Source CT Research Articles

SU‐E‐I‐110: Do Reconstruction Parameters Affect CT‐Based Kidney Stone Volume Quantification?

Purpose: To investigate the effects of reconstruction field‐of‐view (FOV), slice increment and kernel on the quantification of kidney stone volume. Methods: Three concentrations (200, 400 and 800 mg/cc) and two sizes (21.87 and 99.9 mm3) of hydroxyapatite (HA: Ca5(PO4)OH) cylinders were used to simulate kidney stones of 3 and 5 mm diameter, respectively). These were submerged in a 30 cm wide water phantom and scanned on a dual‐ source CT scanner operating in dual‐energy mode. Reconstruction was performed with a) FOVs from 50 to 300 mm at 50 mm interval; b) image reconstruction increments of 0.1, 0.3 and 0.6 mm; c) 6 recon kernels, from very smooth to medium sharp, all using a 0.6 mm image thickness. CT data were segmented and volumes calculated using volume‐rendered 3D images of each stone, commercial software (Analyze, version10.0) and fixed CT number thresholds (130, 250 and 550 HU for HA200, HA400 and HA800, respectively). Differences in volume between measured and reference values, normalized to the reference values were calculated. The range (maximum — minimum) in normalized volume differences were used to assess the overall influence of various reconstruction parameters. Results: Although normalized volume differences varied with cylinder concentration and size, the differences among FOVs were not substantial (<10.61%). The normalized volume differences among image increments were very small (< 2.45%), except for the lowest concentration/smallest size (differences up to 8.14%), where the smallest image increment was most accurate (differences < 3.02%). Very smooth kernels decreased volume accuracy by as much as 16.19%. Conclusions: Reducing reconstruction FOV did not increase accuracy of volume measurements. Overlap between reconstructed images was beneficial for volume measurement of low concentrations of HA (200 mg/cc) and small size (3 mm diameter) objects. Very smooth kernels were not suitable for volume measurement due to too much edge blurring.

WE‐C‐110‐01: Volume‐of‐Interest Scanning Technique in Multi‐Slice CT: A Feasibility Study

Purpose: To evaluate the feasibility of Volume‐of‐Interest (VOI) scanning in multi‐slice CT to reduce radiation dose while maintaining image quality within the VOI. Methods: Scanning patients with a limited scan field‐of‐ view can substantially reduce radiation dose but usually cause severe truncation artifacts. VOI scanning involves adding a filter in the x‐ray beam to strongly attenuate peripheral photons, so that much lower x‐ray intensity is delivered to the region outside a preselected VOI while allowing the original intensity within the VOI. A semi‐anthropomorphic abdominal phantom was sequentially scanned with 240, 180, 120, 60, 30, 15 and 10 mAs at 120 kVp on a dual source CT scanner. A 10‐cm diameter cylindrical VOI was selected at the center of phantom. Six rods with various attenuation levels were located within the VOI. Projection data outside the VOI were replaced with data acquired with lower mAs to simulate VOI projection data. CT images were reconstructed using same reconstruction kernels as the full dose scan. CT numbers and noise were measured on images with and without the VOI filter. Radiation dose with and without the VOI filter was estimated using 16 projection views over a 180 range where the VOI filter was manually placed in the x‐ray beam.. A simulated VOI study was also performed using patient data. Results: CT numbers and noise inside the VOI were comparable to those obtained with a full dose scan for mAs level outside the VOI as low as 6.25% of the full mAs. Radiation dose outside the VOI was reduced up to 50%‐65%, while dose inside the VOI was reduced by 25%–30%. Diagnostic information within the VOI appeared to be preserved, although only large anatomic structures outside the VOI were visible. Conclusions: VOI scanning is feasible in multi‐slice CT and may substantially reduce patient dose, while preserving image quality inside the VOI.

