phantom and abdominal circumference

8/12/2019 Phantom and Abdominal Circumference

1/7

670 AJR:199 , September 2012

In a previous study [14], with a set of right

cylindric phantoms of various diameters, we

showed that radiation can be reduced by a

combined reduction in kilovoltage and tube

currenttime product, while preserving im-

age noise, and that technique optimization

curves could be created to guide clinical im-

aging. Furthermore, because of wide varia-

tions in pediatric body shape, radiation dos-

es are better calibrated to patient abdominal

circumference (AC) than to age or weight.

However, the cross-sectional shape of the

pediatric abdomen tends to be oval rather thancircular, especially when a child gets older.

Also, in most abdominal CT studies, iodin-

ated contrast agent is used to better visual-

ize certain abnormalities. To take advantage

of iodines K-edge (33 keV), low kilovoltage

should be used in CT [15]. However, lower ki-

lovoltage results in higher image noise; there-

fore, it may not be suitable for imaging large

patients where image noise dominates the

contrast. To better characterize both noise and

Optimization of Kilovoltage andTube CurrentExposure Time

Product Based on AbdominalCircumference: An Oval PhantomStudy for Pediatric Abdominal CT

Frank Dong1

William Davros1

Jessica Pozzuto2

Janet Reid3

Dong F, Davros W, Pozzuto J, Reid J

1Section of Medical Physics, Cleveland Clinic, 17325

Euclid Ave, CL2-MP, Cleveland, OH 44112. Address

correspondence to F. Dong ([email protected]).

2Department of Biomedical Engineering, Case Western

Reserve University, Cleveland, OH.

3Section of Pediatric Radiology, Childrens Hospital

Cleveland Clinic, Cleveland, OH.

Pediatr ic Imaging Original Research

AJR2012; 199:670676

0361803X/12/1993670

American Roentgen Ray Society

Diagnostic x-ray imaging modali-

ties, such as CT, serve an impor-

tant role in patient care [1]. In

2000, TheNew England Journal

of Medicinenamed medical imaging one of

the top 11 innovations for the past 1000 years

[2]. However, the potential risk of radiation-

induced cancers has caused concerns among

patients, physicians, and the general public

[37]. Pediatric patients are more sensitive to

radiation than adults, mainly because they

have more rapidly dividing cells and longer

life expectancy [810]. Therefore, radiationdose reduction in diagnostic imaging proce-

dures, especially CT, has become a major fo-

cus for pediatric radiologists. Image Gently,

an alliance of multiple medical organizations

with a focus on radiation reduction, proposes

reduced tube currentexposure time product

and kilovoltage for pediatric CT [1113], but

there have yet to be standardized guidelines

offered for combined kilovoltage and tube

currenttime product reduction.

Keywords:abdominal circumference, contrast-to-noise

ratio, CT, dose reduction, pediatric imaging

DOI:10.2214/AJR.10.6153

Received November 17, 2010; accepted after revision

January 27, 2012.

OBJECTIVE.This CT study evaluates image noise and radiation dose using a modified

CT dose index phantom to approximate pediatric abdominal shape. Contrast-to-noise ratio

(CNR) and radiation dose were measured.

MATERIALS AND METHODS.The oval shape was simulated by fixing 1000-mL sa-

line bags aside cylindric phantoms with variable circumferences. The doses at the center and

peripheral holes in the phantom were recorded. Measurements were obtained at 50400 mAs

and 80140 kVp. Diluted iodine contrast agent filled the center hole, and distilled water filledthe peripheral holes. CNR was defined as the difference in CT number between diluted iodine

and water divided by the standard deviation (SD) of CT number of water.

RESULTS.Dose increased linearly with increases in tube currentexposure time product

and by a power function (kVpn, where n= 2.643.09) for increases in kilovoltage. A range

of scanning parameters was established for each circumference from which technique opti-

mization curves were created to determine the best tube currenttime product and kilovolt-

age pairs when noise was less than 20 HU and dose was less than 2.5 cGy. CNR increased by

40% as kilovoltage was reduced from 140 to 80 kVp. A dose reduction of 70% was observed

for 140 versus 80 kVp for the same CNR.

