Key words
brain - radiation dose - CT-spiral - ventricular width - hydrocephalus - adults
Introduction
Due to its wide availability, cerebral computed tomography (CCT) is often used in
the clinical routine to monitor the width of the ventricles in patients with hydrocephalus
[1]. CCT provides additional information regarding the position of ventricular drains
and their possible complications. An increased number of follow-up CTs can result
in a considerable cumulative radiation dose, and consequently in an increased stochastic
risk for genetic defects or development of X-ray induced malignomas in the course
of a lifetime [1]
[2]
[3]
[4].
The ocular lens is a radiosensitive organ due to its limited regenerative capacity.
Therefore, particular consideration should be given to CT scans of the head [5]. Despite gantry angulation parallel to the orbitomeatal line and the use of sequential
scanning techniques reducing overbeaming and overranging, there remains a scattered
radiation exposure of up to 10 % compared to CT scans performed without these techniques.
The application of external eye lens protectors or fractional scanning techniques
enables a further dose reduction of approximately 50 % [5]
[6]
[7]. However, several studies showed that there is no evidence of a minimal threshold
dose for radiation-induced cataract [8]. Hence it must be assumed that even low X-ray doses contribute to ocular lens damage.
We hypothesized that it should be possible to decrease radiation dose in follow-up
CCTs in adult patients with hydrocephalus due to considerable attenuation differences
between cerebral white matter and the sharply defined ventricles. Thus, we aimed to
estimate the minimal dose required to reliably measure ventricular width in adults
using a custom-made phantom.
Materials and Methods
Reference values
In order to establish the phantom reference values for the mean attenuation of cerebral
white matter, cerebrospinal fluid and the width of the lateral ventricles were retrospectively
derived from regular non-enhanced CCTs of 30 adults aged between 18 – 85 years randomly
chosen from our digital picture archive. The CT scans were performed with the multislice-CT
scanners, LightSpeed Ultra (GE Healthcare, Little Chalfont, United Kingdom) and Somatom
Sensation (Siemens Healthcare, Forchheim, Germany). A tube voltage of 120 kV and a
tube current of 238 ± 47 mA (mean ± standard deviation of the mean) were used for
the LightSpeed Ultra scanner and a tube voltage of 140 kV and tube current of 241 ± 47 mA
were used for the Somatom Sensation scanner.
The width of the frontal horn and the cella media of the lateral ventricles were determined
with our regular picture archiving and communication system (PACS; IMPAX, AGFA HealthCare,
Mortsel, Belgium) as delineated in [Fig. 1a, b]. In addition, the attenuation of the cerebrospinal fluid in lateral ventricles and
of white matter in the semioval center was determined ([Fig. 1b, c]). All measurements were determined once in consensus decision by two principle investigators.
Fig. 1 Exemplary position of width measurement of the lateral ventricles a, b and the attenuation measurements of the cerebrospinal fluid in the lateral ventricles
and of the white matter in the semioval center b, c in reference patients.
Abb. 1 Exemplarische Position der Weitenmessung der Seitenventrikel a, b und Dichtemessung des Liquors im Seitenventrikel sowie der weißen Substanz im Centrum
semiovale b, c für die Referenzpatienten.
Custom-made phantom
The study was performed using the following custom-made phantom previously described
in the literature [9]: 64 g of gelatine (Platagel, Hela Gewürzwerk, Hermann Laue GmbH & Co. KG, Ahrensburg,
Germany) were dissolved in 800 ml of hot water, 2.6 ml of contrast medium (Ultravist
300, Bayer Vital, Leverkusen, Germany) were added and 400 ml of the solution were
poured into an adult calvarium. The contrast medium was supplied to the gelatine-water
mixture in order to obtain a comparable attenuation to that of the white matter of
humans. The inner fibers of the carrots were shaped with a knife into pieces with
a length of approximately 10 cm and a width of approximately 1 cm. After solidification
of the gelatine, the inner fibers of two carrots representing the lateral ventricles
were placed onto the gelatine. They were then embedded with the remaining gelatine
solution (400 ml).