Reproducibility of coronary plaque detection and characterization using low radiation dose coronary computed tomographic angiography in patients with intermediate likelihood of coronary artery disease (ReSCAN study)

The purpose of this study is to evaluate the interscan, interobserver and intraobserver agreement for coronary plaque detection, and characterization using low radiation dose high-pitch spiral acquisition coronary CT angiography (CTA). Two experienced observers independently evaluated coronary CTA datasets from 50 consecutive patients undergoing two 128-slice dual source CT scans within 12 days. Mean (±SD) estimated radiation exposure was 1.5 ± 0.2 mSv per scan. Observers recorded the presence and characterization of coronary plaques as non-calcified or calcified. A "segment involvement score" (SIS) was computed by summing the numbers of segments with any coronary plaque per patient. Reproducibility was assessed using kappa (κ) statistics, paired t test and Bland-Altman analyses. Interscan, interobserver, and intraobserver agreement (κ-values) for detection of any or calcified plaques were 83-94% (κ-values 0.57-0.85), and 67-84% (0.31-0.67) for non-calcified plaques on a patient level. No significant difference was observed in mean interscan or interobserver SIS. Mean (95% CI) intraobserver SIS difference was -0.88 (-1.25; -0.51), P < 0.001, with limits of agreement from -4.7 to 2.9. Low radiation dose high-pitch coronary CTA permits detection of any or calcified plaques with high interscan, interobserver, intraobserver agreement. However, variability for the detection of non-calcified plaque is substantial.

Commentary on: delayed enhancement imaging of myocardial viability: low-dose high-pitch CT versus MRI

In 2005, Nikolaou et al. [1] published a paper in EuropeanRadiology about the assessment of myocardial perfusionand viability from routine contrast-enhanced 16-detector-row computed tomography. The authors reported promisingresults, however, concluded carefully: “… application ofdelayed enhancement techniques might be useful. …However, this procedure requires a second delayed CTscan, increasing the radiation exposure of the patient.”I remember the first scientific presentations aboutcardiac perfusion imaging and delayed enhancement usingcomputed tomography—it was interesting, but the scientificcommunity considered this topic more or less a scientificgimmick rather than serious work with clinical relevance.Now, in 2011, Alkadhi et al. [2] publish in EuropeanRadiology a paper entitled “Delayed enhancement imagingof myocardial viability: Low-dose high-pitch CT versusMRI.” The authors included 24 patients and could showthat by using the newest generation CT equipment, delayedenhancement by CT has become a reality at a ultra-lowradiation dose in submillisievert range, and is now ready forclinical use: The estimated mean radiation dose was as lowas 0.89±0.07 mSv (range: 0.74–1.02 mSv). This lowradiation dose was obtained by using the “high-pitch”spiral CT technique with a pitch factor of 3.4 andprospective ECG-synchronization; furthermore a 100 kVtube voltage was applied for the delayed enhancement CTdata acquisition, making use of the higher attenuation ofiodine at lower photon energies. In the present study, theauthors perform a standard contrast-enhanced CTA using ahigh-pitch protocol on a commercially available 128-slice,dual-source CT system after the administration of 70 mliodinated contrast media. In order to achieve an adequatedose of iodine, 5 min after the first CT data–acquisition, asecond dose of 70 ml of iodinated contrast media wasadministered and, after a total delay of around 15 min fromthe first injection, a second high-pitch delayed enhancementCT data–set was obtained. The sensitivity, specificity, andaccuracy of CT for the diagnosis of non-viability were90.0%, 92.9%, and 91.7% per patient and 60.7%, 96.8%and 94.4% per segment, respectively, with MRI serving asthe gold standard.What happened in the last 6 years to make the vision ofmyocardial viability imaging a reality?In my opinion, new technical developments such as low-dose gating techniques, prospective ECG-triggering andhigh-pitch CT are all responsible for this present stepforward in cardiac imaging. The Cardiac Imaging ‘hype’started with the introduction of 64-slice CT with arevolution in image quality. This first wave of excitementwas followed by a disillusion: the radiation dose of cardiacCT reached values of 12–14 mSv, even higher than theradiation dose values of a conventional invasive angiog-raphy. However with the introduction of newer CTequipment with 128-, 256-, and 320-slices, as well asdual source CT–machines, faster CT protocols withradically lower radiation dose have become possible. Inthe present paper, the CTA–,data–acquisition, as well asthe delayed enhancement imaging, is performed using a highpitch of up to 3.4, compared with 0.2–0.4 for conventionalnormal retrospective ECG-triggered CT. Excellent imagequality with high pitch has become possible due to recentlyintroduced 128-dual-source CT technology. With this tech-nique, low-dose CT is possible, with a radiation dose lower

Virtual unenhanced second generation dual-source CT of the liver: Is it time to discard the conventional unenhanced phase?