CONCLUSION.Because pediatric patients of the same age and weight come in all shapes

and sizes, abdominal circumference is a useful clinical parameter on which to base CT scan

techniques controlling radiation outputnamely kilovoltage and tube currenttime product.

Low-kilovoltage techniques for patients with small circumference show better iodine CNR.

Dong et al.Use of Abdominal Phantom to Determine Pediatric C T Param-eters

Pediatric ImagingOriginal Research


2/7

AJR:199 , September 2012 671

Use of Abdominal Phantom to Determine Pediatric CT Parameters

iodine contrast, contrast-to-noise ratio (CNR)

is a more relevant image quality metric, which

can be used for size-dependent technique op-

timization [16].

The aim of our work is to explore the re-

lationship between kilovoltage, tube current

time product, noise, and dose and to identi-

fy kilovoltage and tube currenttime product

pairs in a bounded system constrained by a

maximum permissible image noise concur-

rently with maximum permissible dose. These

data were collected using a set of ovoid phan-

toms whose circumferences mimic the clinical

range from newborn to age 18 years.

Materials and MethodsIn this in vitro prospective study, right cylin-

dric polymethylmethacrylate (PMMA) CT dose in-

dex (CTDI) phantoms (model 764244156, Fluke

Biomedical) with diameters of 10, 16, and 32 cm

(or 30-, 50-, and 100-cm circumference) were used

to approximate the diameters of an infant, child,

and adolescent at the waist. The diameter data rang-

es indicated a 2-year-olds range of 1416 cm, a

4-year-olds range of 1519 cm, and an 18-year-

olds range of 2233 cm [17]. To mimic the shape

of the pediatric patients abdomen, a 1000-mL sa-

line bag was fixed to each side of the CTDI phan-

toms (Fig. 1). The AC of each right-circular phan-

tom plus the addition of the saline bags, one to eachside, was determined by measuring the circum-

ference of each configuration, which included the

PMMA phantoms and the saline bags, with a flex-

ible measuring tape. Two saline bags contributed an

additional 12 cm to the original AC of three CTDI

phantoms. However, for simplicity, each oval phan-

tom was still labeled as 30, 50, and 100 cm. The

combined AC was in the final technique optimiza-

tion curve, as explained in the Results section.

The CTDI phantoms had multiple holes cut at

the center and also at the 12-, 3-, 6-, and 9-oclock

locations approximately 1 cm from the phantom

edge. These holes were mainly used for placing the

dosimetry probe.

Using a National Institute of Standards and

Technologycalibrated ion chamber and electrom-

eter (Victoreen 660, Fluke Biomedicals), radiation

doses at the center and 12-oclock locations of the

CTDI phantoms were recorded. Accuracy and re-

producibility were verified according to vendor

specifications. Measurements were obtained for

varying tube currenttime products (50, 100, 200,

300, and 400 mAs) and kilovoltages (80, 100, 120,

and 140 kVp) for each oval phantom. All mea-

surements were acquired on a single 16-MDCT

scanner (Sensation-16, Siemens Healthcare). Scan

control parameters were pitch of 0.95, scan FOV

of 50 cm, image thickness of 3 mm, image-to-im-

age spacing of 3 mm, a reconstruction kernel of

B31f, and display FOVs of 40, 24, and 18 cm for

the 32-, 16-, and 10-cm-diameter phantoms, re-

spectively. There was no automated tube current

control used in either the rotational direction or in

thez-direction for any scans or any phantoms used

in this work; we used fixed tube current techniques.

The clinically acceptable noise value used in

this work was determined by sampling images

deemed to be clinically acceptable for a profes-

sional dictation by experienced board-certifiedpediatric subspecialty radiologists. These images

were deidentified pediatric abdominal cases ac-

quired at similar kilovoltage, tube currenttime

product, and other scan parameters used in the

phantom study portion of this work. In these im-

ages, we chose to measure noise in the liver. This

was done for several reasons: it is a clinically sig-

nificant organ prone to many diseases; it is large

enough to get a reasonable region of interest (ROI)

circle onto; and it is reasonably homogeneous.