Phantom measurement
The phantom was examined on two different CT scanners (LightSpeed Ultra and Somatom
Sensation). Both CT scanners were calibrated daily prior to the examination of the
phantom.
For the LightSpeed Ultra scanner the following scan parameters were used: number of
detector rows: 8, collimation: 8 × 1.25 mm, pitch: 1 (table increment: 10 mm per rotation),
rotation time: 1 sec, reconstructed slice thickness: 5 mm, number of reconstructed
slices: 8, reconstruction kernel: “standard”, field of view: 220 mm, matrix size:
512 × 512 pixel, tube current: 380 mA with a tube voltage of 140 kV, and tube currents
of 400 mA, 350 mA, 300 mA, 250 mA, 200 mA, 150 mA und 100 mA with tube voltages of
140 kV, 120 kV, 100 kV and 80 kV. At a tube voltage of 140 kV, only a maximum tube
current of 380 mA could be selected on the LightSpeed Ultra scanner. Therefore, 380
mA was the highest tube current used by 140 kV on this scanner.
For the Somatom Sensation scanner the following scan parameters were selected: number
of detector rows: 64, collimation: 24 × 1.2 mm, pitch: 1 (table increment: 28.8 mm
per rotation), rotation time: 1 sec, reconstructed slice thickness: 4.8 mm, number
of reconstructed slices: 6, reconstruction kernel: “H 31s”, field of view: 220 mm,
matrix size: 512 × 512 pixel, tube current: 400 mA with a tube voltage of 140 kV,
and the tube currents of 400 mA, 350 mA, 300 mA, 250 mA, 200 mA, 150 mA and 100 mA
with tube voltages of 140 kV, 120 kV, 100 kV and 80 kV, respectively.
All possible combinations of tube current and tube voltage were tested in each case
for both scanners ([Table 1]) and the CTDIVol values in mGy were specified. 11 scan series on each CT scanner and for each of the
28 different single doses were performed with the phantom.
Table 1
Overview of the different combinations of tube voltage and tube current used with
the LightSpeed Ultra and Somatom Sensation scanner arranged according to descending
radiation doses. The corresponding scores of corrected ventricular width measurements
are shown.
Tab. 1 Überblick über die beim LightSpeed Ultra und Somatom Sensation verwendeten Kombinationen
von Röhrenspannung und -strom geordnet nach absteigender Strahlendosis. Angegeben
ist weiter die jeweilige Anzahl richtig gemessener Ventrikelweiten.
|
LightSpeed Ultra
|
Somatom Sensation
|
|
kV/mA
|
CTDIVol (mGy)
|
mean score
|
kV/mA
|
CTDIVol (mGy)
|
mean score
|
|
140 / 380
|
109.69
|
4.00 ± 0.00
|
140 / 400
|
83.76
|
4.00 ± 0.00
|
|
140 / 350
|
101.04
|
3.77 ± 0.53
|
140 / 350
|
73.29
|
3.64 ± 0.58
|
|
140 / 300
|
86.60
|
3.50 ± 0.51
|
140 / 300
|
62.82
|
3.27 ± 0.75
|
|
120 / 400
|
84.70
|
3.09 ± 0.53
|
120 / 400
|
55.04
|
3.09 ± 0.68
|
|
120 / 350
|
74.11
|
3.18 ± 0.66
|
140 / 250
|
52.35
|
3.46 ± 0.74
|
|
