Determination of the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams

PurposeTo determine the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams.MethodsAn AGMS-DM+ semiconductor detector was calibrated in terms of air kerma measured with an ionization chamber. Air kerma was measured for 20 x-ray beams with tube voltages of 50–140 kVp and a half-value layer (HVL) of 2.2–9.7 mm Al for given quality index (QI) values of 0.4, 0.5, and 0.6, and converted to the surface dose. Finally, the air kerma and HVL measured by the AGMS-DM+ detector were expressed as a ratio of the surface dose for 10 × 10 and 20 × 20 cm² fields. The ratio of both was represented as a function of HVL for the given QI values and verified by comparing it with that calculated using the Monte Carlo method.ResultsThe air kerma calibration factor, CF, for the AGMS-DM+ detector ranged from 0.986 to 1.016 (0.9% in k = 1). The CF values were almost independent of the x-ray fluence spectra for the given QI values. The ratio of the surface dose to the air kerma determined by the PTW 30,013 chamber and the AGMS-DM+ detector was less than 1.8% for the values calculated using the Monte Carlo method, and showed a good correlation with the HVL for the given QI values.ConclusionIt is possible to determine the surface dose of a water phantom from the air kerma and HVL measured by a semiconductor detector for given QI values.     

Institutional Diagnostic Reference Levels and Peak Skin Doses in selected diagnostic and therapeutic interventional radiology procedures

PurposeInstitutional (local) Diagnostic Reference Levels for Cerebral Angiography (CA), Percutaneous Transhepatic Cholangiography (PTC), Transarterial Chemoembolization (TACE) and Percutaneous Transhepatic Biliary Drainage (PTBD) are reported in this study.Materials and methodsData for air kerma-area product (P_KA), air kerma at the patient entrance reference point (K_a,r), fluoroscopy time (FT) and number of images (NI) as well as estimates of Peak Skin Dose (PSD) were collected for 142 patients. Therapeutic procedure complexity was also evaluated, in an attempt to incorporate it into the DRL analysis.ResultsLocal P_KA DRL values were 70, 34, 189 and 54 Gy.cm² for CA, PTC, TACE and PTBD respectively. The corresponding DRL values for K_a,r were 494, 194, 1186 and 400 mGy, for FT they were 9.2, 14.2, 27.5 and 22.9 min, for the NI they were 844, 32, 602 and 13 and for PSD they were 254, 256, 1598 and 540 mGy respectively. P_KA for medium complexity PTBD procedures was 2.5 times higher than for simple procedures. For TACE, the corresponding ratio was 1.6. PSD was estimated to be roughly 50% of recorded K_a,r for procedures in the head/neck region and 10% higher than recorded K_a,r for procedures in the body region. In only 5 cases the 2 Gy dose alarm threshold for skin deterministic effects was exceeded.ConclusionProcedure complexity can differentiate DRLs in Interventional Radiology procedures. PSD could be deduced with reasonable accuracy from values of K_a,r that are reported in every angiography system.     

Conversion factors for effective dose and organ doses with the air kerma area product in patients undergoing percutaneous transhepatic biliary drainage and trans arterial chemoembolization

Conversion factors used to estimate effective (E) and organ doses (H_T) from air Kerma area product (KAP) are required to estimate population doses in percutaneous transhepatic biliary drainage (PTBD) and trans arterial chemoembolization (TACE) interventional procedures.In this study, E and H_T for ten critical organs/tissues, were derived in 64 PTBD and 48 TACE procedures and in 14,540 irradiation events from dosimetric, technical and geometrical information included in the radiation dose structured report using the PCXMC Monte Carlo model, and the ICRP 103 organ weighting factors. Conversion factors of: 0.13; 0.19; 0.26 and 0.32 mSv Gy⁻¹ cm⁻² were established for irradiation events characterized by a Cu filtration of 0.0; 0.1; 0.4 and 0.9 mm, respectively. While a single coefficient of conversion is not able to provide estimates of E with enough accuracy, a high agreement is obtained between E estimated through Monte Carlo methods and E estimated through E/KAP conversion factors accounting separately for the different modes of fluoroscopy and the fluorography component of the procedures.An algorithm for the estimation of effective and organ doses from KAP has been established in biliary procedures which considers the Cu filtration in the X-ray irradiation events. A similar algorithm could be easily extended to other interventional procedures and incorporated in radiation dose monitoring systems to provide dosimetric estimates automatically with enough accuracy to assess population doses.     

