Validation of pediatric thyroid phantom using Single-Energy and Dual-Energy Computed Tomography

Researcher: Supervisor:

Co-supervisors:

MOAYYAD ALSSABBAGHDR. RAFIDAH ZAINONPROF ABD AZIZ TAJUDDINDR. MAHAYUDDIN BIN ABDUL MANAP

1. PROBLEM STATEMENT• To study the doses received by patient, several materials are used to

simulate the human tissue and organs for long time.

• Each Tissue Equivalent Material(TEM) has different mass attenuation coefficient as it is the case for different body tissue.

• Phantoms are needed nowadays in radiation dosimetry measurement since the dose deposited in the body cannot be measured directly.

• The CT numbers, which measured in Hounsfield Unit (HU), is essential to differentiate between different tissues in the body by evaluating their attenuation values

• Using the CT numbers to determine the mass attenuation values from a phantom material is not considered very common due to many factors affect the accuracy of the measured CT number, such as the model and number of detectors.

2. OBJECTIVE

• The main aim of this study was to validate a fabricated paediatric thyroid phantom by measuring the attenuation values of the phantom material using the Single-Energy and Dual-Energy Computed Tomography.

• High density Polyethylene was used to fabricate a thyroid phantom of a paediatric age of 9 years, with width of 1.2 cm and 2.5 cm in height for each lobe [1,2] (Figure 1.a).

• The thyroid phantom inserted into a container made of Acrylate (PPMA) to simulate the position of the thyroid inside the neck as shown in figure 1.b

Figure 1:(a) The fabricated paediatric thyroid

phantom (b) The container (neck) and the thyroid

phantom positioned inside it.(a) (b)

3. Materials and Methodology

• Siemens single source dual-energy CT was used to evaluate the CT numbers of the phantom. (IPPT)

• The same CT machine was adapted to be used as a Single-energy scanner.

• The current (mAs) was set automatically using the tube current modulation (CARE Dose 4D).

• The machine also automatically set the slice width to 5mm and the scanning time to 1.69 s.

3. Materials and Methodology (Cont.)

• To obtain the mass attenuation values, two methods were performed. 1. Single-energy CT (SECT) was used to produce a single

images at five different voltages (70, 80, 100, 120 and 140 kV).

2. The dual-energy CT technique at only two voltages (80 kV and 140 kV) was used to produce the perfusion images.

3. Materials and Methodology (Cont.)

3. Materials and Methodology (Cont.)

• The CT numbers of the thyroid phantom material in water and air were evaluated in each image from both scanning modes.

• Finally, The mass attenuation coefficients (m/r) of the thyroid phantom material were obtained in each scanning mode and then compared with the National Institute of Standards and Technology’s (NIST’s) tables.

2. Thyroid Phantom inside the container filled with water1. Thyroid Phantom in air

• Scanning the thyroid phantom in Air and water in both modes (DECT and SECT)

• The Linear Attenuation Coefficient has obtained from the evaluated CT numbers in each image

• The m divided by the density of the phantom material (Polyethylene

= 0.92 g/cm3) [3] to obtain the Mass Attenuation Coeff. (m/r) .

• The (m/r) of the thyroid Phantom in water and air that obtained from the SECT were compared with the (NIST’s) tables as listed in table 1.

4. Results and Discussion

4. Results and Discussion (Cont.)

• Table 1 shows No significant difference between the results of the phantom material in water and air with NIST values .

Table 1: The mass attenuation coefficients of the phantom material obtained in air and water from the SECT

kV

In Air In Water NIST

mpantom/r (cm2/g) % Difference mpantom/r (cm2/g)

% Difference m/r (cm2/g)

70 0.1878 0.97 0.1884 0.66 0.189780 0.1806 0.92 0.1816 0.41 0.1823

100 0.1704 0.89 0.1714 0.31 0.1719120 0.1635 0.62 0.1650 0.33 0.1645140 0.1562 0.60 0.1579 0.49 0.1571

Figure 1: The obtained mass attenuation coefficient from SECT compared with the values of NIST.

4. Results and Discussion (Cont.)• The obtained mass attenuation coefficients (m/r) from SECT in

water and air showed very good match with the NIST’s tables as shown in figure 1.

65 75 85 95 105 115 125 135 1450.15

0.17

0.19

0.21in Air in Water

Energy (kVp)

Mas

s att

enua

tion

coeffi

cien

t m/r

(cm

2/g)

• The mass attenuation coefficient of the phantom material obtained from the perfusion image using the DECT showed perfect match with the NIST’s values.

* The quality of the perfusion image from 80kV & 140kV equal to 120kV. (4-6)

kVIn Air In Water NIST

mpantom/r(cm2/g)

mpantom/r(cm2/g)

m/r(cm2/g)

80 0.1827 0.1812 0.1823120 0.1652 0.1646 0.1645140 0.1579 0.1575 0.1571

Table 2: The obtained mass attenuation coefficient of the Perfusion images using the DECT

4. Results and Discussion (Cont.)

4. Results and Discussion (Cont.)

75 85 95 105 115 125 135 1450.15

0.17

0.19

Air water

Energy (kV)

Mas

s att

enua

tion

coeffi

cien

t (cm

2/g)

Figure 2: The obtained mass attenuation coefficient from DECT compared with the values of NIST.

• The percentage difference found to be very small and can be neglected.

5. Conclusion

• According to our study:1. the phantom in water showed perfect match with NIST’s

values than the phantom in air. This is due to the additional attenuation of the x-ray beams inside the water.

2. The single-energy and dual-energy CT can be used to determine the mass attenuation coefficients of the thyroid phantom material in water and air.

3. The perfusion image obtained from the dual-energy CT can be used as a new method to obtain the mass attenuation coefficients as it showed perfect match with the NIST’s values

Questions Thank you

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