140 / 250
|
72.17
|
3.27 ± 0.55
|
120 / 350
|
48.16
|
3.46 ± 0.60
|
|
120 / 300
|
63.52
|
3.14 ± 0.94
|
140 / 200
|
41.88
|
3.32 ± 0.65
|
|
140 / 200
|
57.73
|
3.18 ± 0.80
|
120 / 300
|
41.28
|
3.36 ± 0.66
|
|
100 / 400
|
54.49
|
2.55 ± 1.06
|
120 / 250
|
34.40
|
3.27 ± 0.63
|
|
120 / 250
|
52.93
|
2.95 ± 0.95
|
100 / 400
|
32.88
|
3.14 ± 0.83
|
|
100 / 350
|
47.68
|
2.59 ± 0.85
|
140 / 150
|
31.41
|
3.09 ± 0.61
|
|
100 / 300
|
40.87
|
2.91 ± 1.23
|
100 / 350
|
28.77
|
2.50 ± 0.96
|
|
140 / 150
|
40.58
|
3.14 ± 0.77
|
120 / 200
|
27.52
|
2.91 ± 0.61
|
|
120 / 200
|
39.68
|
2.82 ± 0.91
|
100 / 300
|
24.66
|
2.41 ± 0.85
|
|
100 / 250
|
34.06
|
2.59 ± 1.14
|
140 / 100
|
20.94
|
3.14 ± 0.89
|
|
80 / 400
|
30.50
|
2.41 ± 0.96
|
120 / 150
|
20.64
|
2.82 ± 0.96
|
|
120 / 150
|
29.76
|
2.86 ± 0.71
|
100 / 250
|
20.55
|
2.55 ± 0.67
|
|
140 / 100
|
27.05
|
3.05 ± 1.13
|
100 / 200
|
16.44
|
2.50 ± 0.91
|
|
80 / 350
|
26.68
|
2.09 ± 1.11
|
80 / 400
|
16.04
|
2.59 ± 0.80
|
|
100 / 200
|
25.53
|
2.64 ± 1.22
|
80 / 350
|
14.04
|
2.46 ± 1.18
|
|
80 / 300
|
21.43
|
2.50 ± 1.10
|
120 / 100
|
13.76
|
2.18 ± 1.01
|
|
120 / 100
|
19.84
|
2.55 ± 0.80
|
100 / 150
|
12.33
|
2.18 ± 1.10
|
|
100 / 150
|
19.15
|
2.50 ± 1.06
|
80 / 300
|
12.03
|
1.77 ± 1.15
|
|
80 / 250
|
17.86
|
1.86 ± 0.99
|
80 / 250
|
10.03
|
2.14 ± 1.21
|
|
80 / 200
|
14.29
|
2.09 ± 0.81
|
100 / 100
|
8.22
|
2.05 ± 1.17
|
|
100 / 100
|
12.76
|
2.36 ± 1.09
|
80 / 200
|
8.02
|
2.27 ± 0.77
|
|
80 / 150
|
10.72
|
1.82 ± 0.96
|
80 / 150
|
6.01
|
1.86 ± 0.83
|
|
80 / 100
|
7.15
|
2.18 ± 1.10
|
80 / 100
|
4.01
|
1.86 ± 1.08
|
The helical scan technique was chosen in order to be able to optionally investigate
the influence of slice thickness on the measurements. The slice orientation was coronal
and axial with respect to the long axis of the carrots.
Data analysis
For each combination of tube current and voltage, the mean attenuation of the gelatine
and that of the outer parts of the carrots were measured on 10 consecutive axial slices
in a total of 11 regions of interest (ROIs) placed as delineated in [Fig. 2a]. The width of the inner fibers of the carrots was determined on coronal slices at
four sites ([Fig. 2b]). All measurements were determined with our regular PACS (IMPAX, AGFA HealthCare,
Mortsel, Belgium). Images acquired at the highest radiation dose (tube voltage of
140 kV and tube current of 380 mA / 400 mA) served as a reference. To ensure that
corresponding measurements were carried out exactly at the same position in each dataset,
the measurement lines of the reference scans were copied and pasted into each of the
other datasets. The measurement lines were adjusted to the edges of the carrots in
consensus decision of two principle investigators blinded to the scan parameters.
