that most examinations can be accomplished with effective doses well below 5 mSv, with the exception of imaging of some obese individuals. It is noteworthy that our patient population is generally overweight, a substantial fraction qualifying as obese. However, more work is needed to validate and refine the appropriate levels of dose reduction for contrast-enhanced CT that maintain an acceptable level of diagnostic accuracy. On the basis of our preliminary findings, our current thinking is to maintain the higher levels of dose reduction (80–90%) for unenhanced CT indications but to back off slightly to approximately 60–70% of the standard dose for IV contrast-enhanced studies.
We acknowledge a number of limitations to this study. This study was an interim analysis of an ongoing prospective trial and was not intended to resolve any indication-specific issues. Given the small sample size and heterogeneous techniques applied to this preliminary cohort, we would caution against any firm conclusions regarding the accuracy of ultralow-dose abdominal CT, especially because lesion-specific matching was not included. Furthermore, we did not address the possibility of pseudolesions (false-positive findings) among focal lesions detected at the ultralow dose. Larger homogeneous patient cohorts that group common indications and study techniques are needed to better assess focal lesion detection and diagnostic accuracy. We intentionally kept the handling of focal lesion detection fairly generic for this initial report and allowed pooling of the data. Lesion characterization and clinical relevance were also not considered for this first analysis. We did not specifically address whether spatial resolution is preserved with the various reconstruction methods at ultralow doses, but we do intend to investigate this issue using a phantom. We did not investigate the use of dualenergy or low-kilovoltage imaging, nor did we acquire the ultralow-dose series in the arterial or more delayed phases of contrast enhancement. These issues can be addressed at a later date as part of the ongoing prospective trial. Finally, the percentage dose reduction relative to standard in our study was lessened by the use of the ASIR-driven protocols and would have been greater if the older FBP protocols had been used.
In summary, MBIR shows great promise for substantially reducing radiation doses at abdominal CT in clinical practice, whereas ASIR and FBP appear to be of limited value for the aggressive radiation dose levels targeted in this study. Further investigation is needed to determine the optimal indication-specific dose levels that maintain adequate diagnostic performance with MBIR. An important finding was that objective and subjective image quality measures do not necessarily correlate with diagnostic performance at ultralow-dose CT.