Real-time patient radiation dosimeter for use in interventional radiology

There is currently no effective real-time patient dosimeter available for use in interventional radiology (IR). We conducted a feasibility study in a clinical setting to investigate the use of the new dosimeter using photoluminescence sensors during procedures. Reference dosimeters were set at almost the same position of the prototype dosimeter sensors.We found excellent correlations between the reference measurements and those of the prototype dosimeter (r² = 0.950). The sensor of the new dosimeter does not interfere with the IR procedure. The new dosimeter will be an effective tool for the real-time measurement of patient skin doses during IR.     

Calibration of 192Ir high dose rate brachytherapy source using different calibration procedures

Aim

To calibrate Ir-192 high dose rate (HDR) brachytherapy source using different calibration methods and to determine the accuracy and suitability of each method for routine calibrations.

Background

The source calibration is an essential part of the quality assurance programme for dosimetry of brachytherapy sources. The clinical use of brachytherapy source requires an independent measurement of the air kerma strength according to the recommendations of medical physics societies.

Materials and methods

The Ir-192 HDR brachytherapy source from Gammamed plus machine (Varian Medical Systems, Palo Alto, CA) was calibrated using three different procedures, one using the well-type ionization chamber, second by the in-air calibration method and third using solid water phantoms. The reference air kerma rate (RAKR) of the source was determined using Deutsche Gesellschaft fur Medizinische Physik (DGMP) recommendations.

Results

The RAKR determined using different calibration methods are in good agreement with the manufacturer stated value. The mean percentage variations of 0.21, −0.94, −0.62 and 0.58 in RAKR values with respect to the manufacturer quoted values were observed with the well-type chamber, in-air calibration, cylindrical phantom and slab phantom measurements, respectively.

Conclusion

Measurements with a well-type chamber are relatively simple to perform. For in-air measurements, the indigenously designed calibration jig provides an accurate positioning of the source and chamber with minimum scatter contribution. The slab phantom system has an advantage that no additional phantom and chamber are required other than those used for external beam therapy dosimetry. All the methods of calibration discussed in this study are effective to be used for routine calibration purposes.     

Determination of backscatter factors based on the quality index for diagnostic kilovoltage x-ray beams

PurposeThis study aims to investigate the relationship between backscatter factors and Al-half-value-layers (Al-HVL) by making the quality index (QI) a parameter for diagnostic kilovoltage x-ray beams.MethodsBackscatter factors, B_w, for x-ray fluence spectra were calculated from the weighted average of B_w for monoenergetic photons of between 8 and 140 keV with field sizes of 10 cm × 10 cm to 40 cm × 40 cm. The value of B_w for monoenergetic photons was calculated from the ratio of the water kerma at the surface of a water phantom and that at the same point free-in-air using the EGSnrc/cavity code. The weighted averaged backscatter factors were validated by comparing them with those of direct Monte Carlo calculations for the x-ray fluence spectra. The B_w for the x-ray fluence spectra were classified by a QI of 0.35, 0.4, 0.5, 0.6, and 0.7 specified by the ratio of the effective energy and maximum energy. The relationship between B_w and Al-HVL was evaluated for the given QI values. The x-ray fluence spectra were generated for tube voltages of 40–140 kVp with Al-HVLs of 0.5–13.2 mm using the SpekCalc program.ResultsThe weighted averaged backscatter factors for x-ray fluence spectra agreed within 0.7% with those of the direct Monte Carlo calculations. The backscatter factors were represented by the fitting curves of R² > 0.99 with Al-HVL for the given QI values.ConclusionsIt is possible to obtain B_w more accurately by using QI specified by the measured Al-HVL.     