Fig. 2 Size and position of the regions of interest (ROIs) which were used to measure the
mean attenuation (in Hounsfield Units) of the carrots (ROIs labeled with white arrows)
and gelatin (ROIs labeled with black arrows) on 10 consecutive axial slices acquired
with the LightSpeed Ultra scanner a. Mean attenuation values of ROIs placed at 12 and 6 o’clock (dashed circles) in the
carrots are not shown for the sake of clarity. Width measurements of the carrots at
4 sites on each side acquired with the LightSpeed Ultra scanner b.
Abb. 2 Größe und Position der Messfelder (Regions of Interest, ROIs), mit denen die mittlere
Röntgendichte (in Hounsfield-Einheiten) von Möhren (mit weißen Pfeilen markiert) und
Gelatine (mit schwarzen Pfeilen markiert) auf 10 konsekutiven axialen Schnittbildern
mit dem LightSpeed-Ultra-Scanner bestimmt wurde a. Die mittleren Dichtewerte der bei 6 und 12 Uhr in den Möhren platzierten Messfelder
(gestrichelte Kreise) sind zur besseren Übersichtlichkeit nicht abgebildet. Breitenmessung
der Möhren mit dem LightSpeed-Ultra-Scanner an 4 Stellen auf jeder Seite b.
The measured width of the carrots was regarded as within an acceptable range of error,
if it did not differ from the reference values by more than ± 0.5 mm. This range was
determined after consultation with our neurosurgeons who consider a change in ventricular
size by 1 mm as relevant for their therapeutic decisions. Measurements within this
range were judged as a “correct measurement” and resulted in a score of one point
each. For each carrot, a maximum score of 4 could be obtained, if measurements at
all four sites within one carrot were within the accepted range. If none of the four
width measurements was correct compared to the reference, a score of 0 was assessed.
The mean score ± standard deviation was determined for each combination of tube current
und tube voltage for both scanners.
In each case, the attenuation of the gelatine and the carrots as mean ± standard deviation
in Hounsfield Units (HU) as well as the mean scores for the width measurement of carrots
± standard deviation were determined.
The relationship between the number of correct width measurements of the carrots and
radiation dose (CTDIVol values) was statistically analyzed using the Spearman’s rank correlation coefficient.
Based on visual inspection of the scatter plots, a quadratic curve fit was used to
describe the mathematical relationship of these parameters and calculate the goodness
of fit (R2).
Results
Reference measurements in adult patients
Reference measurements in 30 randomly selected CCTs performed in adult patients revealed
a mean attenuation of white matter in the semioval center of 27.3 ± 1.2 HU and a mean
attenuation of cerebrospinal fluid in the lateral ventricles of 5.9 ± 2.2 HU, resulting
in a mean difference of attenuation between cerebral white matter and cerebrospinal
fluid of 21.4 HU. The mean widths of the frontal horn and the cella media of the lateral
ventricles were 0.8 ± 0.3 cm and 1.2 ± 0.4 cm, respectively.
Phantom measurements
In the phantom, the mean attenuation of the carrots representing the lateral ventricles
was 22.1 ± 5.1 HU as measured with the LightSpeed Ultra scanner and 23.9 ± 5.2 HU
as measured with the Somatom Sensation scanner. The mean attenuation values for gelatine
representing white matter were 41.0 ± 0.7 HU (LightSpeed Ultra) and 45.2 ± 0.9 HU
(Somatom Sensation). The differences in the attenuation of gelatine and carrots (white
matter and cerebrospinal fluid) were 18.9 HU (LightSpeed Ultra) and 21.3 HU (Somatom
Sensation). There were no significant changes in attenuation over time or with different
tube voltages and currents. The width measurements of the carrots yielded values between
5.52 and 11.75 mm.
