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Evaluation of CT Dose and Image Quality in Recife, Brazil

Helen Jamil Khourya*

, Marcos Ely Almeida Andradea, Richard Kramer

a,

Simone Kodlulovich Diasb

a Federal University of Pernambuco, Nuclear Energy Department – DEN/UFPE Av. Prof. Luiz Freire, 1000, Cidade Universitária – Recife-PE, Brasil.

b Radioprotection and Dosimetry Institute – IRD/CNEN Av. Salvador Allende, Jacarepaguá – Rio de Janeiro-RJ, Brasil

Abstract. The objective of this work was to perform a dosimetry study of six CT scanners located in Recife, Brazil, and to evaluate the image quality of these equipments. In this work, the volume CT air kerma index (CVOL) and the air kerma length product (PKL,CT) were estimated. These quantities were calculated using normalized weighted air kerma ind ices in CT standard dosimetry phantoms (nCW), supplied by the ImPACT group for several CT scanners, and the scanning parameters of 15 adult examinations of the head, chest, abdomen and abdomen & pelvis of each institution. The image quality tests were performed using the phantom and accreditation protocol from the American College of Radiology (ACR). The results showed a wide variation of air kerma values for the six evaluated institutions. The CVOL values for head scans varied between 11 and 59 mGy and the PKL,CT, from 49 to 545 mGy.cm. The chest examinations presented CVOL values varying from 6 to 15 mGy, and PKL,CT values between 120 and 466 mGy.cm. For abdominal scans, the estimated CVOL values varied between 5 and 14 mGy, and the PKL,CT values varied from 96 to 425 mGy.cm. The wide variation of air kerma between different centres is related to the type of scanner and also to the scanning protocol. The results also showed that, although the CVOL and PKL,CT values are compatible to the European reference levels, the image quality did not attend all ACR CT accreditation requirements. KEYWORDS: Computed Tomography; image quality; volume CT air kerma index. 1. Introduction Computed Tomography (CT) is an important diagnostic imaging method and is been widely used. It is based on the acquisition of thin axial images of the patient body, resulting in little over lap of anatomical structures, better contrast and better spatial resolution than conventional radiography. In spite of these advantages, the patient dose in CT procedures is much higher than conventional radiologic procedures. According to UNSCEAR 2000 [1], CT scans represent 34% of the annual collective dose of all X-ray diagnostic procedures. The currently recommended quantities for CT dosimetry are the volume CT air kerma index (CVOL) and the air kerma-length product (PKL,CT), which is determined from Cvol CVOL is a useful indicator of CT dose, considering specific information about each acquisition protocol. The PKL,CT supplies the total CT scan air kerma-length product [2]. In CT dosimetry, apart from estimations of CVOL and PKL,CT , it is also important to evaluate the image quality, that depends on parameters like slice thickness, gantry rotation time, table axial increment or helical pitch, reconstructions algorithms etc. According to ICRP 102 [3], the image noise in CT is the image quality parameter that is most related to the patient dose. A dose reduction increases the noise and deteriorates the low contrast resolution. The effects of these parameters on image quality and air kerma ind ices can be assessed quantitatively by phantom measurements. The aim of this work was to evaluate the CT image quality and to estimate the volume CT air kerma index and the air kerma length product for CT scanners located in Recife, Brazil.

* Presenting author, E-mail: hjkhoury@gmail.com

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2. Materials and Methods

Six CT scanners located in Recife, Brazil, were evaluated. Table 1 shows type and model and the

installation year of each scanner. Table 1: Main characteristics of the evaluated CT scanners.

Institution Manufacturer / scanner model Scanner type Installation year

1 Philips CT Aura SSCT 2001 2 Toshiba Auklet SSCT 2001 3 GE HiSpeed FX/i SSCT 2003 4 GE HiSpeed FX/i SSCT 2003 5 Toshiba Asteion SSCT 2003 6 Siemens Somatom Sensation 64 MSCT 2006

OBS: SSCT – Single-Slice CT Scanner; MSCT – Multislice CT Scanner.

