A new SPECT/CT reconstruction algorithm: reliability and accuracy in clinical routine for non-oncologic bone diseases

xSPECT Bone® (xB) is a new iterative reconstruction algorithm developed by Siemens® for bone single photon emission computed tomography (SPECT). Unlike classic SPECT reconstructions, xB uses ordered subset conjugate gradient minimization algorithm (OSCGM). Its originality consists of constraining counts in computed tomography (CT) based on bone segmentation (Fig. 1) and providing a quantitative reconstruction [1, 2].

This innovation, like the progress of image acquisition and reconstruction, could convey a higher diagnostic confidence through an enhanced bone uptake location. Studies have reported early planar images with good sensitivity yet poor specificity. The latter was improved when using SPECT reconstructions with negative predictive value while maintaining an excellent sensitivity [3‐5]. Moreover, the use of CT improved the specificity of SPECT [4], particularly concerning small lesions. Besides, physical limitations such as attenuation or Compton scattering have also benefited from corrections integrated directly into reconstruction algorithms, leading to less artifacts and shorter reconstruction time. Then, the “side-by-side” display of SPECT and CT images (SPECT + CT) was replaced by fused SPECT/CT images [6‐10]. In this manner, Römer et al. were able to identify 90% of SPECT findings classified as indeterminate [11]. These authors also indicated that exact matching of functional and anatomic data may be necessary, especially for imaging of small anatomic structures.

That said, taking into account patient’s clinical data should also be regarded as a mainstay in enhancement of overall diagnostic confidence of scintigraphy.

In the end, in non-oncological context, the objective of both clinician and health care provider is to reduce additional imaging that could delay patient management, increase stress, and induce additional irradiation.

The objectives of this study were:

The Siemens® xB tomographic image reconstruction is a new way of bone image reconstruction theoretically being suggested to provide better bone contrast, thus high-quality images compared with conventional reconstructions. This hypothesis should be assessed for confirmation. To our knowledge, this study is the first to evaluate the diagnostic reliability and accuracy of this novel reconstruction in routine clinical practice. The study includes a large number of patients and their follow-up and concludes to a high inter-observer agreement and a similar diagnostic confidence as compared with F3D.

A high kappa index for xB (0.89) [95% CI 0.84–0.93] showed a very strong IOA, highlighting the reliability of interpretation, between junior and senior expert readers.

Thirty-one IRDD (14.9%) were observed among the 207 SPECT/CT. Of the 31 IRDD, 21 were classified as major (10.1%). A diagnosis was made according to the follow-up in most IRDD cases (16/21). With a better spatial resolution to observe smaller SPECT abnormalities and a better bone to soft tissue contrast, xB may theoretically allow increased detection and better visualization of weakly evolving or small abnormalities that could go unnoticed with F3D. However, according to our clinical experience, detecting smaller or weakly evolving abnormalities did not have a major clinical relevance and did not lead to a therapeutic modification.

Indeed, the higher the IRDD proportion, the higher is the IOD percentage (i.e., high IRDD 41.9% vs. low IRDD 19.8%): borderline bone scan abnormalities were most likely interpreted subjectively (i.e., between physicians with different experience) and therefore more likely to induce IOD, with less clinical relevance.

Of the 16 IRDD, 10 diagnoses done by F3D vs. 6 by xB were confirmed through follow-up. This difference was explained in particular by a higher number of false positives for xB, 4 against none for F3D (Fig. 2). However, the number of false negatives was almost equivalent, 5 for F3D and 4 for xB (Fig. 3). Finally, three radiopharmaceutical uptake errors were observed: two in xB and one in F3D. One radiopharmaceutical uptake error in xB was due to patient movements between the SPECT and the CT acquisitions (Fig. 4). These were not detectable on xB/CT fused images alone but were detectable on F3D/CT images. Aberrant xB images due to location uptake errors are easy to identify. However, when the movements are minimal, they can be undetectable and lead to diagnostic error. This suggests the necessity to systematically take a look at the F3D/CT slices in order to control the accurate registration of SPECT and CT slices in xB reconstruction. Nevertheless, the good spatial resolution of xB can ease the reading and thus change the diagnosis. This is illustrated in Fig. 5: a joint disorder was diagnosed between talus and trigonum bones using xB (and confirmed by follow-up) and as a talus contusion using F3D.

It should also be noted that the study was not carried out by comparing only the SPECT reconstructions but rather by comparing the registered images SPECT/CT with knowledge of the clinical context. The use of CT slices and the knowledge of pain mechanism may have an impact on the diagnostic confidence of the scintigraphy. Thereby, Vija et al. [13] demonstrated significantly higher accuracy of xB used without CT slices compared to F3D. However, the difference was no longer statistically significant between the two reconstructions when fused with the CT slices.

Finally, the striking innovation of the xB reconstruction is the esthetic aspect of SPECT images, which could ease visualization and interpretation of anomalies on MIP images. This combined with clinical and paraclinical findings may enhance patient management and treatment.

Thus, the ease of interpretation provided by xB could bring an added value to the scintigraphic examination given the resultant high-quality images. Most clinicians pay close attention to the images and this often without reading the acquisition report [14].

Given the technological advancement in bone scintigraphy, clinicians and health care provider’s objectives are to highlight diagnostic confidence thus limiting the use of additional imagery.

All together, we believe that prospective studies are warranted to reach more conclusive results in regard with xB reconstruction reliability and accuracy in bone imaging. This further step can help at reaching robust clinical evidence as well as diagnostic consensus.

Similarly, to repeat this study in a multicentric way would limit the interpretation bias observed in our study. Our diagnostic decisions were not independent given that the junior physician was trained by the senior physician from our nuclear medicine department. Nevertheless, the inter-observer agreement scores are comparable to those observed in the literature (0.87–0.97) [8, 15‐18].

Our study demonstrated that xB reconstruction algorithm was a reliable tool in diagnosis of non-oncological bone diseases, providing high inter-observer agreement and similar diagnostic confidence compared with F3D. Moreover, it may improve SPECT/CT images quality thanks to a striking esthetic aspect. Moreover, the proper registration between SPECT and CT slices needs to be checked systematically in F3D images.