Of interest was the magnitude of noise content

in abdominal soft organs; this was determined

to be approximately 20 HU. When our phantom

was scanned in the clinical technique range that

yielded doses of 2.02.5 cGy (CTDI), the mag-

nitude of the noise was approximately eight times

greater than that found in clinical cases. This high

noise value is the result of three causes: first, the

saline bags create long attenuation paths; second,

the PMMA has a higher attenuation coefficient

than that of water; and third, our geometry creat-

ed structural noise from sharp angles and fluid-air

interfaces. In order for us to scale how changing

exposure (coulombs per kilogram) changed the

amount of noise in an image, we chose to create a

scaling factor that, when applied to phantom data,

would yield an approximate noise value in clini-

cal images. For our setup, including materials and

techniques, this factor was about eight, meaning

that our phantom images had about eight times as

much noise as a similarly acquired clinical image.

Noise was recorded as the SD of pixel values at

each ROI within multiple images from each scan

with an ROI size of 100 mm2. The ROI was placed

at the same location (close to the center of the image)

using image-processing software (MATLAB ver-

sion 7.9.0, MathWorks). We used the noise values

from a random sample of deidentified clinical imag-

es with acceptable image quality from pediatric CTscans acquired at the same kilovoltage, effective tube

currenttime product (defined as [tube current in mil-

liamperes exposure time in seconds] / pitch fac-

tor), and other scan parameters. Because of the high

noise introduced by the saline bags, a noise calibra-

tion factor of 8.0 was determined and applied to the

phantom data to create equivalent clinical data. Ac-

ceptable image quality was determined by a pediat-

ric radiologist as the maximal tolerated image noise.

A

Fig. 1CT dose index (CTDI) phantom.A,Saline bags are affixed to each side of cylindrical CTDI phantom (16-cm-diameter phantom shown) to mimic oval shape of pediatric abdomen.B,CT image of 16-cm-diameter CTDI phantom with saline bags.

B


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Dong et al.

For the CNR measurement, iodinated contrast

agent (iopromide; Ultravist 300, Bayer Schering

Pharma) was diluted to match the clinically rel-

evant CT number of approximately 300 HU, and

this diluted contrast agent was injected into the

center holes of the oval phantoms. Distilled water

was also injected into one of the peripheral holes

as a reference. The image contrast was measured

on the basis of the difference of mean CT num-

ber between iodine contrast agent and water. The

CNR was computed by taking the difference be-

tween the mean CT number of diluted iodine and

water and dividing this difference by the noise

measured in water.

Reproducibility of the dose measurements was

assessed by collecting 10 sets of dose data at thecenter of the 32-cm diameter phantom scanned

at 200 mAs and 120 kVp at a 12-mm collimation

width. The noise measurement error was also as-

sessed by placing the ROI near the center of the

phantom images (away from the dose probe) using

the MATLAB-based image processing tool. Im-

age noise was given as the SD of the pixel values

in the ROI.

A one-tail paired Student ttest was used as the

statistical method to determine whether the dif-

ference in the measurements from two separate

sets of the scan parameters was significant, where

p less than 0.05 was considered statistically sig-

nificant. A linear regression method was used to

curve-fit the measured dose versus tube current

time product. For the measured dose at different

kilovoltages, a 10-based logarithm was taken be-

fore the linear regression was applied, because the

relationship between the dose and kilovoltage was

better described by a power function.

Results

For all phantom circumferences, doses in-

creased with increases in tube currenttime

product (Fig. 2A) and kilovoltage (Fig. 2B).

The relationship was linear for tube cur-

renttime product but increased by approxi-

mately the nth power of kilovoltage, with nof 2.643.09 (Table 1). As dose increased,

noise decreased for all circumferences with

a sharp decrease at lower doses and a pla-

teau effect at higher doses (Fig. 3). For ex-

ample, if the dose increased from 2 to 4 cGy,

noise decreased by 5.0 HU, whereas when

the dose increased from 6 to 8 cGy, noise de-

creased by only 1.4 HU. While holding ki-

lovoltage constant at 120 kVp and increas-

ing tube currenttime product, the absorbed

dose increased as circumference decreased

(Fig. 2A). The increase in dose at two small-

er circumference phantoms (30- and 50-cm

AC) was significant compared with the larg-

er 100-cm AC phantom, and it appeared not

to be linearly proportional to the decrease in

AC. As kilovoltage increased, significant in-

creases in the absorbed dose for the same AC

resulted (p< 0.03, based on one-tail paired

Student t test) (Fig. 4). The combination of

dose reduction from tube currenttime prod-

uct and kilovoltage is tabulated in Table 2.

