CT bone density analysis of low-impact proximal femur fractures using Hounsfield units

AimTo quantify and compare changes in bone mineral density (BMD) via CT analysis in patients with and without spontaneous femoral fractures.Materials and methodsConsecutive series of patients with CT imaging for spontaneous femoral fractures were compared to the age and gender matched controls. Bone density fixed region of interest measurements were obtained at the site of the fracture, proximally at the femoral head, and distally at the lesser trochanter in fracture patients and controls. Inter- and intrapatient comparisons were performed, including Chi-square and t-test analyses.Results24 spontaneous fractures and 25 controls were analyzed with no significant differences in mean age, gender, or body mass index. There were differences in the bone density between the fracture and contralateral non-fracture sides at (p = 0.0001) and distal (p < 0.0001) to the fracture. Proximal and distal bone density differences existed between case fracture and control non-fracture sites (p < 0.0001, p = 0.0001), and between the case non-fracture and control non-fracture sites (p < 0.0001, p < 0.0001). The reliability for measurements was good to excellent proximally (ICC = 0.63–0.87), moderate to excellent at the fracture site (ICC = 0.43–0.78), and fair to good distal (ICC = 0.24–0.68) to the fracture site.ConclusionPatients with spontaneous femoral fractures exhibit lower bone density than the asymptomatic controls. Bone insufficiency is best demonstrated proximal or distal to, rather than at the fracture site.     

A technique for using CT images in attenuation correction and quantification in SPECT

A technique is described for using computed tomography (CT) images for attenuation correction and quantification in SPECT. The CT images are aligned with the corresponding SPECT slices and the Hounsfield units are converted to linear attenuation coefficient values for the SPECT radionuclide. The attenuation coefficient map thus produced is used to provide the attenuation correction required in the SPECT reconstruction. The technique has been evaluated in both a non-anatomical and an anatomical phantom giving a mean accuracy in quantifying activity of various features in the phantoms of 2.6% (range 0.3%-4.0%). The value of performing scatter correction prior to attenuation correction in obtaining accurate quantification is demonstrated. The practicalities of applying the technique in patient studies are discussed.     

Deriving Hounsfield units using grey levels in cone beam CT: a clinical application

Objective

To present a clinical study demonstrating a method to derive Hounsfield units from grey levels in cone beam CT (CBCT).

Methods

An acrylic intraoral reference object with aluminium, outer bone equivalent material (cortical bone), inner bone equivalent material (trabecular bone), polymethlymethacrylate and water equivalent material was used. Patients were asked if they would be willing to have an acrylic bite plate with the reference object placed in their mouth during a routine CBCT scan. There were 31 scans taken on the Asahi Alphard 3030 (Belmont Takara, Kyoto, Japan) and 30 scans taken on the Planmeca ProMax 3D (Planmeca, Helsinki, Finland) CBCT. Linear regression between the grey levels of the reference materials and their linear attenuation coefficients was performed for various photon energies. The energy with the highest regression coefficient was chosen as the effective energy. The attenuation coefficients for the five materials at the effective energy were scaled as Hounsfield units using the standard Hounsfield units equation and compared to those derived from the measured grey levels of the materials using the regression equation.

Results

In general, there was a satisfactory linear relation between the grey levels and the attenuation coefficients. This made it possible to calculate Hounsfield units from the measured grey levels. Uncertainty in determining effective energies resulted in unrealistic effective energies and significant variability of calculated CT numbers. Linear regression from grey levels directly to Hounsfield units at specified energies resulted in greater consistency.

Conclusions

The clinical application of a method for deriving Hounsfield units from grey levels in CBCT was demonstrated.     