Scanning parameters for adults were collected for head, chest and abdomen & pelvis examinations. For each procedure and each institution 15 examinations, randomly selected, were evaluated and the following parameters were registered: tube voltage and current, gantry rotation time, collimation and slice thickness, pitch or increment, scan length and the scanner dose index indication. Institution 2 makes only head scans and institution 3 does not perform abdominal scans.

Volume CT air kerma indices were calculated using the following equation:

W

VOL

CC

p= (mGy), (1)

where CW is the weighted CT air kerma index and p is the selected pitch. The pitch factor is the ratio between the distance moved by the patient support table (between consecutive axial scans or per rotation in helical scanning) and the total beam width (number of slices per rotation multiplied by the slice thickness). For axial scans, CVOL can be calculated by multiplying the CW value by the ratio between the beam width and the scan increment [4]. The CW values were obtained from the weighted CT air kerma index, normalized to mAs, nCW, as given by the ImPACT CT Patient Dose Calculator [5]. The nCW values are normalized to 100 mAs and depend on the following specifications: scan model, phantom (head or body), tube voltage and beam collimation. PKL,CT has been calculated by multiplying CVOL by the scan length and is given in units of mGy⋅cm [2]. To evaluate the image quality, the ACR CT accreditation protocol [6] and the ACR CT accreditation phantom, manufactured by Gammex , were used. Figure 1 shows the ACR-CT phantom, which has been designed to examine a broad range of image quality parameters. These include: positioning accuracy, CT number accuracy, slice thickness, low contrast resolution, high contrast spatial resolution, CT number uniformity and image noise.

Fig. 1: ACR CT Accreditation phantom.

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The ACR CT accreditation phantom is a solid body containing four modules, each having 4 cm thickness and 20 cm diameter. Typical acquisition parameters were used for testing, normally selected at the centres, for the examinations of head (cerebrum), adult abdomen and high-resolution chest. The phantom was positioned at the center of the gantry, aligned with the use of the scanner’s laser indicators and a spirit level. The first module is used to evaluate the slice thickness and CT number calibration. It contains five cylinders made from different materials. Using circular regions of interest (ROI’s), it was possible to evaluate the CT number corresponding to each material. The reference CT numbers for each material are listed below: - Polyethylene: from -107 to -87 HU (Hounsfield Units) - Water: from -5 to +5 HU - Acrylic: from +110 to +130 HU - Bone equivalent: from +850 to +970 HU - Air: from -1005 to -970 HU The CT numbers for water are recommended to be between ±5 HU for any nominal voltage or slice thickness. To evaluate the low contrast resolution, the image at the center of Module 2 was analyzed. There are sets with 4 cylinders each in this region, having the following diameters: 2, 3, 4 5 and 6 mm. The smallest diameter, for which all the four cylinders could be visualized, was recorded. The cylinder set with 6 mm should always? be clearly visualized. CT number uniformity, image noise and the presence of artifacts were evaluated based on the image of Module 3. Five circular ROI’s of approximately 400 mm2 were defined, one being located at the center of the image and four towards the edges. The uniformity value was calculated as absolute value of the difference between the ROI at the center of the CT image and ROI at the edge point. Image noise was measured as the standard deviation (in HU) of a central ROI in a homogeneous (typically water) phantom. Finally, the fourth phantom module was used to evaluate the high contrast spatial resolution through observation of bar patterns with different spatial frequencies. Patterns corresponding to 5 lp/cm (lines pairs per cm) and 6 lp/cm should at least be clearly visible for the abdominal and the high-resolution parameters, respectively. Figure 2 shows the Modules 2 to 4 of the ACR phantom with its structures

Fig. 2: Representations of ACR CT phantom Modules 2, 3 and 4. The diameters of the cylinder sets in Module 2 and the spatial frequency of the bar patterns Module 4 are indicated in the images [7].