The influence of operator position,height and body orientation on eye lens dose in interventional radiology and cardiology: Monte Carlo simulations versus realistic clinical measurements

ObjectiveThis paper aims to provide some practical recommendations to reduce eye lens dose for workers exposed to X-rays in interventional cardiology and radiology and also to propose an eye lens correction factor when lead glasses are used.MethodsMonte Carlo simulations are used to study the variation of eye lens exposure with operator position, height and body orientation with respect to the patient and the X-ray tube. The paper also looks into the efficiency of wraparound lead glasses using simulations. Computation results are compared with experimental measurements performed in Spanish hospitals using eye lens dosemeters as well as with data from available literature.ResultsSimulations showed that left eye exposure is generally higher than the right eye, when the operator stands on the right side of the patient. Operator height can induce a strong dose decrease by up to a factor of 2 for the left eye for 10-cm-taller operators. Body rotation of the operator away from the tube by 45°–60° reduces eye exposure by a factor of 2. The calculation-based correction factor of 0.3 for wraparound type lead glasses was found to agree reasonably well with experimental data.ConclusionsSimple precautions, such as the positioning of the image screen away from the X-ray source, lead to a significant reduction of the eye lens dose. Measurements and simulations performed in this work also show that a general eye lens correction factor of 0.5 can be used when lead glasses are worn regardless of operator position, height and body orientation.     

Effect of air cavities on the dose delivered to the lung during high-dose brachytherapy

In the treatment of lung cancer using the radiotherapy technique of intracavitary brachytherapy with an¹⁹²Ir source, the lung is normally assumed to be entirely composed of a homogenous mass of soft tissue. The aim of this study is to investigate whether there is the possibility that the air cavities in the lung influence the dose delivered to the lung at a prescribed distance from the source. The Monte Carlo code MCNP-4A was used to model the dose delivered by both¹⁹²Ir and¹⁹⁸Au as a function of treatment medium, density and composition, photon energy, and distance from the source. The suitability of MCNP-4A for this study was tested by producing depth-dose profiles for photons in water and comparing these to calculated profiles produced using well-documented methods.     

Efficiency of personal dosimetry methods in vascular interventional radiology

PurposeThe aim of the present study was to determine the efficiency of six methods for calculate the effective dose (E) that is received by health professionals during vascular interventional procedures.MethodsWe evaluated the efficiency of six methods that are currently used to estimate professionals’ E, based on national and international recommendations for interventional radiology. Equivalent doses on the head, neck, chest, abdomen, feet, and hands of seven professionals were monitored during 50 vascular interventional radiology procedures. Professionals’ E was calculated for each procedure according to six methods that are commonly employed internationally. To determine the best method, a more efficient E calculation method was used to determine the reference value (reference E) for comparison.ResultsThe highest equivalent dose were found for the hands (0.34 ± 0.93 mSv). The two methods that are described by Brazilian regulations overestimated E by approximately 100% and 200%. The more efficient method was the one that is recommended by the United States National Council on Radiological Protection and Measurements (NCRP). The mean and median differences of this method relative to reference E were close to 0%, and its standard deviation was the lowest among the six methods.ConclusionsThe present study showed that the most precise method was the one that is recommended by the NCRP, which uses two dosimeters (one over and one under protective aprons). The use of methods that employ at least two dosimeters are more efficient and provide better information regarding estimates of E and doses for shielded and unshielded regions.     

Web-based platform for patient dose surveys in diagnostic and interventional radiology in Bulgaria: Functionality testing and optimisation

In the period 2013–2016 the National Centre of Radiobiology and Radiation Protection (NCRRP) at the Ministry of Health of Bulgaria has developed a web based platform for performing national patient dose surveys and establishing Diagnostic Reference Levels (DRLs). It is accessible via internet browser, allowing the users to submit data remotely. Electronic questionnaires, specific for radiography, fluoroscopy, image guided interventional procedures, mammography and CT, were provided. Short and clear manuals were added to guide users and minimise human errors. The web-based data collection platform is functional and is currently being used for performing the third national dose survey in Bulgaria, launched in 2016. Data analysis is facilitated due to the standardisation of collected data and their storing. Using the platform, the participating facilities can establish their typical dose levels based on the median value, and compare them to DRLs. A disadvantage of the platform is the need to enter data manually, but it is opened for future upgrades for automatic data harvesting and analysis. Various practical approaches were used to overcome the lack of qualified human resources and insufficient understanding of the DRL and dose tracking concept and to motivate facilities to submit data.     