Using a noise threshold of 20 HU and a dose

threshold of 2.5 cGy (dose measured with

16-cm diameter CTDI phantom with 50-cm

equivalent AC), a range of imaging param-

eters was established for each AC (Table 2).

From these data, a technique optimizationcurve was established to determine optimal

tube currenttime product and kilovoltage

pairs for equivalent AC (Fig. 5). Here, be-

cause of the saline bag on each side, 12 cm

has been added to the equivalent AC. Esti-

mated optimal parameters were as follows:

for the small patients (equivalent AC, 4060

cm), 80 kVp and less than 120 mAs; for the

medium pediatric patients (equivalent AC,

6080 cm), 100 kVp and less than 65 mAs;

and for larger size pediatric patients (equiva-

lent AC, 80100 cm), 120 kVp and less than

65 mAs. For large pediatric patients (equiv-

alent AC, > 110 cm), either 140 kVp can beused to increase the x-ray penetration with

less than 50 mAs, or 120 kVp with tube cur-

renttime product increased to 70 mAs.

Figure 6 shows how CNR increases with

lower kilovoltage for a constant dose lev-

el (0.65 cGy, measured with a 16-cm diam-

eter phantom). At 140 kVp, the CNR was

26; when kilovoltage decreased to 80 kVp,

the CNR increased to 46 (> 70% improve-

100-cm AC

50-cm AC

30-cm AC

00

5

10

15

20

30

25

100 200 300 400 500

Tube CurrentTime Product (mAs)

Do

se(cGy)

100-cm AC linear fit

50-cm AC linear fit

30-cm AC linear fit

Measured 100 cm

Measured 50 cm

Measured 30 cm

600

5

10

15

20

30

40

35

25

70 80 90 100 110 120 130 140 150

Tube Voltage (kVp)

Do

se(cGy) Power-fit 100 cm

Power-fit 50 cm

Power-fit 30 cm

A

Fig. 2Absorbed dose in center of CT dose index phantom.A,Graph shows center absorbed dose versus tube currenttime product for three abdominal circumferences (ACs) at 10 kVp.B,Graph shows center absorbed dose versus kilovoltage for 50-cm AC at 400 mAs.

B

TABLE 1: Linear Regression Coefficients (a, b, and R2) for Dose VersusEffective Tube CurrentTime Product (mAs) and Linear RegressionCoefficients (a, n, and R2) for Dose Versus Kilovoltage (kVp)

AbdominalCircumference (cm)

Dose = (a mAs) + b Dose = a (kVp)n

a b R2 a n R2

30 0.0627 0.269 0.996 8.19 105 2.64 0.986

50 0.0558 0.331 0.997 3.17 105 2.81 0.984

100 0.0131 0.0527 0.997 1.92 106 3.09 0.980


4/7



ment). Figure 7 shows that the dose can be re-

duced while maintaining the same level of the

CNR (i.e., 40) if lower kilovoltage can be used.

For example, at 80 kVp, 0.5 cGy was needed

to achieve a CNR of 40. However, at 140 kVp,

1.75 cGy was needed to achieve the same level

of CNRa 250% dose increase.

Repeatability measures for exposure data

gathered yielded approximately 1.5% repro-

ducibility at all phantom sizes, kilovoltages,

and tube currenttime products. This high

level of reproducibility resulted in stat istical-

ly significant differences (p< 0.05) in doses

for data shown in Figures 2, 4, and 5.

The noise measurement error was as-

sessed by placing the ROI near the center

of the phantom images (away from the doseprobe). The statistical calculations resulted

in mean ( SD) noise of 35.23 2.25 HU.