Dual-energy dual source CT can almost simultaneously image patients using two different tube potentials, allowing material decomposition and creation of 'virtual unenhanced' (VU) images from post-contrast series. 75 patients undergoing triple-phase liver CT examinations were imaged using a second generation dual-source CT machine with tube potentials 140/100 kVp. Post-processing VU series were derived from arterial and portal phases. Regions-of-interest from liver parenchyma and within fat ('noise' assessment) were drawn to compare VU series to conventional unenhanced (CU) series. Subjective analysis assessed image quality and the suitability of VU to replace CU series. Mean Hounsfield unit (HU) values of liver were higher in the VU series: portal 51.9 (SD = 10.29), arterial 51.1 (SD = 10.05), compared to the CU series 49.2 (SD = 9.11); P<0.001. However, Pearson's correlation of the VU and CU series remained excellent: 0.838 (portal), 0.831 (arterial). Bland-Altman plots also showed good agreement between both VU and the CU datasets. Noise measurements were significantly lower in both VU series (P<0.001). For subjective analysis, image quality was rated as very good/excellent in 100% of CU images, 93.3% of portal VU and 88.7% of arterial VU series. Overall, portal VU and arterial VU images were acceptable replacements for the CU series in 97.4% and 96.1%, respectively. Post-processing was noted to create a number of artefacts in VU images--knowledge of these is essential for interpretation. Portal and arterial-derived VU images objectively correlate to CU images and demonstrate good image quality and acceptability. VU image sets could replace the conventional unenhanced images in the vast majority of cases, significantly reducing radiation dose.

Image fusion in dual energy computed tomography for detection of various anatomic structures – Effect on contrast enhancement, contrast-to-noise ratio, signal-to-noise ratio and image quality

The purpose of this study was to evaluate image fusion in dual energy computed tomography for detecting various anatomic structures based on the effect on contrast enhancement, contrast-to-noise ratio, signal-to-noise ratio and image quality. Forty patients underwent a CT neck with dual energy mode (DECT under a Somatom Definition flash Dual Source CT scanner (Siemens, Forchheim, Germany)). Tube voltage: 80-kV and Sn140-kV; tube current: 110 and 290 mAs; collimation-2×32×0.6 mm. Raw data were reconstructed using a soft convolution kernel (D30f). Fused images were calculated using a spectrum of weighting factors (0.0, 0.3, 0.6 0.8 and 1.0) generating different ratios between the 80- and Sn140-kV images (e.g. factor 0.6 corresponds to 60% of their information from the 80-kV image, and 40% from the Sn140-kV image). CT values and SNRs measured in the ascending aorta, thyroid gland, fat, muscle, CSF, spinal cord, bone marrow and brain. In addition, CNR values calculated for aorta, thyroid, muscle and brain. Subjective image quality evaluated using a 5-point grading scale. Results compared using paired t-tests and nonparametric-paired Wilcoxon-Wilcox-test. Statistically significant increases in mean CT values noted in anatomic structures when increasing weighting factors used (all P≤0.001). For example, mean CT values derived from the contrast enhanced aorta were 149.2±12.8 Hounsfield Units (HU), 204.8±14.4 HU, 267.5±18.6 HU, 311.9±22.3 HU, 347.3±24.7 HU, when the weighting factors 0.0, 0.3, 0.6, 0.8 and 1.0 were used. The highest SNR and CNR values were found in materials when the weighting factor 0.6 used. The difference CNR between the weighting factors 0.6 and 0.3 was statistically significant in the contrast enhanced aorta and thyroid gland (P=0.012 and P=0.016, respectively). Visual image assessment for image quality showed the highest score for the data reconstructed using the weighting factor 0.6. Different fusion factors used to create images in DECT cause statistically significant differences in CT value, SNR, CNR and image quality. Best results obtained using the weighting factor 0.6 for all anatomic structures used in this study.

