CT number variations due to different image acquisition and reconstruction parameters: a thorax phantom study.

A humanoid thorax phantom containing six compartments was scanned with two different computed tomography (CT) scanners using various image acquisition and reconstruction parameters. The differences of CT numbers were statistically significant between the two CT scanners for each compartment (p<0.001) except for the "air" compartment. The variabilities of the CT numbers are described for the different parameters. The mean CT numbers of the "water" compartment, for instance, ranged from 1 to 15HU (Hounsfield Units), those of the "air" compartment varied from -962 to -990HU. Knowledge of these CT number variabilities is necessary when CT numbers are used for tissue characterization.     

Dose Calculation on KV Cone Beam CT Images: An Investigation of the Hu-Density Conversion Stability and Dose Accuracy Using the Site-Specific Calibration

Precise calibration of Hounsfield units (HU) to electron density (HU-density) is essential to dose calculation. On-board kV cone beam computed tomography (CBCT) imaging is used predominantly for patients' positioning, but will potentially be used for dose calculation. The impacts of varying 3 imaging parameters (mAs, source-imager distance [SID], and cone angle) and phantom size on the HU number accuracy and HU-density calibrations for CBCT imaging were studied. We proposed a site-specific calibration method to achieve higher accuracy in CBCT image-based dose calculation. Three configurations of the Computerized Imaging Reference Systems (CIRS) water equivalent electron density phantom were used to simulate sites including head, lungs, and lower body (abdomen/pelvis). The planning computed tomography (CT) scan was used as the baseline for comparisons. CBCT scans of these phantom configurations were performed using Varian Trilogy? system in a precalibrated mode with fixed tube voltage (125 kVp), but varied mAs, SID, and cone angle. An HU-density curve was generated and evaluated for each set of scan parameters. Three HU-density tables generated using different phantom configurations with the same imaging parameter settings were selected for dose calculation on CBCT images for an accuracy comparison. Changing mAs or SID had small impact on HU numbers. For adipose tissue, the HU discrepancy from the baseline was 20 HU in a small phantom, but 5 times lager in a large phantom. Yet, reducing the cone angle significantly decreases the HU discrepancy. The HU-density table was also affected accordingly. By performing dose comparison between CT and CBCT image-based plans, results showed that using the site-specific HU-density tables to calibrate CBCT images of different sites improves the dose accuracy to ～2%. Our phantom study showed that CBCT imaging can be a feasible option for dose computation in adaptive radiotherapy approach if the site-specific calibration is applied.     

Renal arterial blood flow measurement by breath‐held MRI: Accuracy in phantom scans and reproducibility in healthy subjects

This study evaluates reliability of current technology for measurement of renal arterial blood flow by breath‐held velocity‐encoded MRI. Overall accuracy was determined by comparing MRI measurements with known flow in controlled‐flow‐loop phantom studies. Measurements using prospective and retrospective gating methods were compared in phantom studies with pulsatile flow, not revealing significant differences. Phantom study results showed good accuracy, with deviations from true flow consistently below 13% for vessel diameters 3mm and above. Reproducibility in human subjects was evaluated by repeated studies in six healthy control subjects, comparing immediate repetition of the scan, repetition of the scan plane scouting, and week‐to‐week variation in repeated studies. The standard deviation in the 4‐week protocol of repeated in vivo measurements of single‐kidney renal flow in normal subjects was 59.7 mL/min, corresponding with an average coefficient of variation of 10.55%. Comparison of renal arterial blood flow reproducibility with and without gadolinium contrast showed no significant differences in mean or standard deviation. A breakdown among error components showed corresponding marginal standard deviations (coefficients of variation) 23.8 mL/min (4.21%) for immediate repetition of the breath‐held flow scan, 39.13 mL/min (6.90%) for repeated plane scouting, and 40.76 mL/min (7.20%) for weekly fluctuations in renal blood flow. Magn Reson Med 63:940–950, 2010. © 2010 Wiley‐Liss, Inc.     

