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Low kV rotational 3D X-ray imaging for improved CNR of iodine contrast agent
Dirk Schäfera, Martin Ahrensa,b, Michael Grassa,
a: Philips Research Europe, Hamburg, Germany
b: Philips Research Europe, Aachen, Germany
Abstract:
The contrast of iodine to soft tissue (water) decreases with higher tube voltage in reconstructed 3D X-ray
images. Improved acquisition protocols with a tube voltage of about 80 kV for imaging iodine have been
proposed earlier for diagnostic CT imaging. We investigate the contrast-to-noise ratio (CNR) and the CNR-todose ratio (CDR) for different concentrations of iodinated contrast agent inserts in water background. The
tube voltage of the protocol is lowered from 123 kV to 83 kV in 10 kV steps. A series of measurements with
16 different settings of tube voltage, current and filter settings are investigated. The weighted computed
tomography dose index CTDIW for the new protocol settings is measured.
Four protocols with tube voltages between 83 kV and 103 kV and similar X-ray dose are compared to the
original protocol. A low contrast phantom, containing a water filled cylinder with 5 tubes of different mixtures
of iodine contrast inside a 32 cm PMMA ring, is imaged with each protocol. Increased contrast of the iodine
filled tubes to the water background is clearly visible in the reconstructed volumes for lower tube voltage and
less copper filtering. The best results are obtained with the (83 kV, 561 mA, 0.4 Cu) – protocol. This protocol
may improve iodine contrast agent visibility in various 3D imaging applications. For large patients a higher
tube voltage, e.g. the (103 kV, 325 mA, 0.4 Cu) – protocol, may be used to avoid tube power limitations at
83 kV. This protocol still has improved iodine imaging compared to the 123 kV protocol and a larger tube
power reserve.
1
Introduction
The contrast of iodine to soft tissue (water) decreases with higher tube voltage in reconstructed 3D X-ray
images. Improved acquisition protocols with a tube voltage of about 80 kV for imaging iodine have been
proposed for diagnostic CT imaging [1]. We investigate the contrast-to-noise ratio (CNR) and the CNR-todose ratio (CDR) for different concentrations of iodinated contrast agent inserts in water background. The
tube voltage of the protocol is lowered from 123 kV to 83 kV in 10 kV steps. A series of measurements with
different settings of tube current and filter settings for each tube voltage are investigated. The weighted computed tomography dose index CTDIW for the new protocol settings is measured to compare changes of image
quality at similar X-ray dose.
2
Methods
The XperCT abdominal low dose protocol of a standard Philips Allura Xper FD20 system (Philips Healthcare,
Best, The Netherlands) with a tube voltage of 123 kV has been used as a starting point. Projections are
acquired in 10 seconds over an angular range of 205 degree with a frame rate of 30 Hz. The tube voltage
was reduced in steps of 10 kV and the tube current range was increased to compensate the lower X-ray flux
at lower kV. Measurements with three different copper filters of 0.1, 0.4, 0.9 mm have been performed. In
total 16 different configurations have been examined and the weighted CTDI dose index has been measured
with a dedicated body phantom and a dosimeter (Unfors, Billdal, Sweden). Five rotational scans have been
performed with the dosimeter positioned in the four peripheral and the central position for each configuration.
Four protocols with a similar X-ray dose compared to the original protocol are summarized in the first four
columns in Table 1 and considered in the following.
A low contrast phantom has been imaged with all protocols. The phantom is shown in Figure 1. It consists of
the body ring from the CTDI-phantom with a PMMA ring of 32 cm outer diameter and 16 cm inner diameter. A
water filled cylinder is placed inside the PMMA ring containing 5 tubes with different mixtures of iodine contrast agent and distilled water as summarized in Figure 1.
Figure 1: The low contrast phantom for CNR measurements.
The contrast-to-noise-ratio (CNR) and the CNR-to-dose ratio (CDR) [2] are used as quality measure.
(1)
,
,
where SROI is the averaged signal value in Hounsfield units inside the iodine tube under consideration and
σROI the corresponding standard deviation. SH2O is the averaged signal in the water background.
3
Results
Transaxial slices of the reconstructed volumes for the five different protocols are shown in Figure 2.
Figure 2: Transaxial slices of reconstructed volumes, level 0 HU, window 400 HU.
The increased contrast of the iodine filled tubes to the water background for lower tube voltage and less
copper filtering is clearly visible. The corresponding data for contrast and signal standard deviation are given
in Table 1. The values for CNR and CDR are given in Table 2 and displayed in Figure 3.
Table 1: Measured contrast and signal standard deviation for iodine tubes.
Table 2: CNR and CDR for iodine tubes
The quantitative CNR and CDR measurements confirm the visual impression that lower tube voltage gives
improved iodine-water image contrast. The best contrast and CNR values are achieved for the (83 kV,
325 mA, 0.1 Cu) – protocol. However, the best CDR values, and hence the best dose utility is obtained with
the (83 kV, 561 mA, 0.4 Cu) – protocol. The stronger copper filtering reduces the amount of very low energy
photons not contributing to the image contrast but only to the dose.
Figure 3: Graphical visualization of CNR and CDR of Table 2.
4
Conclusion
Better visibility of iodine-water contrast is achieved on interventional C-arm systems with lower tube voltages.
For the modified low contrast CTDI phantom the best results are obtained with the (83 kV, 561 mA, 0.4 Cu) –
protocol. This protocol may improve iodine contrast agent visibility in various 3D imaging applications. For
large patients a higher tube voltage, e.g. the (103 kV, 325 mA, 0.4 Cu) – protocol, may be used to avoid tube
power limitations at 83 kV. This protocol still has improved iodine imaging compared to the 123 kV protocol
and a larger tube power reserve.
5
This work has not been presented elsewhere
References:
[1] W. Kalender et al., Application- and patient size-dependent optimization of X-ray spectra for CT Med.
Phys., Vol. 36(3), 993-1007, 2009.
[2] T. Nishino, et al., Thickness of Molybdenum Filter and Squared Contrast-to-Noise Ratio per Dose for
Digital Mammography, AJR 2005; 185 : 960–963