3 Results and discussion Table 2 shows the irradiation parameters for the studied examinations and for all evaluated institutions. It is important to observe that the inter-slice distances (increment) for head scans are higher than the recommended values. Contiguous slices (increment = thickness) are recommended for any serial (axial) procedure. The slice thickness and pitch for chest examinations are also higher than the European recommended values, which are: slice thickness of 7 to 10 mm; pitch from 1.0 to 1.5 [7].

ROI

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The other parameters correspond to the European guidelines for both SSCT [7] and MSCT [8].

Table 2: Irradiation parameters intervals used at the studied CT examinations.

Institution Parameter Head Chest Abdomen & pelvis

1

kVp 120-130 120 120-130 mAs 200-240 100 100 Thickness (mm) 3.0-7.0 5.0 5.0-7.0 Increment (mm) or pitch 5.0-7.0 1.25 1.25-1.75 Scan length (cm) 3.2-10.5 26.0-31.5 31.8-44.0

2

kVp 120 - - mAs 110-121 - - Thickness (mm) 3.0-10.0 - - Increment (mm) or pitch 5.0-10.0 - - Scan length (cm) 4.0-11.0 - -

3

kVp 120 120 - mAs 300 170 - Thickness (mm) 3.0-7.0 5.0 - Increment (mm) or pitch 4.0-8.0 1.50 - Scan length (cm) 3.6-9.5 18.0-30.5 -

4

kVp 120 120-140 120 mAs 160-300 130-150 140-200 Thickness (mm) 3.0-7.0 5.0 5.0-7.0 Increment (mm) or pitch 4.0-8,0 1.00-1.80 1.50-1.80 Scan length (cm) 4.0-11.1 15.5-36.5 26.6-46.9

5

kVp 120 120-135 120 mAs 90-160 112-158 135-158 Thickness (mm) 3.0-10.0 5.0-7.0 5.0-7.0 Increment (mm) or pitch 5.0-10.0 1.40-1.80 1.40-1.70 Scan length (cm) 4.3-9.0 23.0-33.6 21.0-47.3

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kVp 120 120 120 mAs 190-220 135-291 176-366 Thickness (mm) 3.0-9.0 1.00 1.0-5.0 Collimation (mm) 30 x 0.6 64 x 0.6 64 x 0.6 Increment (mm) or pitch 18.0 1.50 1.35-1.50 Scan length (cm) 5.1-9.9 23.0-39.5 25.2-45.4

The CVOL values for routine head scans were estimated for the both regions of the examination: posterior fossa and cerebrum. The maximum values, shown in Figure 3, are lower than the United Kingdom reference levels [2], which are 65 mGy (posterior fossa) and 55 mGy (cerebrum) for examinations performed with single slice CT scanners. For multi-slice scanners, the reference values are 100 mGy (posterior fossa) and 65 mGy (cerebrum), which are also higher than the estimated value for all six institutions. The lowest CVOL values for head scans were observed at institutions 2 and 5, that use current-time products lower than 200 mAs. On the other hand, it was observed that the Cvol values for institution 6, which has a MSCT scanner, were lower than those found for the other three single slice CT scanner. This result is in accordance with information given in the ICRP 102 [3], which states that the MSCT dose can be lower than the SSCT dose, depending on the selected parameters.

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a) Posterior Fossa b) Cerebrum

Fig. 3: Calculated Cvol for head examinations performed at the six institutions.

The CVOL values for routine chest examinations are presented in Figure 4, as box and whiskers diagram. Each of the rectangles shown in this diagram is limited on the upper and lower sides by lines that represent the first and third quartile of the CVOL distribution. Therefore, the rectangle covers 50% of the values at each point and the length of the lines that extend upwards and downwards from each rectangle gives the exact idea of how the 25% highest and the 25% lowest values are distributed around the median. All the average CVOL values are lower than 10 mGy, which is the reference value for CT chest examination performed at SSCT, while 13 mGy, represents the reference value for MSCT [2].

Fig. 4: Box and whiskers diagram presenting the distribution of the CVOL values estimated for chest scans.