Feasibility of setting up generic alert levels for maximum skin dose in fluoroscopically guided procedures

PurposeThe feasibility of setting-up generic, hospital-independent dose alert levels to initiate vigilance on possible skin injuries in interventional procedures was studied for three high-dose procedures (chemoembolization (TACE) of the liver, neuro-embolization (NE) and percutaneous coronary intervention (PCI)) in 9 European countries.MethodsGafchromic® films and thermoluminescent dosimeters (TLD) were used to determine the Maximum Skin Dose (MSD). Correlation of the online dose indicators (fluoroscopy time, kerma- or dose-area product (KAP or DAP) and cumulative air kerma at interventional reference point (K_a,r)) with MSD was evaluated and used to establish the alert levels corresponding to a MSD of 2 Gy and 5 Gy. The uncertainties of alert levels in terms of DAP and K_a,r, and uncertainty of MSD were calculated.ResultsAbout 20–30% of all MSD values exceeded 2 Gy while only 2–6% exceeded 5 Gy. The correlations suggest that both DAP and K_a,r can be used as a dose indicator for alert levels (Pearson correlation coefficient p mostly >0.8), while fluoroscopy time is not suitable (p mostly <0.6). Generic alert levels based on DAP (Gy cm²) were suggested for MSD of both 2 Gy and 5 Gy (for 5 Gy: TACE 750, PCI 250 and NE 400). The suggested levels are close to the lowest values published in several other studies. The uncertainty of the MSD was estimated to be around 10–15% and of hospital-specific skin dose alert levels about 20–30% (with coverage factor k = 1).ConclusionsThe generic alert levels are feasible for some cases but should be used with caution, only as the first approximation, while hospital-specific alert levels are preferred as the final approach.     

The novel application of Benford's second order analysis for monitoring radiation output in interventional radiology

Benford's law is an empirical observation which predicts the expected frequency of digits in naturally occurring datasets spanning multiple orders of magnitude, with the law having been most successfully applied as an audit tool in accountancy. This study investigated the sensitivity of the technique in identifying system output changes using simulated changes in interventional radiology Dose-Area-Product (DAP) data, with any deviations from Benford's distribution identified using z-statistics.The radiation output for interventional radiology X-ray equipment is monitored annually during quality control testing; however, for a considerable portion of the year an increased output of the system, potentially caused by engineering adjustments or spontaneous system faults may go unnoticed, leading to a potential increase in the radiation dose to patients. In normal operation recorded examination radiation outputs vary over multiple orders of magnitude rendering the application of normal statistics ineffective for detecting systematic changes in the output. In this work, the annual DAP datasets complied with Benford's first order law for first, second and combinations of the first and second digits. Further, a continuous ‘rolling’ second order technique was devised for trending simulated changes over shorter timescales. This distribution analysis, the first employment of the method for radiation output trending, detected significant changes simulated on the original data, proving the technique useful in this case. The potential is demonstrated for implementation of this novel analysis for monitoring and identifying change in suitable datasets for the purpose of system process control.     

A retrospective comparison of organ dose and effective dose in percutaneous vertebroplasty performed under CT guidance or using a fixed C-arm with a flat-panel detector

PurposeTo compare the organ-dose and effective-dose (E) delivered to the patient during percutaneous vertebroplasty (PVP) of one thoracic or lumbar vertebra performed under CT guidance or using a fixed C-arm.MethodsConsecutive adult patients undergoing PVP of one vertebra under CT-guidance, with optimized protocol and training of physicians, or using a fixed C-arm were retrospectively included from January 2016 to June 2017. Organ-doses were computed on 16 organs using CT Expo 2.4 software for the CT procedures and PCXMC 2.0 for the fixed C-arm procedures. E was also computed with both software. Dosimetric values per anatomic locations for all procedures were compared using the paired Mann-Whitney-Wilcoxon test.ResultsIn total, 73 patients were analysed (27 men and 46 women, mean age 78 ± 10 years) among whom 35 (48%) underwent PVP under CT guidance and 38 (52%) PVP using a fixed C-arm. The median E was 11.31 [6.54; 15.82] mSv for all PVPs performed under CT guidance and 5.58 [3.33; 8.71] mSv for fixed C-arm and the differences was significant (p<0.001). For lumbar PVP, the organ doses of stomach, liver and colon were significantly higher with CT-scan than with the fixed C-arm: 97% (p=0.02); 21% (p=0.099) and 375% (p=0.002), respectively. For thoracic PVP, the lung organ dose was significantly higher with CT-scan than with the fixed C-arm (127%; p<0.001) and the oesophagus organ doses were not significantly different (p = 0.626).ConclusionThis study showed that the E and the organ dose on directly exposed organs were both higher for PVP performed under CT-guidance than with the fixed C-arm.     