Computed tomography densitometry of the lung: a method to assess perfusion defects in acute pulmonary embolism

OBJECTIVE: To evaluate the potential of spiral computed tomography (CT) densitometry of the lung to assess segmental perfusion defects in patients with acute pulmonary embolism. MATERIALS AND METHODS: Ten patients with known segmental or lobar perfusion defects on ventilation/perfusion scintigraphy and with normal findings in the contralateral lung segment underwent spiral CT of the thorax before and after the administration of contrast material. Regions of interest were defined in 14 segments with normal perfusion and in 14 segments with reduced perfusion. Three consecutive densitometry measurements were performed in each segment. RESULTS: Those segments with reduced perfusion showed a significantly lower mean CT value on the enhanced scans (-813.4 +/- 57.1 Hounsfield units (HU) vs -794.0 +/- 44.8 HU, P = 0.01) and a significantly decreased contrast enhancement (12.3 +/- 18.2 HU vs 29.8 +/- 16.6 HU, P <0.01) when compared to segments with normal perfusion. Measurements from the unenhanced CT scans were not statistically different between segments with reduced and normal perfusion. CONCLUSIONS: Spiral CT densitometry allows the assessment of at least segmental perfusion defects in patients with acute pulmonary embolism.     

A cardiac phantom and pulsatile flow pump for magnetic resonance imaging studies

Fast scan magnetic resonance imaging (MRI) acquisitions are a rapid noninvasive means of evaluating the cardiovascular system. Because the appearance of flowing blood is highly variable, the interpretation of these images is sometimes difficult. A nonferromagnetic phantom that could generate lifelike pulsatile flow and also simulate the motions of the beating heart would facilitate image interpretation. This paper describes an MRI-compatible cardiovascular phantom that mimics the motions of the heart and also creates physiologic pulsatile flow. The phantom consists of a ventricle and an air pump that drives it. The pump is connected to the ventricle with seven meters of air hose so that the pump (which has ferromagnetic parts) can be placed outside the magnet room. The ventricle is placed in an airtight Plexiglas cylinder and the pump alternately pressurizes and depressurizes the cylinder, driving fluid in and out of the ventricle. The motions of the ventricular wall simulate the motions of the heart, and the pulsatile flow generated is of physiologic velocities and volumes. This phantom also can be used with other methods of evaluating cardiovascular function, such as MUGAS, angiography, and Doppler, allowing correlation between MRI and other modalities. Finally, the phantom can be used to study almost any aspect of cardiovascular function from pulsatile flow velocity to ventricular studies (ejection fractions, cardiac output, wall motion) and even studies of stenotic or regurgitant valves.     

Visualization of moving fluid: quantitative analysis of blood flow velocity using MR imaging

A new method for the measurement of blood flow using magnetic resonance imaging has been developed. The flow velocities are calculated from the distances that the fluid has moved. The distances are directly visualized by a new pulse sequence. In a phantom study, the measured flow rates showed very good correlation with actual flow rates of up to 20 l/min (3 m/sec). In a volunteer study, pulsatile flow velocities of a large artery were measured with electrocardiographic gating. The flow pattern of a cardiac cycle at the abdominal aorta is similar to that revealed by other methods of measurement, such as Doppler ultrasound. This method allows reasonably accurate quantitative analysis of blood flow in the large arteries.     

Quantitative perfusion imaging with pulsed arterial spin labeling: a phantom study.