[Table 1] shows the CTDIVol values with different combinations of tube current and tube voltage that were applied
with the LightSpeed Ultra and Somatom Sensation scanner. The mean CTDIVol values of the LightSpeed Ultra scanner were higher by the factor of 1.56 ± 0.14 than
the one of the Somatom Sensation scanner. The scanned distance with the LightSpeed
Ultra scanner was 1.39 times longer than with the Somatom Sensation scanner due to
the different collimation of both scanners. Arranging the different combinations of
tube current and tube voltage according to descending radiation doses showed a different
sequential arrangement for both scanners ([Table 1]). Doses of 109.69 mGy (with 140 kV / 380 mA, LightSpeed Ultra) and 83.76 mGy (140 kV,
400 mA, Somatom Sensation) served as a reference.
When the tube voltage was reduced at a given tube current, the CTDIVol values decreased and the pixel noise increased exponentially ([Fig. 3]). By reducing the tube voltage from 120 kV to 80 kV, the mean CTDIVol values decreased by a factor of 2.83 ± 0.09 (LightSpeed Ultra) and 3.43 ± 0.00 (Somatom
Sensation).
Fig. 3 Pixel noise of gelatine in Hounsfield Units (HU) measured with LightSpeed Ultra and
Somatom Sensation in the regions of interest (shown in Fig. 1) at different combinations
of tube voltage und tube current (open dots and diamonds with error indicator represent
mean values ± standard deviation of the LightSpeed Ultra and the Somatom Sensation
scanner, respectively).
Abb. 3 Pixelrauschen der Gelatine in Hounsfield-Einheiten (HE), die in Messfeldern (Regions
of Interest) mit dem LightSpeed Ultra und Somatom Sensation bei verschiedenen Kombinationen
von Röhrenstrom und -spannung gemessen wurden (offene Punkte und Rauten mit Fehlerindikator
geben die Mittelwerte ± Standardabweichung für den LightSpeed-Ultra- und den Somatom-Sensation-Scanner
an).
A reduction of the tube current at a given tube voltage resulted in a linear decrease
of CTDIVol values ([Fig. 4]) and an exponential increase in pixel noise ([Fig. 3]). With a reduced tube current from 380/400 mA to 100 mA, the CTDIVol values decreased by a factor of 4.22 ± 0.11 (LightSpeed Ultra) and 4.00 ± 0.00 (Somatom
Sensation).
Fig. 4 Radiation doses (CTDIVol values) measured at different combinations of tube voltage and tube current on the
LightSpeed Ultra (open dots) and Somatom Sensation scanner (diamonds).
Abb. 4 Strahlendosen (CTDIVol-Werte), die mit dem LightSpeed Ultra (offene Punkte) und dem Somatom Sensation (Rauten)
bei den verschiedenen Kombinationen und Röhrenspannung und -strom gemessen wurden.
At standard values of 120 kV and 400 mA for a CCT, the applied doses were 84.70 mGy
with the LightSpeed Ultra scanner and 55.04 mGy with the Somatom Sensation scanner.
Measurement of carrot (ventricular) width
When reducing the radiation doses (CTDIVol), the score for the correct measurement of carrot (ventricular) width decreased at
both scanners by a quadratic correlation ([Fig. 5], [6]).
Fig. 5 Correlation between scores of correct ventricular width (X-axis) and radiation doses
(Y-axis) measured for the LightSpeed Ultra scanner (labeled with open dots).
Abb. 5 Korrelation zwischen dem Punktwert der korrekten Ventrikelweitenmessung (X-Achse)
und der Strahlendosis (Y-Achse) für den LightSpeed-Ultra-Scanner (markiert mit offenen
Punkten).
Fig. 6 Correlation between scores of correct ventricular width (X-axis) and radiation doses
(Y-axis) measured for the Somatom Sensation scanner (labeled with diamonds).
Abb. 6 Korrelation zwischen dem Punktwert der korrekten Ventrikelweitenmessung (X-Achse)
und der Strahlendosis (Y-Achse) für den Somatom-Sensation-Scanner (markiert mit Rauten).