SPECT/CT physical principles and attenuation correction

Using nuclear medicine techniques, physiologic activity and processes can be identified in a way that is unique from other modalities. Oftentimes it is helpful to know the exact location of the physiologic uptake that is visualized on a scan. Knowing the exact location can sometimes help to distinguish normal from abnormal physiologic uptake. When an abnormality has been identified, knowing the exact location can then be helpful in treatment planning. The ability to provide precise localization of physiologic data from nuclear medicine studies is now possible with hybrid SPECT/CT systems. Additionally, these systems provide an accurate attenuation correction of the nuclear medicine image data. After reading this article, the technologist will be able to list and describe the inherent problems associated with SPECT image acquisition and reconstruction, briefly explain how data acquired from the CT scanner are used to provide attenuation correction data for SPECT and anatomic information for diagnostic purposes, list and briefly describe the different types of clinical SPECT/CT systems, and discuss the importance of accurate CT and SPECT image registration.     

PET attenuation coefficients from CT images: experimental evaluation of the transformation of CT into PET 511-keV attenuation coefficients

The CT data acquired in combined PET/CT studies provide a fast and essentially noiseless source for the correction of photon attenuation in PET emission data. To this end, the CT values relating to attenuation of photons in the range of 40-140 keV must be transformed into linear attenuation coefficients at the PET energy of 511 keV. As attenuation depends on photon energy and the absorbing material, an accurate theoretical relation cannot be devised. The transformation implemented in the Discovery LS PET/CT scanner (GE Medical Systems, Milwaukee, Wis.) uses a bilinear function based on the attenuation of water and cortical bone at the CT and PET energies. The purpose of this study was to compare this transformation with experimental CT values and corresponding PET attenuation coefficients. In 14 patients, quantitative PET attenuation maps were calculated from germanium-68 transmission scans, and resolution-matched CT images were generated. A total of 114 volumes of interest were defined and the average PET attenuation coefficients and CT values measured. From the CT values the predicted PET attenuation coefficients were calculated using the bilinear transformation. When the transformation was based on the narrow-beam attenuation coefficient of water at 511 keV (0.096 cm(-1)), the predicted attenuation coefficients were higher in soft tissue than the measured values. This bias was reduced by replacing 0.096 cm(-1) in the transformation by the linear attenuation coefficient of 0.093 cm(-1) obtained from germanium-68 transmission scans. An analysis of the corrected emission activities shows that the resulting transformation is essentially equivalent to the transmission-based attenuation correction for human tissue. For non-human material, however, it may assign inaccurate attenuation coefficients which will also affect the correction in neighbouring tissue.     

Respiratory average CT for attenuation correction in myocardial perfusion SPECT/CT

Objective

Cine average CT (CACT) and interpolated average CT (IACT) have been proposed to improve attenuation correction (AC) for PET/CT in oncologic and cardiac studies. This study aims to evaluate their effectiveness on myocardial perfusion SPECT/CT using computer simulation and physical phantom experiments.

Methods

We first simulated normal male with ^99mTc-sestamibi distribution using digital XCAT phantom with respiratory motion amplitudes of 2, 3, and 4 cm. Average activity and attenuation maps represented static SPECT and CACT, while the attenuation maps of end-inspiration and end-expiration represented two helical CTs (HCTs), respectively. Sixty noise-free and noisy projections were simulated over 180° using an analytical parallel-hole projector. We then filled 673 MBq ^99mTc into an anthropomorphic torso phantom with normal heart or heart with a defect which placed on a programmable respiratory platform to model various respiratory amplitudes. Sixty projections were acquired over 180° using a clinical SPECT/CT scanner. The CACT, standard HCT, and 2 HCTs at extreme phases were acquired. Interpolated CT phases were generated between them using affine plus b-spline registration, and IACT was obtained by averaging the interpolated phases and the 2 original extreme phases for both simulation and phantom experiments. Projections were reconstructed with AC using CACT, IACT, and HCTs, respectively. Polar and 17-segment plots were analyzed by relative difference (RD) of the uptake. Two regions-of-interest (ROI) were drawn on the defect and background area to obtain the intensity ratio (IR).