PURPOSE: Pulsed arterial spin labeling (PASL) is a magnetic resonance (MR) method for measuring cerebral blood flow. Although several validation studies for PASL in animals and humans have been reported, no reports have detailed the fundamental study of PASL using a flow phantom. We compared the true and theoretical flow rates in a flow phantom to confirm the analytical validity of quantitative perfusion imaging with Q2TIPS sequence. METHODS: We built a flow phantom consisting of a 40-mm diameter plastic syringe filled with plastic beads and small plastic tubes 4 mm in diameter. Gd-DTPA-doped 8L water solution (0.1 mM) was circulated between the syringe and a tank through a plastic tube by a constant flow pump while the flow rate was adjusted between 0 and 2.61 cm/s. Q2TIPS sequence parameters were TI(1)=50 ms and TI(2)=1400 ms. Five imaging slices of 50 subtraction images were acquired sequentially in a distal-to-proximal direction using a single-shot echo planar imaging (EPI) technique. The theoretical flow rate calculated based upon the previously reported kinetic model for Q2TIPS was compared with the true flow rate. RESULTS: A good linear relationship was observed between the theoretical, F', and true flow rates, F, in a flow rate range of 1.43 to 1.95 cm/s (F'=1.024*F-1.915, R(2)=0.902). The ratio of theoretical to true flow rate was 92 (+/-) 4%. CONCLUSION: Flow rate was quantified with reasonable accuracy when the entire amount of labeled bolus within the phantom could be recovered. Our experiment confirmed the analytical validity of Q2TIPS and suggested that blood flow measurement may be feasible using the Q2TIPS pulse sequence and kinetic model of the PASL equation.     

Pulsatile flow index for qualitative measurements of blood flow with duplex ultrasound. An experimental study

We determined the accuracy of Doppler blood flow measurements in an experimental investigation using a tissue-simulating phantom, pulsatile flow pumps and heparinized blood. A new index for qualitative assessment of blood flow, the pulsed flow index (PFI) is described. The PFI takes advantage of the area under the flow velocity curve between the true zero line and the diastolic baseline. Under conditions of continuous flow, the PFI ranged from 0.82 to 0.94 (mean value 0.90). The PFI was found to be relatively independent of the transducer/vessel angle (+/- 8%) and the inter/intra-operator variation was small (+/- 7.5%, or +/- 7%, respectively).     

Ability of calibration phantom to reduce the interscan variability in electron beam computed tomography

OBJECTIVE: To test the hypothesis that a calibration phantom would improve interpatient and interscan variability in coronary artery calcium (CAC) studies. METHODS: We scanned 144 patients twice with or without the calibration phantom and then scanned 93 patients with a single calcific lesion twice and, finally, scanned a cork heart with calcific foci. RESULTS: There were no linear correlations in computed tomography Hounsfield unit (CT HU) and CT HU interscan variation between blood pool and phantom plugs at any slice level in patient groups (p > 0.05). The CT HU interscan variation in phantom plugs (2.11 HU) was less than that of the blood pool (3.47 HU; p < 0.05) and CAC lesion (20.39; p < 0.001). Comparing images with and without a calibration phantom, there was a significant decrease in CT HU as well as an increase in noise and peak values in patient studies and the cork phantom study. CONCLUSION: The CT HU attenuation variations of the interpatient and interscan blood pool, calibration phantom plug, and cork coronary arteries were not parallel. Therefore, the ability to adjust the CT HU variation of calcific lesions by a calibration phantom is problematic and may worsen the problem.     

Faster flow quantification using sensitivity encoding for velocity-encoded cine magnetic resonance imaging: in vitro and in vivo validation

PURPOSE: To test the agreement between conventional and sensitivity-encoded (SENSE) velocity encoded cine (VEC) MRI in a flow phantom and in subjects with congenital and acquired heart disease. MATERIALS AND METHODS: Flow measurements were performed in a 1.5 T scanner using a segmented k-space VEC MRI sequence and then repeated with a SENSE factor of 2. The flow phantom used a piston pump to generate physiologic arterial waveforms (0.5-4.9 L/min). In the subjects, flow measurements were performed in the ascending aorta (N = 33) and/or the main pulmonary artery (N = 24). RESULTS: Utilization of SENSE reduced the scan time by 50%. In the phantom, measurements without and with SENSE agreed closely with a mean difference of 0.01 +/- 0.08 L/min or 0.12% +/- 3.8% (P = 0.68). In the subjects, measurements without and with SENSE also agreed closely with a mean difference of 0.08 +/- 0.36 L/min or 1.3% +/- 7.2% (P = 0.08). Compared with standard imaging, the use of SENSE reduced the signal-to-noise ratio (SNR) by 28% in the phantom (N = 10) and 27% in vivo (N = 22). CONCLUSION: VEC MRI flow measurements with a SENSE factor of 2 were twice as fast and agreed closely with the conventional technique in vitro and in vivo. VEC MRI with SENSE can be used for rapid and reliable quantification of blood flow.     

