Download Iterative reconstruction in single source dual-energy CT

Iterative reconstruction in single source dual-energy CT
pulmonary angiography: Is sufficient to achieve a radiation
dose as low as state-of-the-art single-energy CTPA?
Poster No.:
B-0300
Congress:
ECR 2014
Type:
Scientific Paper
Authors:
M. Ohana, M.-Y. Jeung, A. Labani, S. El-Ghannudi, C. Roy;
Strasbourg/FR
Keywords:
Pulmonary vessels, Thorax, Radioprotection / Radiation dose,
CT-Angiography, Technology assessment, Comparative studies,
Embolism / Thrombosis
DOI:
10.1594/ecr2014/B-0300
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Page 1 of 22
Purpose
Single source dual energy CT pulmonary angiography (DE-CTPA) induces an increase
of radiation dose going up to 40% when compared to a single energy examination
carried out on the same scanner.
The main objective of this study was to nullify this increase by using iterative
reconstruction (Adaptive Statistical Iterative Reconstruction - ASIR), and thus validate
the diagnostic quality of DE CTPA made at the same radiation dose that state-of-the-art
single energy CTPA.
Dual Energy Computed Tomography (DECT) is based on a simultaneous acquisition
at low and high kilovoltage (usually 80 and 140kV), with two different approaches on
commercially available scanners (Fig. 1 on page 3):
•
•
Dual Source technique (Somatom Definition FLASH, Siemens), using two
separates couples of x-ray tube/detectors angled at 95°;
Single Source technique (Discovery CT750HD, GE Healthcare), using
one x-ray tube with fast kilovoltage switching between 80 and 140kV every
0,5msec and detectors with fast response and very short afterglow.
The main advantage of DECT is the material-based decomposition. The higher the
molecular weight of a material, the greater the absorption differences between high and
low kilovoltage are, due to the increased likelihood of a photoelectric effect at a lower kV.
This is particularly marked for iodine.
A post-processing system uses these differential data to generate:
•
•
Monochromatic reconstructions, with real-time image reconstruction at
a desired energy level, ranging from 40 to 140 keV. Lowering keV allows a
dramatic increase in iodine contrast, as seen in Fig. 2 on page 4.
Materiel density images, obtained through the decomposition of dual
energy acquisitions using the attenuation spectrum of two basis materials,
thus generating two sets of material-density images. In CT angiography, the
winning pair for material-based decomposition is the iodine/water couple
(Fig. 3 on page 5), from which iodine-density images corresponding to
a cartography of contrast uptake and water-density images equivalent to
an unenhanced acquisition can be obtained.
Page 2 of 22
Low keV monochromatic reconstructions and iodine-density images are therefore
particularly useful in CT pulmonary angiography, as they maximize iodine contrast
and offer an appreciation of lung parenchymal perfusion.
However, these advantages come at the expanse of an increased radiation dose:
•
•
with dual source DECT, it is possible to set a different tension for each
tube, i.e. fix higher mA for lower kV and vice-versa, so as to optimize the
radiation dose delivered. In the literature, CTPA radiation dose ranges from
280 to 405mGy.cm.
with single source DECT, it is currently impossible to set apart the tube
current between high and low kV, or modulate it during the acquisition.
Therefore, mA is fixed and identical for both kV, and cannot be lowered
too much as the intensity of the low kV X-ray must remain sufficient. Dose
Length Product for CTPA is reported going from 330 up to 470mGy.cm.
In single source CTPA, these radiation doses were obtained using classical filtered back
projection (FBP). Introduction of iterative reconstruction for dual energy CT is recent,
and should allow as for single energy examinations an increase in image quality and
in signal-to-noise ratio, therefore permitting to reduce radiation dose to attain the same
quality level.
We hypothesize that thanks to iterative reconstruction, we can cut the radiation dose
of DE-CTPA to the one of a single energy examination, while maintaining at least the
same image quality that a full dose FBP DE-CTPA.