Results

No substantial difference was observed on the polar plots generated from different AC methods, while the quantitative RD measurements showed that SPECT_CACT were most similar to the original phantom, followed by SPECT_IACT, with RD_max <8 and <10% in the simulation study. The RD of SPECT_HCTs deviated from the original phantom and SPECT_CACT in various segments, with RD_max of 19.76 and 16.68% in the simulation and phantom experiment, respectively. The IR of SPECT_HCTs fluctuated more from the truth for higher motion amplitude.

Conclusions

Both CACT-AC and IACT-AC reduced respiratory artifacts and improved quantitation in myocardial perfusion SPECT as compared to HCT-AC. The use of IACT further reduced the radiation dose.

     

SPECT／CT图像配准不良对心肌灌注显像CT衰减校正的影响

目的探讨CT与SPECT图像配准不良对MPICT衰减校正（CTAC）的影响。方法99Tcm-MIBIMPI受检者19名,均为行健康体格检查者,其中男11名,女8名,年龄（65．3±9．6）岁。对MPI图像进行CTAC。利用仪器自带的软件对CT图像进行模拟位移：相对心脏位置进行上、下、左、右、前、后6个方向的移动,移动幅度分别为0．5、1．0、1．5、2．0、2．5、3．0、3．5、4．0和4．5em。重建不同位移状态的CTAC心肌断层图像,利用靶心图获得左心室各壁段的放射性计数百分比,比较位移前后左心室各壁段放射性计数的差异和图像差异。采用SPSS13．0软件对数据进行配对t检验。结果当CT图像的移动距离为0．5cm时,所有位移方向的CTAC图像均未见明显可识别的图像伪影。当CT图像的移动距离≥1．0CITI时,左心室各壁段出现不同程度的图像伪影以及放射性计数的改变;CT图像向上、下、左、右、前、后方向位移时,分别对心尖部,前壁和心尖部,间壁,前壁、心尖部和侧壁,侧壁和下后壁,前壁、心尖部和间壁放射性计数的影响最为显著。向下位移时左心室各壁段放射性计数的改变大于向上位移[（-9．68±8．06）％和（-2．04±1．83）％;f=6．573,P〈0．01],向右位移的改变大于向左位移[（-9．02±8．47）％和（-4．38±3．67）％;t=1．987,P〈0．05]。在左心室各壁段中,前壁、心尖部和侧壁的伪影程度明显较下后壁和间壁显著。结论CT与SPECT图像配准不良可使MPICTAC图像出现不同程度的伪影,其伪影出现的部位和严重程度与配准不良的方向和幅度密切相关。     

Peritoneal fluid of low CT Hounsfield units as a screening criterion for traumatic bowel perforation

Purpose

To investigate whether peritoneal fluid of low CT Hounsfield units is an important screening criterion for traumatic bowel perforation.

Materials and methods

We performed a retrospective study on two cohorts of blunt trauma patients who had peritoneal fluid. Intravenous and oral contrast was used for the first cohort (61 patients) as opposed to intravenous contrast only for the second cohort (60 patients). We compared the CT Hounsfield units of peritoneal fluid with bowel perforation. The optimal cutoff value of CT Hounsfield units was determined, and its diagnostic values for bowel perforation were calculated.

Results

The mean CT Hounsfield units (HU) of peritoneal fluid with bowel perforation were significantly lower (30.3 ± 9.0 versus 44.1 ± 13.6 HU, p = 0.008) in the second cohort. The optimal cutoff value was 43 HU, and its sensitivity, specificity, accuracy and positive likelihood ratio were 100.0, 69.2, 73.3% and 3.3, respectively, for bowel perforation. Comparisons of CT HUs of peritoneal fluid with bowel perforation in the first cohort that used additional oral contrast for CT did not show statistically significant differences.

Conclusion

Peritoneal fluid of low CT HU is a sensitive and important CT screening criterion for traumatic bowel perforation.

