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Volume optimization of abdominal CT contrast agent based on lean body weight

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  • Save American Journal of Biomedical Engineer ing 2013, 3(6A): 22-26 DOI: 10.5923/s.ajbe.201310.04 Contrast Medium Volume Optimization in Abdominal CT on Basis of Lean Body Weight Rodrigues Liliana1,*, Sampaio Ricardo2, Coimbra Miguel3 1Escola Superior de Tecnologia da Saúde do Porto, Instituto Politécnico do Porto, Vila Nova de Gaia, and Unidade de Imagiologia, Hospital da Boavista, Porto, Portugal 2Unidade de Imagiologia, Hospital da Boavista, Porto, Portugal 3Instituto de Telecomunicações, Faculdade de Ciências da Universidade do Porto, Portugal Abstract Co mputed tomography (CT) in abdominal imaging has undergone great advances in recent years. The administration of iodinated contrast media (CM) has evolved along with the evolution of the CT equipment. The most common method for calculat ion of the CM volu me in abdominal CT is based on total body weight (TBW), but in this way some patients get a lo wer dose and some other patients get a higher dose that the optimal dose they need. In the literature there are many formulas to calculate the CM volume to be administered in abdomina l CT, but practice shows that the variability of enhancement is wide. The main objective of this study is to verify that the volume of intravenous CM in abdominal CT calculated on basis of lean body weight (LBW) allo ws good liver enhancement with small amplitude of variat ion around the mean, that is, with less variability in enhancement than CM volu me calcu lated on basis of TBW. In conclusion the calculation of the volume of CM on basis of LBW translates into a low variability of liver enhancement–25.50 HU. In our results 96.05% of sample belongs to a good hepatic enhancement. If we stratify sample by groups of BMI, we also verify no significative differences between slims and fat patients when analysed mean hepatic enhancement. Keywords Intravenous contrast mediu m, Co mputed to mography, Lean body weight, Liver enhancement, Abdominal CT 1. Introduction The development of mu ltidetector CT equip ment (MDCT) with increased spatial and temporal resolution, with acquisition of large volu mes in a fraction of time prev iously needed, and the development of double head injectors, allo wed for a revolution in abdominal CT imag ing and improved diagnostic accuracy[1]. A low dose of iodinated CM results in low parenchymal enhancement and hence limits the diagnostic accuracy of the study. A high dose emphasizes lesions but increases cost and the likelihood of contrast media adverse reaction such as contrast-induced nephropathy (CIN). It would therefore be useful if a simp le calculation method that leads to a more uniform hepatic enhancement among patients could be developed[2]. In the literature there are many protocols to calcu late contrast volume ad min istration in abdominal CT. The most common formu lae calculate CM volu me based upon the patient TBW. Other formu lae do the same calculation based on the iodine concentration of the CM, using and iodine ratio of 0.521g per kilogram of body weight[3]. There are also centres that use a fixed volume of CM, no matter the * Corresponding author: leerodri@gm (Rodrigues Liliana) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved patient’s size or body weight[2]. The most recent literature refers that calculat ing the CM volume based on the LBW may reduce patient-to-patient enhancement variability, while maintaining satisfactory hepatic enhancement. This method would allow one to achieve consistent hepatic enhancement[4,5] and may contribute to the estimation of optimal iodine doses to be used in obese patients[6]. A d ifferent approach, using body fat percentage (BFP) as been advocated in one study as a good method of calculating CM volu me, but BFP is correlated with the LBW and essentially assumes the same principle, that fat does not capture significant amounts of CM and can be d isregarded when calcu lating the optimal dose of CM to be injected[7]. To measure hepatic enhancement the portal/venous phase is the best. Liver enhancement occurs 40/50s after aort ic enhancement. So there are two forms to acquire images in portal/venous phase: one uses an empirica lly delay of 70s or a more rigorous one uses a bolus-tracking program to determine the init iation[5, 8]. Literature considers a good hepatic enhancement when the variation between hepatic density before administration of CM and hepatic enhancement after CM ad min istration is around 50-60 HU[3,5,7]. The main object ive of this study is to verify if the volume of intravenous contrast mediu m in abdominal CT calculated on LBW allows good liver enhancement. To achieve this we define some specific American Journal of Biomedical Engineer ing 2013, 3(6A): 22-26 23 objectives: to find if this method avoids low doses in slim patients and overdoses in fat patients; to verify how the method of volume calcu late can influence de final volu me (TBW/ LBW), to verify if exist a correlat ion between hepatic enhancement calculates on basis of LBW and sex or age of individual. double head injector (Medtron Injekron CT2) allo wing saline injection, immediately after in jection of CM. The contrast med ia used was Iobitridol (Xenetix ®), with a concentration of 350 mg I / mL, which mean 767.8 mg I/ mL of solution. This CM is classified as water soluble, non-ionic and low o s mo lality . 