The SD was within 10% of the mean.

The center dose versus tube currenttime

product and kilovoltage was curve-fitted to

the following functions:

dose = a mAs + b (1),

dose = a (kVp)n (2).

Equation (1) shows that the dose has a lin-

ear relationship with tube currenttime prod-

uct. Both a and b are the fitting parameters.Equation (2) indicates that the dose varies with

kilovoltage by an nth power relationship. The

coefficients were generated by a regression

routine (LINEST in Excel, Microsoft) (Table

1). The correlation coefficient (R2) is great-

er than 0.98 for all regressions, indicating it

was reasonable to use the proposed functions

in Equations (1) and (2). The exponent (n) in

the power-fitting curve for the dose versus ki-

lovoltage was 2.64 for the smallest phantom

and 3.09 for the largest phantom, indicating

the beam-hardening effect was stronger with

the large phantom (Table 1).

Discussion

Radiation dose reduction is a key focus in

the medical and lay press [3, 4, 16]. The vari-

able standards across the country prompted

formation of Image Gently, a consortium of

experts in diagnostic imaging; the groups

have been very successful in disseminating

information and solutions for dose reduc-

tion to medical professionals and laypeople

worldwide [1113].

Pediatric radiology has led the way in cre-

ating weight-based charts for tube currenttime product reduction for CT [18]. Some use

age as their defining parameter [19]; however,

Kleinman et al. [20] caution against using age,

because predicted patient size does not corre-

late well with age and could lead to under- or

overdosing at CT. In addition, because x-ray

attenuation is related to tissue thickness, two

children with the same weight or age could

differ significantly in height, in which case

their abdominal thicknesses could be vastly

different. According to the current weight-

based standards, children identical in weight

and age would undergo CT of the abdomen

using the same dosing parameters; withoutconsidering the impact height may have on

abdominal thickness, the radiation dose could

be too high for one patient and too low for an-

other. Regardless of the parameter used, most

centers performing pediatric CT now use safe

practices for radiation dose in CT [21].

The data collected that depict how dose

changes as tube currenttime product is in-

creased exhibited the expected linear behav-

ior for a fixed phantom size and ion chamber

position. The data taken using the 30-cm cir-

cumference phantom at 400 mAs were twice

that of data taken at 200 mAs. This trend

also held for the 50- and 100-cm circumfer-

ence phantoms. Regarding how dose changes

at a given tube currenttime product while

phantom diameter varies, it can be seen that

the highest dose is recorded with the small-

est size phantom. This is so because there is

a high proportion of primary photons and

scattered photons reaching the ion chamber

for this size phantom. As the phantom diam-

eter increases, the amount of primary pho-

tons reaching the central axis of the phantom

starts to decrease. This decrease in photon

flux is not completely offset by increasedscatter because of increased phantom vol-

ume irradiated. The number of primary pho-

tons reaching the central axis is smallest in

the largest phantom. The large numbers of

scattered photons created in the first few cen-

timeters of the largest phantom do not fully

replace or completely penetrate down to the

central axis, given that they are scattered in

4 steradians and may, in fact, suffer mul-

tiple Compton events; this results in a lower

dose at the central axis compared with the

peripheral location. This pattern of dose dis-

tribution as a function of size may be impor-

tant clinically because it implies that small-er patients suffer proportionately higher core

doses per tube currenttime product used

than do larger patients. This pattern may

also have long-term consequences related to

radiogenic cancers in subdiaphragmatic or-

gans such as the liver pancreas, and kidneys.

There have been many reports touting the

benefits of lowering kilovoltage, as well as

tube currenttime product, for CT using either

0 5 10 20155

10

15

20

25

30

35

Dose (cGy)

No

ise

(HU)

00

5

10

15

20

30

45

35

40

25

100 200 300 400 500

Tube CurrentTime Product (mAs)

Do

se(cGy)

80 kVp 30-cm AC

100 kVp 30-cm AC

120 kVp 30-cm AC

140 kVp 30-cm AC

Fig. 3Graph shows that center noise versus center dose for 50-cm abdominalcircumference phantom followed power function fitting of noise, calculated as25.51 dose0.518 (R2= 0.984).