Figure 4 shows that the CVOL values from institutions 4, 5 and 6 are presenting higher variation than observed at institutions 1 and 3. This result is due to the variations in exposure parameters used in the three institutions, as shown in table 2. The observed variations at institution 6 are caused by the automatic exposure control (AEC) that modulates the tube current along the patient axis. The CVOL values were estimated using the average of all current values selected during the scan. Institution 2 makes only head scans. Abdominal examinations with contrast medium are usually performed in multiples phases: pre-contrast, arterial, portal and excretion. Figure 5 shows the distributions of CVOL values calculated for each abdominal phase at four of the evaluated institutions.

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Fig. 5: Box and whiskers diagrams presenting the CVOL values estimated for each phase of abdomen & pelvis examinations.

a) Institution 1

b) Institution 4

c) Institution 5

d) Institution 6

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The UK reference CVOL values for abdomen & pelvis examinations are 13 mGy for SSCT and 14 mGy for MSCT scanners [2]. The mean values estimated in all institutions, except hospital 6, were lower than these values. The variation of the CVOL values for the arterial-portal phase at institution 1 is due to the reduction of mAs by the technician with the objective to reduce the tube heating and the waiting time between scan procedures. It can be observed that some institutions modify their protocols for different phases of abdominal examinations, except institution 5. The higher variation observed at institution 6 is due to the AEC, as previously explained. Table 3 shows the PKL values estimated for the head, chest and abdomen & pelvis CT examinations. The reference PKL value for SSCT is 510 mGy⋅cm and for MSCT is 560 mGy⋅cm [2]. All estimated values are lower than this reference and this result was expected since the CVOL values were lower already. Table 3: Intervals of the PKL estimated values (in mGy⋅cm) for the evaluated examinations.

Examination Interval Institution

1 2 3 4 5 6

Head

PF

Min 97.2 59.5 149.9 80.1 48.7 152.1

Max 200.4 65.5 233.1 266.4 90.6 162.5

Mean 167.8 64.8 177.6 212.7 68.1 155.2

C

Min 343.0 161.0 392.7 209.4 98.5 170.0

Max 535.1 221.4 466.3 544.9 178.0 267.1

Mean 464.3 193.1 438.5 373.4 160.0 223.0

Chest

Min 251.7 - 169.3 120.6 133.0 192.2

Max 305.0 - 287.0 467.0 200.0 456.2

Mean 269.0 - 253.0 255.0 153.0 287.0

Abdomen

& pelvis

Min 256.5 - - 209.5 118.1 160.4

Max 425.9 - - 492.5 252.5 425.6

Mean 349.8 - - 322.9 196.3 272.2

OBS: PF – posterior fossa; C – cerebrum; Min – minimum value; Max – maximum value.

Table 4 shows the condition of the equipments in relation to the image quality measurements, according to ACR CT accreditation protocol. Figure 6 shows a graph with the percentages of the CT scanners fulfilling the ACR image quality requirements. All evaluated institutions presented at least one inadequate result, while institution 2 did not comply with about 70% of the ACR requirements. This institution presented the lowest CVOL values for routine head examinations. Institutions 3 and 4 have the same CT scanner model, but the exposure parameters used are different, as observed in Table 2. In spite of presenting higher CVOL values, the image quality at institution 4 is better than the verified at institution 3, indicating that this one needs to optimize its parameters. The newest CT scanner between the evaluated ones is the one used at institution 6. The only image quality inadequacy observed was the image noise (0.56%), which according to the Spanish recommendation is 0.50%. But it is known that the ideal image noise depends on some factors, including the patient size and the clinical indication of the procedure [9]. The noise can also be reduced with smooth filters, without changing the irradiation parameters. The higher noise for MSCT is common , because of the thin slice thickness used.

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Table 4: Results of the ACR image quality tests at the six evaluated institutions.