2. Material and Methods 2.1. Expl ained Methods The data collection phase occurred between January and July 2011 in The Imaging Un it of Hospital da Boavista in Porto, Portugal. 2.2. Patients This study was approved by the admin istration of the Hospital da Boavista in Po rto. The inclusion criteria were: acceptance and signing of an informed consent for all patients, patients proposed to an abdominal CT with administration of contrast media in a venous/portal phase, 20 gauge plastic intravenous catheter placed in an antecubital vein, inject ion rate of 3.5 mL/s and finally patients with orthostatic ability to measured and weighed. We excluded all exams with variations in the init ial protocol, patients with hepatic steatosis and patients without clear v isualization of hepatic veins in the venous phase (low cardiac debit). 2.3. Esti mated LB W For each patient a sheet was created which recorded height and weight. The initial phase of the protocol was determine the value of LBW, estimated using the “Clinical Calculator” – The Medcalc©, wh ich is based on the following formu la: LBW (Men) = (1.10 x weight (kg)) - 128 (weight2 / (100 x height (m)) 2; LBW (female) = (1.07 x weight (kg)) - 148 (weight2 / (100 x height (m)) 2. 2.6. Acquisition Protocol All acquisitions were done using a protocol previously record in the CT console. The examination includes a control scan in the liver region and utilizat ion of a bolus-tracking program to determine the initiat ion time of the venous/portal phase. A region-of-interest (ROI) with 1 cm2 was placed in the abdomina l aorta and the image acquisition 40 s after ROI measure 150HU. The literature says that a real venous/portal phase occurs 40 s after arterial phase[8]. The CM volu me was calculated based on LBW with an inject ion of 40 mL of saline solution and a rate of 3.5 mL/s. 2.7. Data Analysis Image analysis included three main stages, as described below. The first step consisted in checking all data sheets, confirming the values recorded on the data sheet (lean weight, calculat ing the CM volu me) and also analysing all observations noted in the record susceptible to modify the result of the examination, for example, change in the protocol. The second was analysing all control scans (images without admin istration of CM) for measuring the density of the liver and spleen for evidence of hepatic steatosis (Figure 1). This analysis was performed with Rad iAnt DICOM Viewer®. We used four ROI with 1 cm2, three in the liver and one in the spleen, so we can compare the liver density with the spleen density. The patients with liver density inferior to 40 HU and all patients with liver density superior to 40 HU but with less of 10 HU of spleen density were excluded[9, 10]. 2.4. Calculation of CM Vol ume We adopted the ratio of Kondo (2010), adapted to protocol of the Imaging Unit o f the Hospital da Boavista. In Kondo’s study the author´s used 600 mg I /kg of TBW and reached a value of 812 mg I / kg of LBW[5]. Protocol of Hospital da Boavista uses 525 mg I /kg of TBW (1,5 mL/kg ). Using the same reasoning our new value is 718 mg I /kg LBW (2 mL/ kg). 2.5. Equi pment and Material The imag ing was performed through a multidetector CT scanner (Siemens So mato m Sensation 40), sections at 1.2 mm detector collimation, 1.5 mm per rotation table feed and 1 p itch. Gantry rotation time was 0.5 s. We used Care Dose 4D, with 250 effective mAs, 120 kV and the acquisition was with a soft kernel filter (kernel 30F med iu m s mooth) and abdominal window. The injection was performed by using a Figure 1. Example of measuring the density of the liver and spleen for evidence of hepatic steatosis, in images without CM In a third stage we analysed all images after CM administration to get our main variable. We selected a slice where liver appears in the major extension. We marked four 24 Rodrigues Liliana et al.: Contrast M edium Volume Optimization in Abdominal CT on Basis of Lean Body Weight ROI’s (1cm2), three in the peripheral region of the liver, thereby preventing large vessels, and one in the spleen (Figure 2). Data was then entered into the database. The "hepatic enhancement" variable is the average of these three meas ure ments . 3.2. Intra-cl ass Correlation Coefficients Since all measurements were performed by the same person, we calculated the intra-class correlation coefficients (ICC). An external evaluator analy zed 30% of the sample, selected randomly. Subsequently, these measurements were compared with the total samp le, obtaining an ICC value of .909 for simple measures and .953 for average measures. We assume the measurements used as valid. 