A parametric study of occupational radiation dose in interventional radiology by Monte-Carlo simulations

This paper presents the results of a parametric study on the occupational exposure in interventional radiology to explore the influence of various variables on the staff doses. These variables include the angiography beam settings: x-ray peak voltage (kVp), added copper filtration, field diameter, beam projection and source to detector distance. The study was performed using Monte-Carlo simulations with MCNPX for more than 5600 combinations of parameters that account for different clinical situations. Additionally, the analysis of the results was performed using both multiple and random forest regression to build a predictive model and to quantify the importance of each variable when the variables simultaneously change. Primary and secondary projections were found to have the most effect on the scatter fraction that reaches the operator followed by the effect of changing the x-ray beam quality. The effect of changing the source to image intensifier distance had the lowest effect.     

Incident air kerma to absorbed organ dose conversion factors for breast and lung in PA thorax radiography: The effect of patient thickness and radiation quality

PurposeConverting the measurable quantities to patient organ doses in projection radiography is usually based on a standard-sized patient model and a specific radiation quality, which are likely to differ from the real situation. Large inaccuracies can therefore be obtained in organ doses, because organ doses are dependent on the exposure parameters, exposure geometry and patient anatomy. In this study, the effect of radiation quality and patient thickness on the organ dose conversion factors were determined.MethodsIn this study, the posterior–anterior projection radiograph of the thorax was selected in order to determine the effect of radiation quality (tube voltages of 70–130 kV and total filtrations of 3 mmAl to 4 mmAl + 0.2 mmCu) and patient thickness (anterior–posterior thicknesses of 19.4–30.8 cm) on the breast and lung dose conversion factors. For this purpose, Monte Carlo simulation programs ImpactMC and PCXMC were used with computed tomography examination data of adult male and female patients and mathematical hermaphrodite phantoms, respectively.ResultsCompared to the reference beam quality and patient thickness, the relative variation range in organ dose conversion factors was up to 74% for different radiation qualities and 122% for different patient thicknesses.ConclusionsConversion factors should only be used with comprehensive understanding of the exposure conditions, considering the exposure parameters, exposure geometry and patient anatomy they are valid for. This study demonstrates that patient thickness-specific and radiation quality-specific conversion factors are needed in projection radiography.     

Validation of a dose tracking software for skin dose map calculation in interventional radiology

PurposeValidate the skin dose software within the radiation dose index monitoring system NEXO[DOSE]® (Bracco Injeneering S.A., Lausanne, Switzerland). It provides the skin dose distribution in interventional radiology (IR) procedures.MethodsTo determine the skin dose distribution and the Peak Skin Dose (PSD) in IR procedures, the software uses exposure and geometrical parameters taken from the radiation dose structured report and additional information specific to each angiographic system. To test the accuracy of the software, GafChromic® XR-RV3 films, wrapped under a cylindrical PMMA phantom, were irradiated with different setups. Calculations and films results are compared in terms of absolute dose and geometric accuracy, using two angiographic systems (Philips Integris Allura FD20, Siemens AXIOM-ArtisZeego).ResultsCalculated and film measured PSD values agree with an average difference of 7% ± 5%. The discrepancies in dose evaluation increase up to 33% in lower dose regions, because the algorithm does not consider the out-of-field scatter contribution of the neighboring fields, which is more significant in these areas. Regarding the geometric accuracy, the differences between the simulated dose spatial distributions and the measured ones are<3 mm (4%) in simple tests and 5 mm (5%) in setups closer to clinical practice. Moreover, similar results are obtained for the two studied angiographic system vendors.ConclusionsNEXO[DOSE]® provides an accurate skin dose distribution and PSD estimate. It will allow faster and more accurate monitoring of patient follow-up in the future.     