Measurement of human myocardial perfusion by double-gated flow alternating inversion recovery EPI.

This paper presents a flow-sensitive alternating inversion recovery (FAIR) method for measuring human myocardial perfusion at 1.5 T. Slice-selective/non-selective IR images were collected using a double-gated IR echoplanar imaging sequence. Myocardial perfusion was calculated after T1 fitting and extrapolation of the mean signal difference SI(Sel - SI(NSel). The accuracy of the method was tested in a porcine model using graded intravenous adenosine dose challenge. Comparison with radiolabeled microsphere measurements showed a good correlation (r = 0.84; mean error = 20%, n = 6) over the range of flows tested (0.9-7 ml/g/min). Applied in humans, this method allowed for the measurement of resting myocardial flow (1.04+/-0.37 ml/g/min, n = 11). The noise in our human measurements (SE(flow) = 0.2 ml/g/min) appears to come primarily from residual respiratory motion. Although the current signal-to-noise ratio limits our ability to measure small fluctuations in resting flow accurately, the results indicate that this noninvasive method has great promise for the quantitative assessment of myocardial flow reserve in humans.     

Magnetic resonance velocity imaging using a fast spiral phase contrast sequence

Time-resolved velocity imaging using the magnetic resonance phase contrast technique can provide clinically important quantitative flow measurements in vivo but suffers from long scan times when based on conventional spin-warp sequences. This can be particularly problematic when imaging regions of the abdomen and thorax because of respiratory motion. We present a rapid phase contrast sequence based on an interleaved spiral k-space data acquisition that permits time-resolved, three-direction velocity imaging within a breath-hold. Results of steady and pulsatile flow phantom experiments are presented, which indicate excellent agreement between our technique and through plane flow measurements made with an in-line ultrasound probe. Also shown are results of normal volunteer studies of the carotids, renal arteries, and heart.     

Phantom testing of peripheral artery. Absolute blood flow measurement with digital arteriography

A technique to measure absolute arterial blood flow in ml/min has been developed utilizing software programs with high-speed digital recording for the off-line analysis of intra-arterial injections of contrast medium. Measurements of pulsatile flow in a phantom for a physical flow model showed that calculations with the final upgrades were within 10% of known flow, using a recording rate of 30 frames/s, a diameter tubing of 5 mm, and flow rates of 300 to 400 ml/min. Quantitative absolute flow of a peripheral artery such as the internal carotid artery may be obtained during routine cerebral arteriography for comparison with anatomic data.     

Prediction of anemia on unenhanced computed tomography of the thorax.

OBJECTIVE: To determine if anemia can be predicted on unenhanced computed tomography (CT) of the thorax. METHODS: Hemoglobin and hematocrit levels were obtained within 24 hours of the unenhanced CT scan of the thorax of 200 patients. Anemia was defined as a hemoglobin level less than 140 g/L for men and less than 120 g/L for women. Regions of interest were placed on the left ventricular cavity, aorta and the interventricular septum if visualized. The attenuation of the interventricular septum and left ventricular cavity were correlated with the presence or absence of anemia. RESULTS: When the interventricular septum was not visualized, for every 1 Hounsfield unit (HU) increase in left ventricular attenuation, hemoglobin increased by 0.435 g/L (SE = 0.253, p < 0.001). Failure to visualize the interventricular septum did not exclude the presence of anemia in either sex. When the interventricular septum was visualized, 100% of males and 89% of females met the criteria for the diagnosis of anemia. The prediction of anemia by visualization of the interventricular septum alone yielded a sensitivity of 75.4% and a specificity of 90.3%, with 80% of patients correctly predicted. The multiple regression analysis model yielded a sensitivity of 94.2% and a specificity of 67.7%, with 86% of patients correctly predicted. CONCLUSION: The diagnosis of anemia should be suggested whenever the interventricular septum is visualized on unenhanced CT.     