Case Report–Spontaneous Splenic Rupture–A Rare Complication of Haematological Malignancies

IntroductionSpontaneous splenic rupture is an abdominal catastrophewith patient presenting with haemodynamic instability. Thisrequires urgent diagnosis and emergency surgery. Themajority of the cases of splenic rupture are those ofdiseased spleen and are called pathological rupture ofspleen [1]. True spontaneous rupture which is very rare, isthe rupture of normal spleen. Haemato-oncological malig-nancies are often incriminated as a cause of pathologicalrupture of spleen [1]. Splenomegaly is a very commonpresentation of chronic myeloid leukaemia. However theprogression of splenomegaly in chronic myeloid leukaemiais usually slow and insidious that many patients may not beaware of it until spleen has become huge in size [2].Pathogenesis of rupture includes congestion of theparenchyma by blast cells and intrasplenic hemorrhagecaused by coagulation abnormalities and splenic infarction.Splenic ruptures usually occur in enlarged spleens [1].Here we present a case of 40 year male who was a knowncase of chronic myeloid leukaemia who presented in emergen-cy with left upper quadrant paint and abdominal distention.Patient was haemodynamically unstable. After doing ultra-sound and computed tomography diagnosis of spontaneoussplenic rupture was made. Patient was operated and spleenec-tomy was done. Patient was stable post operatively.Case ReportWe present a case of 40 year old male who wasdiagnosed as chronic myeloid leukaemia 6 months backon basis of peripheral blood film and bone marrow. Hisspleen was enlarged in size. Patient did not receive anytreatment for this.He presented in the emergency with left upper quadrantpain and progressively increasing abdominal distention. Onexamination patient was mildly febrile and pale. He washaving cold and clumsy skin. His pulse was 136/min andBP was 94/60. Heart and lungs were normal. No pasthistory of trauma was there.Perabdominal ExaminationAbdomen was distended with left upper quadrant tender-ness guarding and rigidly were present. Spleen wasenlarged in size. Investigations showed low Hb of 7.4 gmwith increased TLC of 1,24,000 with neutrohils 84% andPCVof 21.6%. After the examination provisional diagnosisof acute pancreatitis was made. Ultrasonography andcomputed tomography was done.Ultrasound showed marked splenomegaly and largeperisplenic and intra splenic haemotoma (Fig. 1)Computed TomographyComputed tomography was performed on a multi detectorduel source CT Scanner by Siemens (128 slice) usingstandard protocols.Computed tomography revealed a large intraperitonealhematoma in the left upper quadrant extending from leftsubphrenic space till lumbar region. Spleen was notseparately visualized. Splenic arlery was not opacified

A transient current based busbar protection scheme using Wavelet Transforms

This paper deals with application of Wavelet Transform for detection of busbar faults and to discriminate them from external faults. The fault indexes of differential current and that of a source CT current are obtained over narrow moving windows based on their respective detail coefficients. The fault indexes of both current signals obtained are compared with their respective threshold values to detect the internal faults. The time shift of differential current detail coefficients compared to those of source current due to saturation of CT is used to discriminate the external faults from internal faults. The scheme is tested successfully for different types of external and internal faults.

Factors influencing delay time and coronary arterial density during coronary angiography with DSCT

CT angiography (CTA) plays an important role in diagnosing coronary arterial disease. Delay time and density of the coronary arteries related with patient-specific factors are essential for getting an optimal CTA image. To investigate various factors influencing delay time and coronary arterial density during coronary CTA with dual source CT. One hundred and sixteen consecutive subjects who underwent cardiac DSCT with retrospective ECG-gating were included. Factors including gender, age, height, weight, transversal cardiac diameter (TCD), transversal thoracic diameter (TTD), heart rate (HR), body surface area (BSA = [weight × height/3600](1/2)) and cardiothoracic ratio (CTR = TCD/TTD) were recorded, measured and calculated before administration of contrast media during coronary CT angiography. Delay time was determined as duration from the beginning of the injection to the density in the descending aorta at the level of right main pulmonary artery reaching a threshold of 100 HU. Coronary arterial density was measured at the mid portion of the right coronary artery. Regression analysis and stepwise regression analysis were used to investigate the influence of these factors on delay time and coronary arterial density. Delay time decreased with an increasing HR and it was shorter in women than men. Delay time increased with an increasing TCD. Delay time could be predicted by the formula: DT = 16.651-0.110 × HR + 1.902 × gender + 0.394 × TCD (where DT is abbreviation for delay time, gender is 0 for women and 1 for men). Coronary arterial density decreased with an increasing HR and weight. Coronary arterial density could be predicted by the formula: CAD = 923.42-4.099 × HR-3.293 × weight (CAD = coronary arterial density). There was no relationship between the other factors mentioned above and delay time or coronary arterial density. Delay time is influenced by HR, gender and TCD. Coronary arterial density also changes with HR and weight. So HR, gender, TCD and weight can be used to adjust flow rate and dosage of contrast media before administration of contrast media during coronary CT angiography.