Patient exposure data and operator dose in coronary interventional procedures: Impact of body-mass index and procedure complexity

PurposeThe aim of this study was to assess patient exposure data and operator dose in coronary interventional procedures, when considering patient body-mass index and procedure complexity.MethodsTotal air kerma area product (P_KA), Air-Kerma (AK), Fluoroscopy time (FT), operator dose and patient body-mass index (BMI) from 97 patients’ procedures (62 coronary angiography (CA) and 35 Percutaneous Coronary Intervention (PCI) were collected for one year. For PCI procedures, also the complexity index-CI was collected. Continuous variables for each of the 2 groups procedures (CA and PCI) were compared as medians with interquartile range and using Mann-Whitney U test. Multiple group data were compared using Kruskal-Wallis test (significance: p < 0.05).ResultsMedian P_KA was 63 and 125 Gy cm² for CA and PCI respectively (p < 0.001); FT was 3 and 14 min, respectively (p < 0.001). P_KA and FT significantly increased (p < 0.05) with BMI class for CA procedures. P_KA and FT also increased in function of CI class for PCI, thought significantly only for FT (p < 0.001), possibly because of the low number of PCI procedures included; cine mode contributed most to P_KA. Significant dose variability was observed among cardiologists for CA procedures (p < 0.001).ConclusionsDose references levels for P_KA and FT in interventional cardiology should be defined - on a sufficient number of procedures- in function of CI and BMI classes. These could provide an additional tool for refining a facility’s quality assurance and optimization processes. Dose variability associated with cardiologists underlines the importance of continuous training.     

Critical success factors for implementation of an incident learning system in radiation oncology department

AimThe aim of this study was to analyze critical success factors (CSFs) for implementation of an incident learning system (ILS) in a radiation oncology department (ROD) and evaluate the perception of the staff members along this process.BackgroundImplementing an ILS is a way to leverage learning from incidents and is a tool for improving patient safety, consisting of a cycle of reporting and analyzing events as well as taking preventive actions. ILS implementation is challenging, requiring specific resources and cultural changes.Materials and methodsAn ILS was designed and implemented based on the CSF identified in the literature review. Before starting the ILS implementation, a structured survey was applied to assess dimensions of patient safety culture. After the period of implementation (7 months), the survey was applied again and compared with the initial assessment, and interviews were performed with staff members to evaluate the overall satisfaction with ILS and CSFs.ResultsStatistically significant improvements were observed in 5 dimensions (12 totals) of the safety culture survey, considering time points before and after the ILS implementation. According to interviewees, “Facilitating committee”, “Efficient data collection”, “Focus on improvement”, “Just culture” and “Feedback to users” were the most relevant CSFs.ConclusionsThe ILS designed and implemented at ROD was perceived as an important tool to support quality and safety initiatives, promoting the improvement in safety culture. The ILS implementation critical success factors were identified and have shown good agreement between the results of the literature and the users' practical perception.     

基于亚像元估测的城市硬化地表景观格局分析

城市硬化地表不仅是影响城市生态环境质量重要因子,也是定量描述城市地表物理特征,进行城市景观分类的基础.基于多种分辨率遥感影像亚象元分类提取硬化地表成为近年来的研究热点.利用TM/ETM＋和Quickbird不同分辨率遥感数据,以北京市中心城区为研究区域,对比分析回归树法和多元回归法的估测精度,选出预测硬化地表指数（Impervious surface index,简称为ISI）最优估测模型,并进行景观分类与城市景观格局分析.结果表明：（1）回归树亚象元估测法是提取硬化地表信息的一种有效的方法（最大相关系数＝0.94）,不同季节遥感影像可以挖掘地物在不同时期光谱差异,提高分类精度.（2）根据硬化地表指数划分城市用地类型,提供了量化分类的依据;（3）北京城市硬化地表景观格局表现出极强的空间梯度性,从北京市中心到郊区,ISI逐渐降低：城市二环以内,ISI平均值为67.32%,集中分布在高于60%范围;二环-四环分布比较相似,平均值分别为65.91%、66.13%;四环-五环区域ISI下降迅速（ISI=46.42%）,存在两个高峰,分别是低于＜20%和＞70%;六环以外区域,非硬化地表成为主要类型（ISI=9.32%）;（4）北京市景观格局在不同区域差异巨大：从市中心到市郊,景观破碎化程度加强,平均斑块面积逐渐增加,高密度城市用地比例逐步下降,自然地表平均面积呈现U形分布;中等密度城市用地斑块密度最高,破碎度最高;城市用地形状比自然地表复杂,低密度城市用地形状最为复杂.（5）运用回归树亚象元估测法提取出北京中心城区硬化地表信息,为城市地表景观特征提取与高精度量化分类提供了新的研究方法与研究思路,在此基础上进行了景观分类及景观格局分析,进一步推广并论证了硬化地表在景观生态学研究中的应用价值.