# Measuring Identification and Quantification Errors in Spectral CT Material Decomposition
## Materials and Methods
### Imaging Samples  
Two phantoms were used in this study. The first phantom (as shown in Figure 2) was a calibration phantom (Gd/HA phantom) to demonstrate the application of our proposed technique on two different image acquisition protocols. The second phantom with a biological specimen was used to show the use of the technique to exploit the advantages of monitoring changes in detectability of multiple high-Z materials for their correct identification. The calibration phantom and the two biological specimens were scanned with two imaging protocols as described in Table 1. Phantom preparation details are as follows  
1. _Gd/HA phantom:_ A 31 mm-diameter polymethyl methacrylate (PMMA) phantom was custom-built, containing 6 mm diameter vials for a range of concentrations of gadolinium (1, 2, 4 and 8 mg/mL) and 6 mm diameter solid rods (20 mm long) of calcium hydroxyapatite (HA) (54.3, 211.7 and 808.5 mg/mL) (Quality Assurance in Radiology and Medicine (QRM) GmbH, Moehrendorf, Germany) along with water and a fat surrogate (vegetable oil) as shown in Figure 2a. Hydroxyapatite rods and water serve as bone-like and soft tissue-like material respectively. The gadolinium solutions (1, 2, 4, 8 mg/mL) were prepared by serially diluting Multihance (Gadobenate dimeglumine, Bracco Diagnostics Inc., Princeton, NJ, USA) in water.
2. Demonstration with biological specimens: Biological specimens were scanned to demonstrate the effectiveness of our proposed technique for correct material identification and quantification. Two cylindrical cartilage-bone plugs harvested from the bovine stifile joint, each with a diameter of 8 mm and length of 7 mm, were used to quantify gadolinium uptake in articular cartilage. The sample preparation setup was similar to that used in a previous study using iodinated  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Imaging Protocols** & \begin{tabular}{c} **Tube** \\ **Voltage** \\ **(kVp)** \\ \end{tabular} & \begin{tabular}{c} **Tube** \\ **Current** \\ **(\(\upmu\)A)** \\ \end{tabular} & \begin{tabular}{c} **Exposure** \\ **Time (ms)** \\ \end{tabular} & \begin{tabular}{c} **External** \\ **Filtration** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Intrinsic** \\ **Filtration (mm)** \\ \end{tabular} & \
\begin{tabular}{c} **SDD** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Energy Bins** \\ **(keV)** \\ \end{tabular} \\ \hline Protocol-1 & 118 & 24 & 220/frame & 0.375 Brass & 1.8 (Al equivalent) & 250 & 30–45, 45–60, 60–78 and 78–118 \\ \hline Protocol-2 & 118 & 30 & 130/frame & - & 1.8 (Al equivalent) & 250 & 27–33, 33–49, 49–60 and 60–118 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Imaging parameters of two protocols.  
contrast [9]. Samples were incubated for 24 h in Gd contrast agent solution (100% MultiHance) at 37 \({}^{\circ}\)C and then rinsed in PBS for 30 s. A non-Gd incubated sample was used as a control. Both incubated and control samples were placed side by side in a 15-mL Falcon tube, with PBS infused cotton wool at both ends, sealed to maintain a humid environment during scanning.