Turner Syndrome with aberrant right subclavian artery and partial anomalous pulmonary venous return

We report the case of a 6 year old girl with Turner Syndrome and Aberrant Right Subclavian Artery with an incidental finding of PAPVR on contrast-enhanced, high-pitch, 128-slice, electrocardiographic-gated dual source CT. There is value of using high-pitch DSCT in pediatric patients for diagnostic images with minimal radiation exposure.

Exact dual energy material decomposition from inconsistent rays (MDIR)

Dual energy CT (DECT) allows calculating images that show the spatial distribution of the electron density and the atomic number or, more common, images of two basis material densities. In contrast, the Hounsfield unit that is shown in standard CT images is a measure of the x-ray attenuation, which is a function of the atomic number and electron density. To acquire additional information, DECT measures the object of interest using two different detected x-ray spectra. Most clinical CT scanners realize dual energy CT by fast tube voltage switching or by dual source dual detector arrangements and therefore do not allow measuring geometrically identical lines with each spectrum. Then, it is not possible to preprocess the raw data and calculate dual energy-specific raw data sets. The combination of the information of both spectra rather needs to be carried out in image domain after image reconstruction. Compared to the ideal raw data-based dual energy approaches, those image-based DECT methods are inferior because they are not able to correctly deal with the polychromatic nature of the x-rays. This article proposes a dedicated dual energy reconstruction algorithm for inconsistent rays that correctly accounts for all spectral effects. Material decomposition from inconsistent rays (MDIR) is an iterative method to indirectly perform raw data-based DECT even though different lines were measured for both spectra. Its iterative nature allows treating the x-ray polychromaticity correctly. The iterative process is initialized by density images that were obtained from an image-based material decomposition. Those images suffer from errors that originate from the polychromatic nature of the spectra. These errors are calculated by polychromatic forward projection of each measured line. After correction of the initial material density images, the polychromatic forward projection is repeated with more accurate material density images, yielding a more accurate error calculation. To demonstrate the proposed method, simulations and measurements were performed using clinical and preclinical dual source dual energy CT scanners. Two iterations of MDIR are sufficient to greatly improve the qualitative and quantitative information in material density images. It is shown that banding artifacts, cupping artifacts, and mean density errors can be completely eliminated. Simulations with high geometrical inconsistency between the rays of different spectra indicate that nearly exact material decomposition is possible with MDIR. Furthermore, simulations show that the method works well in the presence of materials with K-edges within the detected spectrum. Phantom measurements using a clinical dual source CT scanner show the elimination of artifacts, which cause up to 4% mean density error. At moderate computational burden, the proposed MDIR algorithm yields images of the same high quality as direct raw data-based DECT methods. In contrast to those, MDIR is applicable to the case of inconsistent rays, as it often occurs in clinical or preclinical CT. Compared to image-based methods MDIR reduces artifacts and improves mean density errors in material density images. All dual energy postprocessing methods that are in use today, such as bone removal, virtual noncontrast images, etc., can be applied to the images provided by MDIR.

Application of prospective electrocardiogram-gated dual-source CT in patients with acute chest pain