正常前列腺MSCT灌注成像的临床研究

目的：探讨正常前列腺CT灌注成像的表现。方法：33例正常前列腺经肘静脉注射对比剂后,对选定的前列腺组织层面行MSCT定层连续扫描40次,将得到的160帧图像输入Functional CT软件内,根据同层参考动脉和前列腺组织的时间-密度曲线和各组织强化值计算各层面内每一像素的灌注指标,包括灌注值、TTP、PEI、BV。结果：正常前列腺组织中央带的灌注值、强化峰值（PEI）、达峰时间（TTP）及血容量（BV）分别为（20.93±6.57）ml/（min·mg）、（24.68±5.04）HU、（26.23±2.97）s、（345.62±93.38）ml/g;外周带的分别为（15.65±4.99）ml/（min·mg）、（18.17±4.01）HU、（25.43±2.64）s、（233.61±66.67）ml/g。中央带与外周带的TDC曲线为平坦型,未见明显上升支。双侧外周带TDC曲线对称,基本重合。结论：正常前列腺组织属于低血供器官,中央带的血流灌注值高于外周带,双侧外周带的血流灌注基本对称。     

Evaluation of variations in absorbed dose and image noise according to patient forms in X-ray computed tomography

Excessive radiation exposure in pediatric computed tomography (CT) scanning has become a serious problem, and it is difficult to select scan parameters for the scanning of small patients such as children. We investigated differences in absorbed dose and standard deviation (SD) in Hounsfield unit (HU) caused by differences in the form of the subject using a body-type phantom with removable body parts. Using four X-ray CT scanners, measurements were made with values from 50 mAs to 300 mAs, with slices of 50 mAs, using scan protocols that were assumed to perform thorough examinations. The results showed that the mAs values and absorbed doses were almost proportional, and the absorbed doses in the phantom without body parts were about 1.1-2.2-fold higher than those of the phantom with body parts at the same points. The SD values obtained indicated that the absorbed doses in the phantom with body parts were 0.3-0.6 times those of the phantom without body parts when the mAs values used were adjusted so that both SD values were the same. The absorbed doses in various patient forms can be estimated from these results, and they will become critical data for the selection of appropriate scan protocols.     

Four-dimensional image processing of myocardial CT perfusion for improved image quality and noise reduction

BackgroundImage noise and multiple sources of artifact may affect the accurate interpretation of myocardial CT perfusion (CTP) studies. Although artifact within the image is often time dependent, tissue characteristics remain unchanged irrespective of cardiac phase.ObjectiveWe assessed a new technique of 4-dimensional, spatiotemporal analysis, using redundant time domain information within additional phase acquisitions to reduce CTP image noise.MethodsFour-dimensional analysis was assessed in a static phantom and in 10 CTP studies with invasive fractional flow reserve (FFR) correlation. For each voxel within the CTP study the distribution of local Hounsfield values was measured in both time and space with the use of a customized program within MATLAB software. These values were filtered to eliminate those likely to represent noise or rapidly changing beam hardening artifact. All CTP images were acquired within a single heartbeat with 320 detector-row CT. Image noise was quantified as the SD of voxel values within myocardial segments. Contrast was measured between normal and abnormal vascular territories as assessed by FFR.ResultsThe mean image noise within the unprocessed CTP images was 30 HU (range, 23–42 HU). After 4-dimensional filtering the mean image noise was 22 HU (range, 15–29 HU). The mean reduction in image noise was 28% (P < 0.001). The mean contrast between normally perfused and ischemic segments was not significantly changed. The mean increase in contrast-to-noise ratio between ischemic territories and the myocardial average was 52% (P < 0.001).ConclusionFour-dimensional analysis of CTP significantly reduces image noise and may assist in the assessment of myocardial perfusion studies.     