Fig. 4Graph shows absorbed dose versus tube currenttime product for 30-cmabdominal circumference (AC) phantom at variable kilovoltage.


5/7


Dong et al.

weight or age as a guide [16, 2227]. The ben-

efits include further dose reduction, improved

image contrast, and better iodinated contrast

agent visualization, the last related to imaging

closer to the inherent K-edge of iodine (33 keV)

[15, 27]. There is sufficient evidence suggest-

ing that combining kilovoltage and tube cur-

renttime product reduction is both wise andachievable in pediatric CT [16], but there are

currently no clinical guidelines for this. The

impetus for the preliminary study [14] was to

define the best clinical parameters to achieve

radiation dose reduction in CT (using both ki-

lovoltage and tube currenttime product) that

would render diagnostic quality images with

acceptable noise. Using right cylindric phan-

toms simulating the infant, child, and adoles-

cent abdomen, image noise and dose were most

closely matched to AC, and from this, dosing

parameters were created to achieve the balance

between image noise and patient dose. One lim-

itation of the study was the use of right cylin-

dric phantoms, where most pediatric abdomens

are not true cylinders but more often ovoid in

shape, especially when a child gets older. In ad-

dition, the study did not assess the CNR effects

of changes in kilovoltage.

Boone et al. [16] conducted a similar phan-

tom study on an MDCT system, looking at the

effects on dose and noise in varying both kilo-

voltage (80140 kVp) and tube currenttime

product (30300 mAs) according to object di-

ameter. The effects of parameter manipulation

on CNR, as it applied to abdominal and head

CT, were reported, but unfortunately, the ta-bles included dose reduction factors that ap-

plied only to tube currenttime product and

were not easily transferable to the clinical are-

na. In addition, that studys results were con-

fined to only one vendor platform.

The current study was designed to better

simulate the pediatric abdominal shape be-

yond infancy, to reproduce the essence of ki-

lovoltage and tube currenttime product tech-

nique charts from the prior study [14], and to

evaluate the CNR as an image quality met-

ric, in addition to noise. As in the prior study,

a noise threshold of 20 HU and dose thresh-

old of 2.5 cGy were used to establish a rangeof parameters to generate technique optimiza-

tion curves. Adaptation of the phantom to an

oval shape led to only minor adjustments in

recommended kilovoltage and tube current

time product pairs for each patient size for the

same dose and noise range. The most strik-

ing findings were related to the impact on the

CNR when a low-kilovoltage technique was

used. For example, lowering the kilovoltageTABLE2:AbsorbedDose

andNoiseValuesatVaryingTubeCurrent

TimeProduct,Kilovoltage

,andAbdominalCircumference(A

C)Settings

AC,Tube

Current

Time

Product

80kVp

100kVp

120kVp

140kVp

CTDI(cGy)

Overall

N

oise(HU)

LiverNoise

(HU)

CTDI(cGy)

Overall

Noise(HU)

LiverNoise

(HU)

CTDI(cGy)

O

verall

Noise(HU)

LiverNoise

(HU)

CTDI(cGy)

Overall

Noise(HU)

LiverNoise

(HU)

100cm

50mAs

0.4

314.0

39.3

1.

0

166.

3

20.

8

1.

4

99.

9

12.

5

2.1

74.7

9.3

100mAs

0.8

181.6

22.7

1.

8

111.

0

13.

9

2.6

61.3

7.7

3.7

4

5.3

5.7

200mAs

1.7

141.

2

17.7

3.6

61.0

7.6

5.1

38.0

4.8

7.6

3

1.8

4.0

300mAs

2.5

120.2

15.0

5.2

46.5

5.8

7.7

31.4

3.9

11.7

2

3.7

3.0

400mAs

3.6

98.1

12.3

7.6

35.9

4.5

11.3

27.0

3.4

15.5

2

0.2

2.5

50cm 50

mAs

1.

0

29.7

3.7

2.1

18.

4

2.