The correlation analysis between CTDIVol and score showed a Spearman rank correlation coefficient of 0.88 with a 95 % confidence
interval of 0.76 to 0.95 for LightSpeed Ultra (p-value < 0.0001). The Spearman’s rank
correlation coefficient for Somatom Sensation was 0.93 with a 95 % confidence interval
of 0.84 to 0.97 (p-value < 0.0001). The goodness of fit of the quadratic regression
yielded an R² of 0.81 for LightSpeed Ultra and an R² of 0.86 for Somatom Sensation.
An increase in tube voltage or tube current leads to an undulant non-linear increase
of the score for the correct measurement of carrot (ventricular) width. The score
of correct measurements is plotted against the scanning parameters in [Fig. 7]. [Fig. 8], [9] give an impression of the image quality of scans using less than 85 % of the radiation
dose compared to scans with a tube voltage of 120 kV and a tube current of 400 mA.
Fig. 7 Scores of correct ventricular width measurements at different combinations of tube
voltage and tube current (open dots and diamonds indicate mean scores of the LightSpeed
Ultra and the Somatom Sensation scanner, respectively).
Abb. 7 Punktwert korrekter Ventrikelweitenmessungen bei verschiedenen Kombinationen von
Röhrenspannung und -strom (offene Punkte und Rauten zeigen die mittleren Punktwerte
für den LightSpeed Ultra und den Somatom-Sensation-Scanner).
Fig. 8 Image quality of scans acquired on the LightSpeed Ultra scanner using less than 85 %
of the radiation dose compared to the reference standard, i. e. a tube voltage of
120 kV and a tube current of 400 mA.
Abb. 8 Bildqualität der Untersuchungen, die der LightSpeed-Ultra-Scanner unter Verwendung
von weniger als 85 % der Strahlendosis lieferte, im Vergleich zum Referenzstandard
von 120 kV und 400 mA.
Fig. 9 Image quality of scans acquired on the Somatom Sensation scanner using less than
85 % of the radiation dose compared to the reference standard, i. e. a tube voltage
of 120 kV and a tube current of 400 mA.
Abb. 9 Bildqualität der Untersuchungen, die der Somatom-Sensation-Scanner unter Verwendung
von weniger als 85 % der Strahlendosis lieferte, im Vergleich zum Referenzstandard
von 120 kV und 400 mA.
Discussion
According to our phantom study, the reduction of radiation exposure is dependent on
the lowest score accepted for the correct ventricular width measurement. If, in a
setting of 140 kV and 380/400 mA, a score of 3 is adopted as the lowest limit, the
dose can be reduced by 57 % for LightSpeed Ultra and by 62 % for Somatom Sensation
based on the quadratic regression analysis. If a score of 2.5 is defined as the lowest
limit for the correct ventricular width measurement, a dose reduction of 76 % for
LightSpeed Ultra and 80 % for Somatom Sensation can be achieved.
In principle, CT radiation dose is directly proportional to the tube current and increases
with the square of tube voltage [2]
[4]
[10]. A reduction of these parameters consequently leads to a decrease in the radiation
dose, but also to an increase in the image (pixel) noise resulting in deteriorated
image quality [11]. While high contrast objects such as bones can be scanned with a significant reduction
of radiation dose (more than 50 %) without any relevant loss of information [2]
[10]
[12], scanning of low contrast objects leads to an impairment of image quality through
increasing image noise [1]
[4]. Low contrast resolution and detection of details is mainly determined by pixel
noise. An increase in pixel noise leads to a decrease in low-contrast detectability.
Therefore, low-contrast details on CT images are superimposed by noise [11].
Low-dose protocols of the neurocranium have several limitations, whereby diagnosis
of diapedesis of cerebrospinal fluid, subarachnoidal hemorrhage, cerebritis or early
stroke may be impaired or completely missed [1]. For the repeated assessment of ventricular width in patients with hydrocephalus,
low-dose scanning protocols are often sufficient due to the large attenuation differences
of cerebrospinal fluid and cerebral white matter [1].