Coronary calcium scoring using 16-row multislice computed tomography: nonenhanced versus contrast-enhanced studies in vitro and in vivo

OBJECTIVES: We sought to assess the agreement of coronary artery calcium score in nonenhanced and contrast-enhanced multislice-spiral computed tomography. MATERIALS AND METHODS: Vessel phantoms and 36 patients underwent nonenhanced and contrast-enhanced cardiac multislice-spiral computed tomography (Sensation 16; Siemens, Germany). Reconstruction-parameters: slice thickness 3 mm, increment 2 mm, kernels B35f and B30f. The Agatston score, calcium mass, and number of lesions were calculated. Images were scored using detection thresholds of 130 Hounsfield units (HU) and 350 HU. Based on the Agatston score, risk stratification was performed. RESULTS: In the phantom and patient study, altering the threshold from 130 to 350 HU led to a significant decrease in the mean Agatston score (phantom: 54.6%, patients: 66.7%) and calcium mass (33.0%, 47.0%) (B35f). Contrast-enhanced studies (threshold: 350 HU) showed an increase of the mean Agatston score (71.0%, 20.7%) and calcium mass (81.0%, 16.0%) when compared with nonenhanced scans (threshold: 350 HU). A total of 57% of all patients were assigned to different risk groups. CONCLUSIONS: Contrast material may simulate calcification; therefore, calculation of the coronary calcium score from contrast-enhanced images is not reliable.     

Magnetic resonance phase velocity mapping through NiTi stents in a flow phantom model

PURPOSE: To assess constant and pulsatile flow velocity within the lumen of a peripheral NiTi stent using phase velocity mapping for comparison with independent assessments of flow velocity in a phantom model. MATERIALS AND METHODS: A 9 x 20-mm stent installed in flexible tubing was placed in a phantom filled with stationary fluid. Constant and pulsatile flow (produced by a pump programmed to produce a simulation of the carotid artery flow) was assessed using phase velocity mapping at 4.1 T (for constant flow) and at 1.5 T (for pulsatile flow). In all cases 256 x 256 gradient echo phase velocity maps were acquired. For the pulsatile flow condition, cine images with acquisition gated to the pump cycle were acquired with 40 msec temporal resolution across the simulated cardiac cycle. Computed flow volume rates were compared with fluid volume collection for the constant flow model, and with ultrasonic Doppler flow meter measurements for the pulsatile model. RESULTS: The data showed that volume flow rate assessments by phase velocity mapping agreed with independent measurements within 10% to 15%. CONCLUSION: Phase velocity mapping of the lumen of peripheral size NiTi stents is possible in an in vitro model.     

High-concentration contrast media in multiphasic abdominal multidetector-row computed tomography: effect of increased iodine flow rate on parenchymal and vascular enhancement

OBJECTIVE: The purpose of this study was to assess the influence of the iodine flow rate on parenchymal and vascular enhancement during multiphasic abdominal multidetector-row computed tomography (MDCT). METHODS: Fifteen patients underwent MDCT at an iodine flow rate of 1.2 g/s as well as 1.6 g/s (group A, protocols 1 and 2), and 90 patients underwent MDCT at an iodine flow rate of 1.2 g/s (group B) or 1.6 g/s (group C). Measurements were performed for all groups in the liver, spleen, pancreas, portal vein, inferior vena cava, and abdominal aorta. RESULTS: Aortal and pancreatic enhancement during the arterial phase was significantly higher with the higher iodine flow rate. The mean difference in aortal enhancement was 60 Hounsfield units (HU) between protocols 1 and 2 of group A, and the mean difference was 70 HU between groups B and C. The mean difference in pancreatic enhancement was 10 HU between protocols 1 and 2 of group A and 17 HU between groups B and C. During the portal and hepatic venous phases, no significant difference in enhancement was observed. CONCLUSION: A high iodine flow rate in multiphasic abdominal MDCT improves enhancement of the aorta and the pancreas during the arterial phase but does not influence later phases.