Objective To evaluate the application of prospective ECG-gated dual source CT (DSCT) in patients with acute chest pain, and compare it's image quality and radiation dose with those of retrospective ECG-gated spiral scan. Methods Thirty consecutive patients (Group A, average HR ≥85 bpm) with acute chest pain were scanned with prospective ECG-gated scan and another 30 consecutive patients (Group B, average HR ≥85 bpm)were analyzed by retrospective ECG-gated scan. Tube voltage and tube current were adapted by the BMI of patients. MPR, MIP, CPR and VR were used to display pulmonary arteries (PA), thoracic aorta and coronary arteries (CA). Image quality as well as radiation dose were assessed in 2 groups. Qualitative image quality was compared with chi-square test between the two groups,while quantitative image quality [the image noise ( IN ), signal-to-noise ratio ( SNR ) and contrast-to-noise ratio(CNR)] and radiation dose were evaluated with x2 test and Student's t test. Results The proportion of valid coronary segments for diagnosis were 379/385 ( 98. 44% ) and 390/396 ( 98.48% ) respectively in Group A and Group B with no significant difference(x2 =0. 002,P =0. 961 ). The IN [( 16. 23 ±5.75)vs ( 16. 31 ±3. 32) HU] ,SNR (26. 85 ±9. 94 vs 24. 78 ±9. 91 ) and CNR (20. 99 ±9. 31 vs 18. 65 ±8. 72)showed no significant differences between 2 groups ( t = 0. 069,0. 908 and 1. 224, P ＞ 0. 05, respectively).The ED was on average ( 8. 37 ± 2. 69 ) mSv in Group A, whereas on average ( 20. 05 ± 5.52 ) mSv in Group B. There was a statistical difference between 2 groups ( t = 9. 401, P = 0. 000). Conclusion Low dose prospective ECG-gated DSCT angiography can show similar image quality as retrospective ECG-gated spiral scan with radiation dose. Key words: Tomography, X-ray computed; Chest pain; Radiation dosage

In vivo CT detection of lipid-rich coronary artery atherosclerotic plaques using quantitative histogram analysis: A head to head comparison with IVUS

BackgroundCoronary atherosclerotic plaque characterisation may contribute to risk stratification for future cardiovascular events. The ability of computed tomography to classify plaques as ‘fibrous’ or ‘lipid-rich’ based on their average CT attenuation has been investigated but is fraught with substantial limitations. In this study, we evaluated the potential of analysing the distribution of CT attenuation values measured in Hounsfield Units (HU) within coronary atherosclerotic plaques to classify non-calcified plaques into fibrous and lipid-rich subtypes. Intravascular ultrasound (IVUS) served as the gold standard. Patients and methodsWe evaluated the data sets of 40 patients (30 males, 59±10 years) who had been referred for invasive coronary angiography for clinical reasons and in whom IVUS was performed in at least one coronary vessel. Using dual source CT, coronary CT angiography was performed as a part of a research protocol within 24h previous to invasive coronary angiography. A contrast-enhanced volume dataset was acquired with retrospective ECG gating (120kV, 400mAs/rot, collimation 2mm×64mm×0.6mm, 60–80ml contrast agent i.v). IVUS was performed using a 40-MHz IVUS catheter (Atlantis, Boston Scientific Corporation, Natick, MA) and motorized pullback at 0.5mm/s. Fifty five corresponding non-calcified plaques within the coronary artery system were identified in both DSCT and IVUS using bifurcation points as fiducial markers. In DSCT data sets, serial parallel cross-sections (1mm slice thickness) were rendered orthogonally to the centre line of the coronary artery for each of the 55 plaques. For each cross section and each plaque, a histogram of CT attenuation values (increments of 10HU) was determined. The percentage of pixels with a density ≤30HU was calculated. Using IVUS as the gold standard, plaques were classified as predominantly fibrous (hyperechoic) or predominantly lipid-rich (hypoechoic). Results15 predominantly fibrous plaques vs. 40 predominantly lipid-rich plaques were identified in IVUS. The mean CT attenuation in both plaque types was significantly different (67±31HU vs. 96±40HU, p=0.006), yet with a wide overlap. For the 15 fibrous plaques identified in IVUS, the mean percentage of pixels ≤30HU in CT was 6±10%. For lipid-rich plaques it was 16±10% (p<0.0001). ROC curve analysis revealed that a cut-off of 5.5% pixels with an attenuation ≤30HU identified lipid rich plaques in CT angiography with a sensitivity of 95% (38/40, 95% CI 83–99) and a specificity of 80% (12/15, 95% CI 52–96) [AUC 0.9, 95% CI 0.7–1.0]. Using this threshold, the negative predictive value was 86% (12/14, 95% CI 57–98) and the positive predictive value was 93% (38/41, 95% CI 80–98). ConclusionLipid-rich coronary atherosclerotic plaques contain a significantly higher percentage of pixels with low CT attenuation as compared to fibrous plaques. Histogram analysis may help to differentiate both plaque types. A cut-off of 5.5% of pixels with an attenuation of ≤30HU allowed identification of lipid-rich plaques with a sensitivity of 95% and a specificity of 80%.