This study showed that distance measurements to assess carrot width (ventricular width)
are more accurate due to the lower image noise at high doses. By reducing the tube
voltage, the pixel noise increases, but at the same time it also leads to an increase
in image contrast and contrast-to-noise ratio [12]
[13]. In daily practice this is mostly used in CT examinations of tissues with a high
atomic number (bone or iodine in vessels) due to the photoelectric effect [13]
[14]. The attenuation difference between carrots and gelatine in our phantom was relatively
low and the average attenuation was in the lower segment of the Hounsfield scale so
that the causal photoelectric effect played a minor part. We observed that accurate
measurements were obtained at a comparable dose when the phantom was investigated
at higher tube voltages and lower tube currents compared with scans at low tube voltages
and higher tube currents. High-energy radiation, which is less weakened in the body
or in the phantom compared to low-energy radiation, is produced at high tube voltages.
This results in more X-ray quanta contributing to image formation, so that image noise
is reduced and distance measurements (scores) are more accurate. Cohnen et al. also
showed that better image quality is achieved at higher tube voltages [12]
[15]. Our data clearly showed an increase in the accuracy of carrot width measurement
at higher tube voltages and constant tube current. An increase of the tube current
at a low kV value cannot fully compensate for the absorption of the photons.
Starting from our standard values of 120 kV and 400 mA, we found that CTDIVol values in the phantom can be reduced by 48 % and by 52 % using the LightSpeed Ultra
and the Somatom Sensation scanner, respectively, when a margin of error of 37.5 %
for the correct width measurement of ventricles is accepted. Other studies showed
that for the determination of ventricular size a dose reduction up to 70 % is possible,
however with significant image quality reduction in some cases. These analyses were
performed in clinical studies and in cadaver or phantom studies and evaluated image
quality by changing tube voltage and current [4]
[12]
[16].
Due to different scanner geometry, filters and interpolation algorithms, sequential
combination of tube current, tube voltage and image quality differ between the two
scanners when arranged by the resulting radiation dose [14]. For this reason, the scores for the two scanners can vary at the same settings.
Thus, the most efficient combination of tube current and tube voltage must be determined
individually for each type of scanner [9].
In CT examinations of the neurocranium, sequential and helical techniques are established
with advantages and disadvantages. The advantage of the sequential technique is the
good gray and white matter differentiation. However, streak artifacts in the posterior
fossa increase in the sequential technique and may limit evaluability. In the helical
CT technique overranging results in an increased dose, which can be reduced by adaptive
collimation by the manufacturer. It was also shown that helical CT results in a lower
lens dose and better image quality in the posterior fossa [17]
[18]. In our study design the helical scan technique was chosen in order to have the
option to investigate the influence of slice thickness on the measurements.
Modulation of the tube current in the direction of the XY- and Z-axis is nowadays
standard in modern scanners and adapts the tube current to the dose absorption of
the patient (patient’s geometry). This normally significantly decreases the radiation
exposure of radiosensitive organs while the image quality stays constant [7]. Due to the round sectional profile of the head, dose reduction by tube current
modulation in the direction of XY is insignificant in CCT. In the direction of Z,
there is little change of X-ray absorption between the skull and the skull base, so
that the extent of dose reduction through tube current modulation in the Z-direction
is clearly limited [19].
While our phantom study examined the most common variation of radiation dose by adjusting
tube current and tube voltage, dose adjustment may also be carried out by varying
pitch, scan time, slice thickness and scan volume [2]
[4]
[10]. Increasing the pitch leads to a reduction of scan time. A helical scanning technique
therefore reduces radiation exposure by a factor of 1/pitch. However, increasing the
pitch results in an increase in image noise and poorer spatial resolution in the direction
of the Z-axis. Our Siemens scanner uses the so-called effective mAs. With an increasing
pitch, the effective mAs increases simultaneously and the radiation dose remains the
same [7]. In our GE scanner a change in the pitch leads to a change in dose. For this reason,
the study was carried out with a pitch of 1 on both scanners to enable comparison.
Regarding the complex anatomy of the ventricular system, increasing slice thickness
or decreasing scanning volume seems not to be helpful in assessing and monitoring
hydrocephalus because of a consecutively reduced spatial resolution.