Test Institution

1 2 3 4 5 6

CT number calibration Polyethylene Water Acrylic

Bone Air

A A A

A A

I I I

I A

A I A

A A

A A A

A A

A A A

I I

A A A

A A

CT number x slice thickness 1.0 mm

2.0 mm 3.0 mm 5.0 mm 7.0 mm 10.0 mm

A

- A A I -

-

I - I - I

I

- I I I -

A

- I A A -

A

- A A A -

-

- A A A -

CT number x kVp 80 kVp 100 kVp 120 kVp 130 kVp

135 kVp 140 kVp

- A A A

- -

- - I -

- -

I - I -

- I

I - A -

- A

- A A -

A -

A A A -

- A

Slice thickness A A A A A A

Low contrast resolution Abdominal parameters Cerebrum parameters

A A

I I

A A

A A

I A

A A

Uniformity A A A A A A

Image noise* I I A A I A

Spatial resolution Abdominal parameters

Hi-res chest parameters

A

A

A

I

A

I

A

A

A

A

A

A OBS: A – adequate result; I – inadequate result. * results compared to the Spanish recommendation [10].

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Fig. 6: Bar graph showing the percentage of scanners adequate to the ACR image quality requirements.

4 Conclusions

The CVOL and PKL values determined in this work for head, chest and abdomen & pelvis CT examinations were lower than the reference levels. But these results do not mean that the scanners are in good condition and that the irradiation parameters selected at the six evaluated centres are correct. . According to the data shown above, the majority of the CT scanners showed poor image quality and many other inadequacies compared to the ACR requirements. The scanners responsible for the lowest CVOL and PKL values had the worst image quality. This result indicates the need to review the acquisition protocols selected at these institutions, aiming to improve the image quality and to maintain the CVOL values as low as reasonably achievable.

Acknowledgements

The authors wish to thank the institutions participating for to allow the collection of data and also thank CAPES and CNPq for the financial support. REFERENCES

[1] UNSCEAR, Sources and effects of ionizing radiation. Vol 1: Sources. United Nations Scientific

Comittee on the Effects of Atomic Radiation (UNSCEAR 2000 – Report to the General Assembly).

[2] SHRIMPTON, P. C., HILLIER, M. C., LEWIS, M. A., DUNN, M., Doses from Computed Tomography (CT) Examinations in the UK. National Radiological Protection Board (NRPB-W67 2003 Review). Health Protection Agency: London, 2005.

[3] ICRP, Managing Patient Dose in Multi-Detector Tomography (MDCT). International Commission on Radiological Protection Publication 102. Elsevier, 2007.

[4] IAEA. Dosimetry in Diagnostic Radiology: An International Code of Practice. International Atomic Energy Agency Technical Report Series (IAEA-TRS) n. 457. Vienna, 2007.

CT number

calibration

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[5] ImPACT. CT Patient Dosimetry Calculator. Version 0.99x. ImPACT Scan, 2006. [6] McCOLLOUGH, C. H.; BRUESEWITZ, M. R.; McNITT-GRAY, M. F.; BUSH, K.;

RUCKDESCHEL, T.; PAYNE, J. T.; BRINK, J. A.; ZEMAN, R. K. The phantom portion of the American College of Radiology (ACR) Computed Tomography (CT) accreditation program: Practical tips, artifacts examples, and pitfalls to avoid. Medical Physics, v. 31, 2004.

[7] EUROPEAN COMMISSION. European Guidelines on quality criteria for computed tomography (EUR 16262 EN). Luxembourg, 1999.

[8] BONGARTZ, G.; GOLDING, S. J.; JURIK, A. G.; LEONARDI, M.; VAN PERSIJN VAN MEERTEN, E.; RODRIGUEZ, R.; SCHNEIDER, K.; CALZADO, A.; GELEIJNS, J.; JESSEN, K. A.; PANZER, W.; SHRIMPTON, P. C.; TOSI, G. European Guidelines for Multislice Computed Tomography. European Commission, 2004.

[9] McCOLLOUGH, C. H.; BRUESEWITZ, M. R.; KOFLER, J. M. CT Dose Reduction and Dose Management Tools: Overview of Available Options. Radiographics, v. 26, 2006.

[10] SOCIEDADE ESPAÑOLA DE RADIOLOGIA. Protocolo Español de Control de Calidad en Radiodiagnóstico: Revisión 1 (Aspectos técnicos). Madrid: Edicomplet, 2002.