     

Prevalence of misregistration between SPECT and CT for attenuation-corrected myocardial perfusion SPECT

Background  Nonuniform attenuation artifacts may reduce the diagnostic accuracy of cardiac single photon emission computed tomography (SPECT) studies. Compensation strategies using an attenuation map (eg, from x-ray tomography) have been reported to improve accuracy. Because the computed tomography (CT) and SPECT images are obtained sequentially, misregistration of the emission and transmission scans can occur. Our objective was to qualitatively assess these misregistration errors. Methods and Results  This study included 60 patients who consecutively underwent CT attenuation-corrected myocardial perfusion studies acquired on a SPECT/CT system equipped with a nondiagnostic CT scanner. The cardiac SPECT/CT and fused images were reviewed and qualitatively assessed for misregistration of the heart between the CT and emission image data sets. The degree of misregistration was qualitatively rated on a 5-point scale. Misregistration was judged to be none in 4 of 55 patients, minimal in 9, mild in 19, moderate in 21, and severe in 2 patients. Five studies could not be assessed because of severe artifacts on CT. Conclusions  Forty-two percent of the CT attenuation-corrected myocardial perfusion studies had moderate to severe cardiac misregistration qualitatively. Our data suggest that careful review of attenuation correction maps and registration is needed to avoid reconstruction artifacts due to misregistration. Preliminary results of this study were presented at the 2005 Society of Nuclear Medicine Annual Meeting, Seattle, Wash, September 29–October 2, 2005.     

Adrenal adenomas: relationship between histologic lipid-rich cells and CT attenuation number

OBJECTIVE: To evaluate the relationship between lipid-rich cells of the adrenal adenoma and precontrast computed tomographic (CT) attenuation numbers in three clinical groups. MATERIALS AND METHODS: Thirty-five surgically resected adrenal adenomas were used. The clinical diagnoses of the patients included 13 cases of primary aldosteronism, 15 cases of Cushing's syndrome, and 7 non-functioning tumors. The number of lipid-rich clear cells was counted using a microscopic eyepiece grid that contained 100 squares. The results were expressed as the percentages of lipid-rich areas. RESULTS: There was a strong inverse linear relationship between the percentage of lipid-rich cells and the precontrast CT attenuation number (R(2)=0.724, P<0.0001). There were significantly more lipid-rich cells in the primary aldosteronism and non-functioning tumor cases compared to cases of Cushing's syndrome (P=0.007 and 0.015, respectively). The CT attenuation numbers of the primary aldosteronism cases were significantly lower than those of Cushing's syndrome (P=0.0052). Furthermore, the CT attenuation numbers of the non-functioning tumor cases were lower than those of Cushing's syndrome cases. CONCLUSION: We showed that adrenal adenomas in primary aldosteronism and non-functioning tumors contain significantly more lipid-rich cells than those in Cushing's syndrome. They also showed significantly lower attenuation than that in Cushing's syndrome on CT scans. Our results suggest that precontrast CT attenuation numbers may be helpful in the differentiation of adenomas from non-adenomatous lesions, which include malignancies.     

Improvements in the two media method for measurements of gamma-ray linear attenuation coefficients.

The two media method has been previously presented as a solution to the problem of measuring gamma-ray attenuation coefficients of odd-shaped samples. We propose that air is chosen as one of the two media. We theoretically demonstrate that this choice simplifies the equation used, as well as the laboratory work, and also reduces some of the terms associated with experimental uncertainty. It also solves one problem raised in a previous study: the difficulty in increasing the number of independent repetitions.     

Impact of CT attenuation correction by SPECT/CT in brain perfusion images

Purpose  

The aim of this study was to elucidate the regional differences between brain perfusion single photon emission computed tomography (SPECT) images reconstructed with a uniform attenuation correction using Chang’s method (AC-Chang) and a non-uniform attenuation correction with CT using SPECT/CT (AC-CT).     