A further approach to improve image quality in low-dose CT protocols consists of iterative
reconstructions, where different calculation procedures reduce image noise [7]. The extent of a possible further dose reduction with the help of iterative reconstructions
in measuring ventricular width should be investigated in further studies.
In CT scans of the neurocranium scan parameters must be adjusted to the individual
patient’s characteristics, especially the skull size, because they influence the radiation
dose and image quality [2]
[3]
[20]. A thickened calvarium, for example in internal hyperostosis or Paget’s disease,
can reduce image quality significantly in low-dose protocols [10].
In general, scanning parameters for each cranial CT scan should be adapted to the
clinical questions and/or suspected pathological conditions, and the patient’s age
and size in order to achieve a balance between image quality and the patient’s radiation
dose [2]
[20]. In accordance with the ALARA (as low as reasonably achievable) principle, the lowest
possible radiation dose should be applied to answer clinical questions [2]
[10]. Of course, imaging modalities without the use of ionizing radiation should be considered
in assessing ventricular width in patients with hydrocephalus. In infants ultrasound
is the imaging modality of choice to measure ventricular width until the closure of
the fontanelles. Beyond this point, magnetic resonance imaging (MRI) should be performed
[1]
[2].
Study limitations
The major limitation of our study is the use of a phantom, which intrinsically only
partially reflects the situation in humans. Although the difference of attenuation
between gelatine and carrots correlates well with the one between white matter and
cerebrospinal fluid of reference patients, the study phantom could be criticized for
the following points:
The mean attenuation of the gelatine was about 2 times higher and the mean attenuation
of the carrots was about 4 times higher than that of white matter and cerebrospinal
fluid in the reference patients, so that a possible influence of hardening artifacts
cannot be completely ruled out but is likely to play a minor role in the low attenuation
difference in the reference patient.
In order to produce a realistic attenuation difference between the lateral ventricles
and the white matter in the phantoms, contrast agent was added to the gelatine. At
decreasing kV values, the iodine contrast increases, so that there is a potential
bias of the carrot width measurements. However, since the mean attenuation difference
between carrots and gelatine was almost identical at different tube voltages, we can
exclude a relevant influence of the small amount of contrast agent on the measurement
results.
In addition, contrast agent added to the gelatine was slightly absorbed by the carrots,
which led to blurred edges compared to the ones of cerebral white matter and cerebrospinal
fluid in the reference patients [9]. The model calvarium did not contain hematopoietic bone marrow and had no surrounding
soft tissue, which results in lower X-ray attenuation as in real patients. Thus, our
data may overestimate the potential of dose reduction in follow-up CCTs of patients
with hydrocephalus [9].
According to our neurosurgeons, even a minimal increase of the size of the lateral
ventricles can be an expression of dysfunctional cerebrospinal fluid circulation.
Therefore, a deviation of ± 0.5 mm from the reference value was defined as a correct
ventricular width measurement. Due to the very small permitted deviation, an influence
on the study results due to measurement inaccuracies cannot be excluded.
As another limitation, it has to be taken into account that additional pathological
findings, e. g. transependymal cerebrospinal fluid diapedesis, which may occur in
patients and cause a certain degree of uncertainty in the measurements, were not present
in our phantom.
Conclusion
Our phantom measurements suggest that lowering the radiation dose by up to 48 % for
LightSpeed Ultra and 52 % for Somatom Sensation compared to the standard protocol
(120 kV and 400 mA) still allows reliable measurements of ventricular widths.
Clinical relevance of the study
-
Patients with hydrocephalus frequently undergo multiple cranial follow-up CT-scans.
-
Due to a considerable attenuation difference between cerebrospinal fluid and brain
parenchyma, low-dose CT protocols are sufficient to assess ventricular size, leading
to a reduction in cumulative radiation dose.
-
With decreasing radiation dose, the accuracy of ventricular width measurement decreases.