Images for this section:
Page 3 of 22
Fig. 1: Dual Energy CT basic principles
Page 4 of 22
Fig. 2: Monochromatic reconstructions
Page 5 of 22
Fig. 3: Material-density images
Page 6 of 22
Methods and materials
This study was approved by our Institutional Review Board, and informed consent was
obtained from all patients.
Fifty adults referred to our department for CTPA in search of potential pulmonary
embolism were prospectively included in this study between March and September 2013.
All their examinations were carried out on a single source dual energy scanner (Discovery
CT750 HD, GE Healthcare, Milwaukee, Wisconsin, USA), using a dual energy protocol
with acquisitions parameters tweaked to target a radiation dose similar to a 100kV single
energy CTPA, i.e. a DLP of 260mGy.cm. For this objective, fixed tube current was
lowered to 275mA and iterative reconstruction was used, with an ASIR level set at 50%.
Thirty patients from a previous prospective study who underwent classical FBP
reconstructed DE-CTPA with full radiation dose (fixed tube current set at 375mA) on
the same machine were used as the reference group.
Apart from the tube current and the use of iterative reconstruction, all other acquisition
parameters were exactly the same between the two groups: energies set at 80 and
140kV, 0.6s rotation time, automatic injection of 50mL of Iomeprol 400mg/ml (Iomeron,
-1
Bracco, Milan, Italy) at 3.5ml.s , 50mL saline chaser, bolus-tracking technique with a
ROI in the main pulmonary artery and a 60 HU threshold.
Exclusion criteria were for both groups the classical contraindications for iodine contrast
media injection (proven allergy, severe renal impairment with MDRD clearance <30
-1
ml.min ), pregnancy and patient refusal. For each patient, height and weight were noted
in order to calculate the Body Mass Index (BMI).
All cases were thoroughly reviewed by two radiologists specialized in thoracic imaging
using an Advantage Workstation in version 4.6 and the GSI Viewer software.
Dynamic interpretation and analysis were conducted with special focus on:
•
A qualitative analysis, with an evaluation of the overall image quality and
examination diagnostic value using a qualitative 5 level scale (Table 1). Final
Page 7 of 22
•
grade was given after a systematic dynamic analysis of the monochromatic
reconstructions between 40 and 80 keV (concept of "spectral-surfing") and
careful review of the iodine-density images in the axial, coronal and sagittal
planes. Discrepant cases, defined by a difference greater than 2 points
between the two readers' grades, were re-evaluated by a third expert.
A quantitative analysis, with HU measurements including standard
deviations obtained by a circular ROI averaging at least 5mm², made
on 65keV monochromatic reconstructions. These systematic measures
interested the main pulmonary artery, a principal pulmonary artery, a lobar
branch (usually the left lower one) and an erector spinae muscle not in fatty
atrophy.
1
Minimal or absent opacification
Nondiagnostic examination
2
Weak opacification
Large arteries visible but no definite
diagnosis possible
3
Limited opacification
Sufficient for a diagnosis
4
Good opacification
To the peripheral pulmonary arteries up to
the 4th division
5
Excellent opacification
To the subsegmental pulmonary arteries
beyond 5th division
Table 1: Qualitative 5 level scale
Qualitative results were analyzed by a Kruskal-Wallis nonparametric test and quantitative
data were compared using a Student's t test. A p<0.05 was considered significant.
Results
Population
Page 8 of 22
-------------------------------------------------------------------------------The population of the two groups shared overall comparable characteristics: age,
sex-ratio and body mass index showed no statistically significant difference. These
demographics are reported in Table 2.
ASIR Group
Reference Group
p
(Full dose with FBP)
Age
64.8 ±16.2
64.4 ±18.6
0,9159
(yo)
(30 - 92)
(21-91)
(Student's t test)
BMI
25.6 ±4.5
26.2 ±4.6
0,5984
(15.2 - 35.3)
(17.6 - 35.4)
(Student's t test)
29M/21W
14M/16W
0.328
Sex ratio
(Kruskal-Wallis)
Table 2: Demographics of the study
Quantitative analysis
-------------------------------------------------------------------------------HU enhancement weren't statistically different between both groups, whereas mean
image noise was significantly lower in the ASIR group. However, signal to noise and
contrast to noise ratios weren't significantly better in the ASIR group (Table 3).