3

3.0

13.8

1.7

4.6

1

2.0

1.5

100mAs

2.1

18.7

2.3

4.2

13.3

1.7

5.7

10.4

1.3

8.3

8.7

1.1

200mAs

3.7

14.4

1.8

7.7

8.8

1.1

11.5

7.9

1.0

16.0

6.1

0.8

300mAs

5.7

12.1

1.5

12.3

7.3

0.9

18.4

6.1

0.8

25.4

5.3

0.7

400mAs

8.0

9.0

1.1

16.4

6.9

0.9

24.4

6.0

0.8

35.5

4.7

0.6

30cm 50

mAs

1.1

16.

3

2.0

2.3

10.3

1.3

3.5

8.9

1.1

4.8

8.3

1.0

100mAs

2.2

10.8

1.4

4.4

8.3

1.0

6.5

7.0

0.9

8.9

7.0

0.9

200mAs

4.3

8.4

1.1

8.9

6.8

0.9

12.4

6.9

0.9

17.5

5.7

0.7

300mAs

6.5

8.1

1.0

13.2

6.1

0.8

18.8

5.3

0.7

28.3

4.4

0.6

400mAs

8.8

8.1

1.0

17.1

5.4

0.7

26.5

5.2

0.7

39.1

4.4

0.6

NoteOptimalvalueswerechosen

onthebasisofoptimalnoise(20HU)anddose(

2cGy),s

howninboldtype.Borderlinenoiseanddoselevels(22.5cGy)areshowninitalictype.C

TD

I=CTdoseindex.


6/7



from 140 kVp (used in the largest patients) to

80 kVp (used in infants and small children)

resulted in a 40% increase in CNR. On the

other hand, a dose reduction of 70% can beachieved from 140 to 80 kVp while still main-

taining a desired CNR. The technique optimi-

zation tables include tube currenttime prod-

uct and kilovoltage recommendations based

on AC for all children.

Efforts to decrease kilovoltage have im-

portant implications in pediatric abdominal

CT in that intrinsic tissue contrast rises with

a decrease in kilovoltage, leading to superi-

or ability to distinguish between two tissues

with similar attenuation properties. In addi-

tion, because the K-edge of iodine is 33 keV,

lower-kilovoltage imaging lies closer to the

iodine K-edge, prompting discussion of po-

tentially lowering the volume of iodinated

contrast agent needed for an individual pe-

diatric CT scan [15]. In a study of 40 adult

patients randomized to one of two low-kilo-

voltage abdominal CT protocols, Nakayama

et al. [27] showed that one could reduce the

iodinated IV contrast bolus by at least 20%

at 90 kVp and achieve solid organ and vas-

cular enhancement that was superior to that

achieved at 120 kVp. In a similar study de-

sign, Marin et al. [28] showed improved sol-

id organ and vascular enhancement in CT for

pancreatic carcinoma using a low kilovoltageand higher tube currenttime product tech-

nique, while lowering the radiation dose by

71% and improving the CNR by 37%. The

double dose reduction achieved in reducing

kilovoltage and IV contrast agent is an ap-

pealing concept for pediatric CT.

Improvements in CNR are related to both

increases in contrast and decreases in noise.

Improved contrast provides the greatest ben-

efit in detecting small low-contrast structures,

such as liver lesions in fungal septicemia. One

must bear in mind that, by lowering the ki-

lovoltage, improved lesion detection throughimproved contrast might be outweighed by

the negative effects of decreased dose (and

increased noise) such that kilovoltage reduc-

tion would have to be paired with a modest

increase in tube currenttime product. In the

case of small-vessel detection with CT angi-

ography, a reduction in kilovoltage provides

its greatest benefit in dose reduction, not low-

contrast lesion detection, because contrast is

already maximized through IV iodinated con-

trast agent administration. In these cases, tube

currenttime product may be further reduced

without significant effect on image quality.

A limitation of the current work was the

inability to adequately address how body

shape changes with age and how this change

can affect girth and, therefore, CT techniques

based on girth. We recognize that, as chil-

dren grow in height and weight from infancy

to adulthood, the patterns of growth can be

nonuniform [17]. Some children will add sig-

nificantly to their weight while increasing in

height marginally for a time. This pattern of

growth will likely add to girth. Other childrenmay grow rapidly in height and marginally in

weight for periods in their growththis pat-

tern, while not affecting girth much, may af-

fect the oval ratio of the childs cross-section.