320-row CT scanning: reduction in tube current parallels reduction in radiation exposure?

320-row CT scanning: reduction in tube current parallels reduction in radiation exposure?

Recent advances in medical physics

Some of the major interests in medical physics over the last few years have concerned the technical advances in Computed Tomography and high field Magnetic Resonance Imaging. This review discusses the introduction of Dual Source CT and explains how it can not only offer faster data acquisition but also operate with lower radiation doses. This provides enormous benefits for all patients, but for cardiac and pediatric examinations in particular. The advances in MRI at 7T esla are also impressive, with better signal to noise; cardiac and musculoskeletal applications are discussed; technical improvements are work-in-progress for other applications.

Iterative reconstruction in image space (IRIS) in cardiac computed tomography: initial experience

Improvements in image quality in cardiac computed tomography may be achieved through iterative image reconstruction techniques. We evaluated the ability of "Iterative Reconstruction in Image Space" (IRIS) reconstruction to reduce image noise and improve subjective image quality. 55 consecutive patients undergoing coronary CT angiography to rule out coronary artery stenosis were included. A dual source CT system and standard protocols were used. Images were reconstructed using standard filtered back projection and IRIS. Image noise, attenuation within the coronary arteries, contrast, signal to noise and contrast to noise parameters as well as subjective classification of image quality (using a scale with four categories) were evaluated and compared between the two image reconstruction protocols. Subjective image quality (2.8±0.4 in filtered back projection and 2.8±0.4 in iterative reconstruction) and the number of "evaluable" segments per patient 14.0±1.2 in filtered back projection and 14.1±1.1 in iterative reconstruction) were not significant different between the two methods. However iterative reconstruction had a lower image noise (22.6±4.5HU vs. 28.6±5.1HU) and higher signal to noise and image to noise ratios in the proximal coronary arteries. IRIS reduces image noise and contrast-to-noise ratio in coronary CT angiography, thus providing potential for reducing radiation exposure.

Strategies for scatter correction in dual source CT

Dual source CT (DSCT) systems utilize two measurement systems (A) and (B) offset by about 90 degrees. A special challenge in DSCT is cross-scattered radiation, i.e., scattered radiation from x-ray tube (B) detected in detector (A) and vice versa. Cross-scattered radiation can produce artifacts and degrade the contrast-to-noise ratio (CNR) of the images. Correction algorithms are mandatory to mitigate the negative effects of cross-scattered radiation. The purpose of this work is to describe and evaluate different methods for cross-scatter correction in DSCT. The authors present two techniques for cross-scatter correction in DSCT. The first technique (1) is model-based. Assuming that cross-scatter is predominantly surface scatter, adequate knowledge about the surface of the scattering object is sufficient to describe the magnitude and distribution of cross-scatter. The relevant surface information is derived from an analysis of the raw-data sinogram during the CT-scan. The correction is performed by a table look-up into previously measured and stored cross-scatter distributions for a variety of objects with different surface characteristics. The second technique (2) is measurement-based. Dedicated sensors outside the penumbra of the fan beam in the z direction on both detectors (A) and (B) are used for an online measurement of both cross-scattered and forward scattered radiation during the CT-scan. In addition to the two scatter-correction techniques, the authors describe a low-pass filter method for the scatter-correction term with the goal to improve the CNR of the corrected images. This filter can be applied to both model-based (1) and measurement-based (2) scatter correction. Both scatter-correction techniques (1) and (2) are quantitatively assessed and the performance of the low-pass filter method is evaluated using DSCT data of phantoms (water cylinders and anthropomorphic phantoms) and DSCT patient scan data. Both scatter-correction techniques restore image contrasts and reduce cross-scatter induced artifacts in DSCT images. The measurement-based technique results in higher CNR than the model-based technique if the proposed low-pass filtering of the scatter-correction term is applied. Low-pass filtering improves the CNR of cross-scatter-correction approaches beyond the limits published in the literature [Engel et al., "X-ray scattering in single- and dual-source CT," Med. Phys. 35(1), 318-332 (2008)]. Both model-based and measurement-based scatter correction can mitigate the negative effects of cross-scatter in DSCT. The application of low-pass filtering to the scatter-correction term improves the CNR whenever the ratio of scattered radiation to total signal is high, as in larger patients.