Comparing slow- versus high-speed CT for attenuation correction of cardiac SPECT perfusion studies

Background

For SPECT, CT-based attenuation correction is preferred. Many different models of CT are available with SPECT/CT systems. Our study compares clinical cardiac SPECT images that were attenuation corrected using slow-rotation CT and high-speed CT transmission scans.

Methods

We evaluated 59 rest/stress perfusion studies from patients who had undergone both a SPECT/CT with a slow-rotation CT and a perfusion study on a PET/CT camera equipped with a high-speed CT scanner. Each SPECT study was reconstructed with transmission maps from both CT scans and the relative perfusion was assessed using semi-automated software. The summed stress/rest/and difference scores (SSS/SRS/SDS) were compared as well as the test classification.

Results

The intraclass correlation coefficients for the SSS, SRS, and SDS were 0.97, 0.96, and 0.80 respectively. There were no significant differences in the mean SSS, SRS, or SDS with the use of either CT for attenuation corrections. Classifying SSS?>?3 as abnormal, there was 97% concordance (???=?0.88). Classifying SDS?>?1 as abnormal, there was 95% concordance (???=?0.54). A McNemar??s test showed no significant differences.

Conclusions

There were no significant differences between using a high-speed CT and using a slow-rotation CT for attenuation correction of SPECT myocardial perfusion images.     

Scatter and attenuation correction for brain SPECT using attenuation distributions inferred from a head atlas.

Sequential transmission scanning (TS)/SPECT is impractical for neurologically impaired patients who are unable to keep their heads motionless for the extended duration of the combined scans. To provide an alternative to TS, we have developed a method of inferring-attenuation distributions (IADs), from SPECT data, using a head atlas and a registration program. The validity of replacing TS with IAD was tested in 10 patients with mild dementia. METHODS: TS was conducted with each patient using a collimated 99mTc line source and fanbeam collimator; this was followed by hexamethyl propyleneamine oxime-SPECT. IAD was derived by deformably registering the brain component of a digital head atlas to a preliminary SPECT reconstruction and then applying the resulting spatial transformation to the full head atlas. SPECT data were reconstructed with scatter and attenuation correction. Relative regional cerebral blood flow was quantified in 12 threshold-guided anatomic regions of interest, with cerebellar normalization. SPECT reconstructions using IAD were compared with those using TS (which is the "gold standard") in terms of these regions of interest. RESULTS: When we compared all regions of interest across the population, the correlation between IAD-guided and TS-guided SPECT scans was 0.92 (P < 0.0001), whereas the mean absolute difference between the scans was 7.5%. On average, IAD resulted in slight underestimation of relative regional cerebral blood flow; however, this underestimation was statistically significant for only the left frontal and left central sulcus regions (P = 0.001 and 0.002, respectively). Error analysis indicated that approximately 10.0% of the total error was caused by IAD scatter correction, 36.6% was caused by IAD attenuation correction, 27.0% was caused by discrepancies in region-of-interest demarcation from quantitative errors in IAD-guided reconstructions, and 26.5% was caused by patient motion throughout the imaging procedure. CONCLUSION: SPECT reconstructions guided by IAD are sufficiently accurate to identify regional cerebral blood flow deficits of 10%, which are typical in moderate and advanced dementia.     

Space-domain non-iterative approach for SPECT/CT systems considering attenuation and space-variant detector response.

A quantitative analysis of emission planar image reconstruction from projections by an object dependent, exact, direct approach in the space-domain considering both object attenuation and space-variant impulse response of SPECT/CT systems is proposed. That approach is compared with iterative methods and non-object-dependent exact methods in both the space domain and the frequency one. Since the mean-projection precorrection method is the concern of some actual 3D methods of compensation for distance-dependent spatial resolution and is thought right for competing with different methods able to quantify the tracer density in the object of interest, it is also examined in the course of the analysis. The direct approach may also augment the simulation power of the Matlab Image Processing Toolbox concerning the direct and inverse Radon transform from parallel projection data, the Toolbox being actually restricted to the ideal transform in the frequency domain.