In each case, girth and cross-sectional shape

can change dramatically as age increases. In

the tall lean child, significant oval shape may

be present, greater than that studied in this ar-

ticle. In the child who adds weight without a

significant increase in height, a more circu-

lar cross-section may be present; this was ad-

dressed in our first article [14]. More work is

needed concerning how different oval ratios

at fixed girth and also fixed oval ratio at vari-

ous girths affect CT techniques to maintain

adequate noise at reasonable radiation doses.

Another limitation was that one vendors CT

platform was used to collect data. A more com-

plete sampling of various manufacturers and

detector geometries with a vendor product list

00.0

0.5

1.0

1.52.0

3.0

3.5

5.0

4.5

4.0

2.5

605040302010 8070 90 100 110 120

AC (cm)

Do

se(cGy)

80 kVp (120 mAs)

100 kVp (65 mAs)

120 kVp (65 mAs)

140 kVp (50 mAs)

Tube Voltage (kVp)

8010

20

30

40

50

100 120 140

CNR

Tube Voltage (kVp)

800.0

0.4

0.8

1.2

1.6

2.0

100 120 140

Dose(cGy)

Fig. 5Technique optimization curve shows optimal kilovoltage and tubecurrenttime product dose pairs (shaded area) for absorbed dose and abdominalcircumference (AC).

Fig. 6Contrast-to-noise ratio (CNR) was reduced with higher kilovoltage whilekeeping dose at 0.65 cGy (measured with 16-cm-diameter CT dose index phantom).

Fig. 7Radiation dosewas measured at differentkilovoltage to maintaincontrast-to-noise ratio at40 (dose measured with16-cm-diameter CT doseindex phantom).


7/7


Dong et al.

is warranted. This is especially true regarding

row number; as detector array width increases,

the effects of scattered radiation into adjacent

rows may have an effect on signal-to-noise ra-

tio, thereby changing our results. No attempt

was made in this article to maximize or even

characterize CNR. In large measure, this was

the result of limitations in our phantom and lackof funding for this work. Future CNR work in

conjunction with other limitations, as already

discussed, is planned. We did not explore in

this work how our technique recommendations

would affect spatial resolution. The phantom

we used did not contain tools adequate to in-

vestigate this question. The issue of selecting

an upper level of tolerable noise in a clinical

setting is not so much a limitation as it is an

unavoidable reality; it is, however, somewhat

subjective. In a more exhaustive setting, other

radiologists would be queried as to what upper

level of noise they would accept. The question

of what upper level of noise is acceptable in CT

images where noise is a limiting factor is under

way and will be reported separately. Finally, a

limitation of this work was to use the liver as

a proxy for all soft abdominal or retroperitone-

al organs. We chose the liver for the following

reasons: it is host to many diseases; it is large

enough (even in small patients) to get an ade-

quate ROI circle from which noise can be as-

certained; and it is reasonably homogeneous. It

is planned that in clinical studies more organs

of interest will be included.

Regarding automated dose reduction tech-

niques available on commercial CT scanners,we acknowledge their presence, use, and po-

tential for dose reduction in certain situations.

The goal of our work was to study the basic

effect that kilovoltage has on dose in the con-

text of adequate image quality, as determined

by a maximum permissible noise value. Our

intent was to use kilovoltage as the primary

driver for dose reduction and to use tube cur-

renttime product as the fine-tuning mecha-

nism within a kilovoltage selection (and girth

range). In later work, we plan to look at in-

plane and z-axis modulation once a kilovolt-

age stop has been selected according to girth.

Many centers performing pediatric CT arenow using safe practices for radiation dose re-

duction using technique charts for tube cur-

renttime product reduction based on patient

weight or age. We have shown that, because

pediatric patients of the same age and weight

come in all shapes and sizes, AC is a useful

clinical parameter on which to base CT scan

techniques controlling radiation output, name-

ly kilovoltage and tube currenttime product.

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