Dose levels at coronary CT angiography—a comparison of Dual Energy-, Dual Source- and 16-slice CT

To compare the dose estimates and image quality of Dual Energy CT (DECT), Dual Source CT (DSCT) and 16-slice CT for coronary CT angiography (cCTA). Sixty-eight patients were examined with 16 - slice MDCT (group 1), 68 patients with DSCT (group 2) and 68 patients using DSCT in dual energy mode (DECT group 3). CT dose index volume, dose length product, effective dose, signal-to-noise, and contrast-to-noise ratio were compared. Subjective image quality was rated by two observers, blinded to technique. The mean estimated radiation dose of all patients investigated on a 16 - slice MDCT was 12 ± 3.59mSv, for DSCT in single energy 9.8 ± 4.77mSv and for DECT 4.54 ± 1.87mSv. Dose for CTA was significantly lower in group 3 compared to group 1 and 2. The image noise was significantly lower in Group 2 in comparison to group 1 and group 3. There was no significant difference in diagnostic image quality comparing DECT and DSCT. cCTA shows better dose levels at both DECT and DSCT compared to 16-slice CT. Further, DECT delivers significantly less dose than regular DSCT or single source single energy cCTA while maintaining diagnostic image quality.

Attenuation-based characterization of coronary atherosclerotic plaque: Comparison of dual source and dual energy CT with single-source CT and histopathology

Objective To compare different CT acquisition techniques regarding for attenuation-based characterization of coronary atherosclerotic plaques using histopathology as the standard of reference. Materials and methods In a post mortem study 17 human hearts were studied with dual-source CT (DSCT) and dual energy CT (DECT) mode on a DSCT as well as with 16-slice single-source CT (SSCT). At autopsy, atherosclerotic lesions were cut at 5 μm sections. Histopathologic classification of the plaques according to the American Heart Association (AHA) criteria was performed by two pathologists. Attenuation values of all plaques were measured in DSCT, DECT and SSCT studies, respectively and classified based on attenuation according to modified AHA criteria. Results 58 coronary plaques were identified at autopsy. Regardless of the CT technique only 52/58 plaques were found at CT (sensitivity = 89.6%). There was no significant difference between the mean attenuation values of different plaque types between DSCT, DECT, and SSCT: type IV: 11 HU/8 HU/19 HU; type Va: 44 HU/45 HU/52 HU; type Vb: 1088 HU/966 HU/1079 HU). The sensitivity for correct classification varied depending on the plaque type (type II = 0%, type III = 0%, type IV = 43%, type Va = 58%, Vb = 97%). Conclusion Independent of the used acquisition technique, SSCT, DSCT and DECT show similar results for attenuation-based characterization of atherosclerotic coronary plaques.

Use of 100 kV versus 120 kV in cardiac dual source computed tomography: effect on radiation dose and image quality

To evaluate the effective radiation dose and image quality resulting from use of 100 vs. 120 kV among patients referred for cardiac dual source CT exam (DSCT). Prospective data was collected on 294 consecutive patients referred for DSCT. For each scan, a physician specializing in cardiac CT chose all parameters including tube current and voltage, axial versus helical acquisition, and use of tube current modulation. Lower tube voltage was selected for thinner patients or when lower radiation was desired for younger patients, particularly females. For each study, image quality (IQ) was rated on a subjective IQ score and contrast (CNR) and signal-to-noise (SNR) ratios were calculated. Tube voltage of 100 kV was used for 77 (26%) exams while 120 kV was used for 217 (74%) exams. Use of 100 kV was more common in thinner patients (weight 166 lbs vs. 199 lbs, P < .001). The effective radiation dose for the 100 and 120 kV scans was 8.5 and 15.4 mSv respectively. Among scans utilizing 100 and 120 kV, there was no difference in exam indication, use of beta blockers, heart rate, scan length and use of radiation saving techniques such as prospective ECG triggering and tube current modulation. The IQ score was significantly higher for 100 kV scans. While 100 kV scans were found to have higher image noise then those utilizing 120 kV, the contrast-to-noise and signal-to-noise were significantly higher (SNR: 9.4 vs. 8.3, P = .02; CNR: 6.9 vs. 6.0, P = .02). In selected non-obese patients, use of low kV results in a substantial reduction of radiation dose and may result in improved image quality. These results suggest that low kV should be used more frequently in non-obese patients.