ASIR Group
Reference
Group
p
(Student's
(Full dose with test)
FBP)
Pulmonary
trunk
Main
pulmonary
artery
365 ±90
402 ±136
(190 - 544)
(207 - 697)
350 ±89
391 ±128
(181 - 522)
(186 - 669)
Literature
reference
t
(50keV FBP)
0.1969
-
0.1366
-
Page 9 of 22
Lower
artery
lobe 337 ±91
(188 - 529)
388 ±133
-
0.1222
463 ±129
0.0022*
28.6 ±8
0.0989
17.4 ±7.1
0.1222
14.7 ±7.5
(203 - 751)
Mean arterial 351 ±88
enhancement
(199 - 532)
394 ±131
Mean noise
22.1 ±6.6
27.2 ±6.9
(13.4 - 43)
(16.4 - 45)
Signal
to 16.9 ±6
Noise ratio
(7.6 - 35.3)
0.0741
(202 - 694)
14.9 ±6.9
(7.7 - 24)
Contrast
to 14.5 ±5.6
12.7 ±4.8
Noise ratio
(6.2 - 29.9)
(5.3 - 22.1)
Table 3: Quantitative enhancement analysis
Qualitative analysis
-------------------------------------------------------------------------------There was no significant difference in the qualitative evaluation between both groups
(p=0.32 - Table 4). All the CTPA examinations were diagnostic (grade # 3), with an overall
excellent image quality.
ASIR Group
Average
4.44 ±0.7
qualitative
(3-5)
score
Table 4: Qualitative analysis
Reference
Group
p
Literature
reference
(Kruskal(Full dose with Wallis)
FBP)
(FBP)
4.6 ±0.5
4.0
0,3175
(4-5)
Radiation dose
--------------------------------------------------------------------------------
Page 10 of 22
Radiation dose was commonsensically very significantly decreased in the ASIR group
by 37% (p<0.0001), with our initial objective even surpassed as the final mean DLP was
below the original target of 260mGy.cm (Table 5).
ASIR Group
Reference
Group
p
(Student's
(Full dose with test)
FBP)
DLP
244 ±33
(mGy.cm)
(183 - 333)
Table 5: Mean radiation dose
388 ±100
<0.0001*
Literature
reference
t
(FBP)
412 ±34
(278 - 625)
Some examples to illustrate the utility of iterative reconstruction and the excellent image
quality obtained at these low radiation doses are provided in Fig. 4 on page 11 and
Fig. 5 on page 12.
Images for this section:
Page 11 of 22
Fig. 4: Utility of Iterative Reconstruction
Page 12 of 22
Fig. 5: Examples of image quality
Page 13 of 22
Conclusion
Our initial objective is fulfilled: this study demonstrates the efficiency of iterative
reconstruction to nullify the increase in radiation dose associated with the use of a
dual energy technique.
Therefore, one is able to realize DE CTPA of excellent diagnostic quality at a radiation
dose (DLP = 244 mGy.cm) equivalent to that of a single energy examination carried out
on the same scanner, without any significant difference in the arterial enhancement, the
signal to noise and contrast to noise ratios or the qualitative aspect, and even with a
lower mean image noise.
Even though image rating was globally very good (4.44 out of 5) and all investigations
were of diagnostic quality, there were still 5 examinations in the ASIR group that
were rated only 3 out of 5 due to a limited opacification. Most of these unsatisfactory
examinations could be related to a difficult patient (poor general condition, polypnea,
pathological underlying lung parenchyma, obesity).
