菅 幹生

PURPOSE: To quantify the bias of shear wave speed (SWS) measurements in a viscoelastic phantom across six different ultrasound (US) systems and to compare the SWS with those from transient elastography (TE) and magnetic resonance elastography (MRE). METHODS: A viscoelastic phantom of stiffness representing fibrotic liver or healthy thyroid was measured with nine (linear probe) and 10 (convex probe) modes of six different US-based shear wave elastography (SWE) systems using linear and convex probes. SWS measurements of three regions of interest were repeated thrice at two focal depths, coupling the probe to the phantom using a jig. An MRE system using three motion-encoding gradient frequencies of 60, 90, and 120 Hz and TE were also used to measure the stiffness of the phantom. RESULTS: The SWS from different SWE systems had mean coefficients of variation of 9.0-9.2% and 5.4-5.6% with linear and convex probes, respectively, in viscoelastic phantom measurement. The focal depth was a less significant source of SWS variability than the system. The total average SWS obtained with US-SWE systems was 19.9% higher than that obtained with MRE at 60 Hz, which is commonly used in clinical practice, and 31.5% higher than that obtained with TE using the M probe. CONCLUSIONS: Despite the measurement biases associated with the SWE systems, biases were not necessarily consistent, and they changed with the probes used and depth measured. The SWS of the viscoelastic phantom obtained using different modalities increased according to the shear wave frequency used.

我々が開発したWGI(Whole Gamma Imaging)の小動物実証機は,世界初のフルリング型コンプトンイメージングシステムであり,これまでに高精細なマウス画像が得られたことを報告した.フルリングにより高感度化を達成したことが要因の一つとして考えられるが,理論的にコンプトン画像再構成の条件としては必ずしもフルリング型である必要性はなく,検出器の削減により,感度は犠牲になるが,コスト削減や設計の柔軟性向上が期待できる.しかしながら,実際には散乱角度の検出限界やブロック型の検出器配置の影響等で,再構成条件は保証されていない.そこで,本研究では,試作装置の測定データを限定することで部分リング化し,フルリングの場合と比較することで,コンプトン画像再構成に必要なジオメトリ条件を実験的に検討した.円筒型ファントムの測定データを用い,再構成画像を視覚的に評価した結果,散乱検出器もしくは吸収検出器に囲まれる範囲が180°未満の領域が物体にある場合,画像にアーチファクトが生じたが,物体全域が散乱検出器と吸収検出器の共に180°以上囲まれている場合には,フルリングと同様にアーチファクトのない画像が得られた.よって,WGIコンプトン画像再構成の完全性条件として,散乱検出器,吸収検出器共に,対象視野全域を180°以上共通の角度範囲として囲む必要があることが示唆された.(著者抄録)

We evaluated the long-term stability of a newly developed viscoelastic phantom made of polyacrylamide (PAAm) gel for magnetic resonance elastography (MRE) and ultrasound-based shear-wave elastography (US SWE). The stiffness of the cylindrical phantom was measured at 0, 13 and 18 months. Storage and loss moduli were measured with MRE, and shear-wave speed (SWS) was measured with US SWE. Long-term stability was evaluated in accordance with the Quantitative Imaging Biomarker Alliance (QIBA) profiles for each modality. The initial storage and loss moduli of the phantom were 5.01±0.22 and 1.11±0.15 respectively, and SWS was 2.57±0.04 m/s. The weight of the phantom decreased by 0.6% over the 18 months. When measured with MRE, the stiffness of the phantom decreased and changes to the storage and loss moduli were -3.0% and -4.6% between 0 and 13 months, and -4.3% and 0.0% between 0 and 18 months. The US measurements found that SWS decreased by 2.4% over the first 13 months and 3.6% at 18 months. These changes were smaller than the tolerances specified in the QIBA profiles, so the viscoelastic PAAm gel phantom fulfilled the condition for long-term stability. This new phantom has the potential to be used as a quality assurance and quality control phantom for MRE and US SWE.