Percentage volume deviations of the patients’ arms compared to the segmented NAC PET images.

Percentage volume deviations of the patients’ arms in the Dixon-VIBE, HUGE and MLAA attenuation correction maps compared to the patient’s arms segmented in the non-attenuation corrected (NAC) PET data. All 32 patients are sorted by increasing body mass index (BMI) from left to right. Note the tendency of increased body volume truncation percentage for patients with higher BMI.</p

Patient example with maximal relative differences.

The 17-segment polar plot of a single patient example shows the relative difference between improved attenuation correction (AC) maps using HUGE truncation correction and bone detection (a) or MLAA truncation correction (b) compared to Dixon-VIBE AC-map serving as reference. In this patient example maximal relative differences between improved AC and standard Dixon-VIBE AC were observable across the entire patient population. Note that especially in segment 17 (apex) maximal difference values with 18.8% for HUGE + bone and 19.8% for MLAA were determined.</p

Different AC-map artifacts.

Red arrows show AC-map artifacts. In 3/32 patients metal artifacts in Dixon-VIBE AC-map (caused by metal wire cerclage) leads to misclassification of surrounding tissue as “air” (a). Note that in this example the thorax is virtually opened due to the metal artifact (a). Consequently, the segmentation algorithm classifies the lung tissue of the right lung as air and assigns a lower attenuation value in this region (a). In 3/32 AC-maps no bone was segmented (b). In 4/32 patients the bone segmentation mismatches the anatomy (c). In 1/32 patient missing arm segmentation in HUGE imaging occurred (d), and in all 32/32 patients overestimation of AC-map volume in MLAA AC-maps was observed (e).</p

Statistical analysis of quantitative impact of improved AC on AC-map volume and PET signal.

Statistical analysis of quantitative impact (global activity) of HUGE field-of-view extension, bone segmentation and MLAA correction on global PET signal over all segments of the left ventricle (a-c). Increase of AC-map volume (d-f) relative to AC-map volume as provided by standard Dixon-VIBE AC imaging. Boxplots (a, d) show the statistical distribution in relative gain of activity and volume from applying improved AC. Correlation graphs (b, e) and Bland-Altman plots (c, e) depict the comparison between HUGE + bone and MLAA in contrast to standard Dixon-VIBE AC serving as reference. In the correlation graphs (b, e) the linear equation with the coefficient of determination (R2) is given.</p

Difference polar plots between standard and improved AC.

The 17-segment polar plot generated from all 32 patient PET data sets shows the average relative difference per cardiac segment between improved attenuation correction (AC) maps using HUGE truncation correction or MLAA truncation correction compared to Dixon-VIBE AC-map serving as reference (a). Polar plots in (b) show results of 25 patients after splitting the patient population into HUGE only and HUGE + bone corrected data to evaluate the effect of bone AC per segment (b). Average relative difference per segment in MLAA corrected data in patients with fasted preparation and non-fasted preparation (c) are shown to study the effect of AC on fasted and viability protocols. Note the homogeneous gain in relative difference due to HUGE and bone AC of 6% across all cardiac segments, whereas in MLAA and HUGE-only corrected data a slight decrease in relative difference inferior-medial is noticeable due to missing bone information. The relative difference over all segments and patients for MLAA is 8%. The patient preparation seems to have rather low impact on overall signal distribution in the left ventricle (c).</p

Overview of AC-maps, cardiac PET data and polar plots.

Attenuation correction (AC) maps based on Dixon-VIBE sequence (a), the extended Dixon-VIBE sequence additionally using HUGE truncation correction and bone information (b), and the extended Dixon-VIBE sequence additionally using PET-based MLAA information (c). Red arrows depict differences between Dixon-VIBE only AC and improved AC-maps using HUGE, bone segmentation, and MLAA, respectively (b, c). Patient example studied in the fasted state (d-f) and patient example studied with a viability protocol (g-i) are shown. PET/MR fusion image (d, g) and PET short axis view (e, h) of the heart. A 17-segment polar plot (f, i) of the short-axis of the left ventricle was generated for each patient and each PET data reconstruction to evaluate the quantitative effect of improved AC on cardiac PET imaging. Numbers 1–17 are assigned to the 17 individual cardiac segments according to the American Heart Association standard for cardiac polar plots.</p

Patient example with bone artifact.

Patient example with bone artefact (red arrow) in the HUGE + bone AC-map (a). Corresponding difference polar plots of HUGE only, HUGE + bone and MLAA corrected PET data in comparison to standard Dixon-VIBE AC (b). Note that HUGE only and MLAA corrected PET data show a slight decrease in relative difference inferior-medial, whereas, in HUGE and bone corrected data (middle polar plot) an increase in relative difference in segments 2–3 and 8–9 is observable.</p

Classification of AC-map artifacts.

Classification of AC-map artifacts.</p

Boxplots of image quality parameter against PET acquisition time.

Boxplots of the image quality score (IQS), which is defined as nondiagnostic = 0, poor = 1, moderate = 2 and good = 3; the image noise (blood pool), the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) against PET acquisition time in minutes per bed (mpb). Note that the image quality slightly but progressively decreases in each boxplot towards lower PET acquisition times. Also the SNR and CNR decrease with lower acquisition times, while the image noise increases.</p

Simulation results.

<p>Simulations were performed with flip angle map (a) to illustrate the artifact (b). (c)-(e) show different sequence variants: flip angle adaptation (c) with the flip angle calculated for minimal signal difference (red line in f), correction factor applied in k-space (d), and low-pass filter-like reordering of k-space (e). The combination of reordering and k-space correction factor was nearly identical to the reordering alone and is thus not shown here. (g) shows artifact signal intensity relative to corresponding signal inside the phantom. (h) shows signal intensity within the phantom relative to the image with artifact (red, green, and dotted black lines) as well as the intensity of the artifact image relative to an image created with strong / weak signal only (dotted and dashed blue lines). Scaling of the images (b-e) was oriented on the corresponding maximum artifact signal. Hence, the signal inside the phantom is cut off and appears homogeneous.</p