We tried to determine if there were a relation between the image quality and the patient's
morphotype using Pearson's correlation test:
•
•
•
there was a strong positive relationship between image quality and signal to
noise and contrast to noise ratios (#= 0.57 and 0.55 respectively - Fig. 6 on
page 16);
there was a strong negative relationship between body mass index and
signal to noise and contrast to noise ratios (#= -0.59 and -0.55 respectively Fig. 7 on page 17);
however, there was only a moderate negative relationship between body
mass index and image quality (#= -0.31 - Fig. 8 on page 17).
This low negative correlation between image quality and BMI is probably due to the lack
of relationship between the whole body weight and the attenuation of the thorax. Indeed,
apart from the extremes (BMI<18 or >40), the BMI is not necessarily a good reflection
of the thorax diameter: it can overestimate it when fat deposits preferentially over the
abdomen or the hip, or underestimate it in big-breasted women. Being far from perfect,
it is still the most easily obtainable morphological information to routinely optimize the
acquisition parameters.
Page 14 of 22
We can propose the following compromises for DE CTPA with iterative reconstruction
(Table 6):
BMI
Target DLP
Fixed Tube Current
(mGy.cm)
(mA)
<20
200
225
20<BMI<30
250
275
30<BMI<35
300
325
>35
350
Table 6: Proposed optimal acquisition parameters
375
This is eventually only an ersatz of automated tube current modulation, as the technique
is not currently available on single source DECT.
Study limitations
-------------------------------------------------------------------------------•
•
•
•
•
Only 50 patients in the ASIR group, which is however enough to
demonstrate the feasibility of the technique.
Evaluation was made only on quantitative and qualitative levels, with no
comparison of diagnostic accuracy to a reference standard. However, CTPA
is now the standard of care for suspected PE in the emergency setting, and
DE-CTPA is already well established as being more efficient than single
energy examinations.
Only one radiation dose (fixed tube current at 275mA with a target DLP at
260mGy.cm) was studied. Test of the other proposed intermediate doses is
currently under way.
Only one level of ASIR (50%) was studied. For CTA, a higher level of
iteration is possible with ASIR pushed up to 70-80%, which could further
increase the quality of the examination.
The bolus-tracking triggering ROI was placed in the main pulmonary artery.
Some authors recommend placing it in the right cardiac cavities, to avoid
missing the bolus in hyperkinetic patients. We favored the main pulmonary
artery to obtain an enhancement (even moderate) of the thoracic aorta,
useful for differential diagnosis in the context of acute chest pain.
--------------------------------------------------------------------------------
Page 15 of 22
CONCLUSION
-------------------------------------------------------------------------------This study demonstrates the efficiency of the iterative reconstruction to nullify the
increase in radiation dose associated with the use of a dual energy technique.
Therefore, single source dual energy CTPA of excellent diagnostic quality can be
acquired at a radiation dose (DLP = 244 mGy.cm) equivalent to that of a single energy
examination.
Patient's morphotype directly impact the image quality, and while automated tube current
modulation remains unavailable on single source DECT, we can advocate a manual
adjustment of the dose delivered in correlation with the BMI.
Images for this section:
Fig. 6: SNR and CNR VS Image Quality
Page 16 of 22
Fig. 7: SNR and CNR VS BMI
Page 17 of 22
Fig. 8: Image Quality VS BMI
Page 18 of 22
Personal information
Dr Mickaël OHANA
Radiologist
Strasbourg University Hospital
For any questions about this work: [email protected]
Follow me on Twitter: @macromik
Dr Mi-Young JEUNG
Radiologist
Strasbourg University Hospital
Dr Aissam LABANI
Radiology Fellow
Strasbourg University Hospital
Dr Soraya El-Ghannudi
Cardiologist
Strasbourg University Hospital
Pr Catherine ROY
Head of Radiology department
Strasbourg University Hospital
Images for this section:
Page 19 of 22
Fig. 9: Aerial view of Strasbourg and the "Nouvel Hôpital Civil"
Page 20 of 22
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