3.3. Hepatic Enhancement Our results show a uniformity of hepatic enhancement in the total analysis. Co mparing hepatic density after CM administration with hepatic enhancement before CM administration, results show an average increase of 54 HU (Table 2). The variation between the maximu m and minimu m values after CM ad ministration was 25.50 HU; the hepatic density variation before CM ad ministration was 27 HU (Table 2). Figure 2. Example of the methodto measure liver enhancement, after CM admin istrat io n 2.8. Statistical Analysis Statistical analysis was performed using adequate software: Statistical Package for Social Sciences (SPSS, version 19.0). We used the analysis of frequencies to describe sample and analyse the distribution of hepatic enhancement. We used a Intraclass Correlation Coefficient (ICC) to validate data. Sa mple T-Test was used to verify how the method of calcu lation (TBW or LBW) influences the volume. To verify whether there was a statistically significant correlat ion between hepatic enhancement and age or sex we use Pearson correlat ion test. 3. Results 3.1. Patients The initial sample included 108 non-consecutive patients that met the enrolment criteria. Of 108 patients, 32 were excluded, due to changes to the protocol (8), unavailab ility of images (7), withdrawal o f a patient participating (1), debit cardiac alterations (8) and hepatic steatosis (8). The final sample consisted of 76 indiv iduals: 46 (60.5%) were male and 30 (39.5%) were female, aged between 19 and 87 years, a body weight between 45 and 93 kg and a height between 140 and 186 cm, accord ing to Table 1. Table 1. Sample characterization Sample characterization N=76 Minimum Maximum Mean Age (year) 19 Weight (kg) 45 Height (cm) 140 87 62.42 93 67.76 186 165 Stan da rd De sviation 15.50 9.87 9.19 Table 2. Hepatic density vs. hepatic enhancement N=76 Minimum He patic density (Afte r CM) 44.00 He patic enhan cement (Before CM) 100.20 Maximum 71.00 125.70 Mean 56.43 110.20 Variation in sample 27.00 25.50 After analyse sample we verify that 96.05% of results belong to a good enhancement (Table 3). Table 3. Sample distribution by three intervals of hepatic enhancement Enhancement intervals n Low enhancement (< 100 HU) 0 Good enhancement (100-120HU) 73 O ver enhancement (> 120 HU) 3 % 0 96.05 3.95 To understand the real difference in CM volu me using this calculation method (LBW) and the most common method (TBW ), we co mpared the volu me of the same patients using different methods. We used the value of CM volume used with LBW and estimate the volume of CM we would use if we use the TBW as the basis for calculat ing, based on the theory of 1.5 mL CM / kg of TBW. With Sample T-Test we verify a lot of variations in CM volu mes with different methods, (t (75) =2.934, p<.05, r=.913). Figure 3 is a specific case of the sample, in which four patients with the same TBW (66 kg), present different values of LBM and consequently different volu mes of CM ad ministered. But if we use the TBW method to calculate, volu me will be always the same 99 mL. To analyse if this method avoid low doses in slim patients and overdoses in fat patients, we div ided the sample in American Journal of Biomedical Engineer ing 2013, 3(6A): 22-26 25 different classes of body mass index (BMI). We defined four classes as described in Table 4, and co mpared the averages of enhancement between each group. We found a uniform enhancement, particularly in patients who are overweight. The results are no significat ive to patients with low weight, because the sample only has one patient with a BM I below 18.4 values. Figure 3. MC Volume TBW/LBW Table 4. Mean of hepat ic enhancement in 4 groups of BMI Groups of BMI N Mean hepatic enhan cement Stan da rd de viation Low weight (≤ 18.4) 1 117.1 - Normal weight (18.5 – 24.99) 40 111.2 6.2 O verweight (25.00 – 19.99) 31 108.8 4.9 Obesi ty (≥ 30) 4 108.1 6 The Pearson correlation coefficient revealed a significative negative correlation between hepatic enhancement and age, (r=.003, p<.05) and no significative correlation between hepatic enhancement and patient´s sex. 4. Discussion Previous studies refer that to achieve an optimal hepatic enhancement the variation of HU will be around 50 – 60[1]. Since the density of normal liver also lies between 50 and 60 HU, we can consider values between 100 and 120 HU belonging to a good range of hepatic enhancement. In Tab le 3 is possible confirm that 96.05% of our results belong this range. Table 2 reveals also that the average variation of hepatic enhancement in our sample was 53.77 HU. If vo lu me of CM is calculated based on TBW, patients with the same body weight receive the same volu me of CM , independently of height and fat percentage[10]. According Figure 3, if we calculate the CM volu me on basis of LBW in four patients with same body weight we obtain different values; in this case volume can vary between 90 and 110 mL. It is also important to observe that this new method does not reduce the volume. It optimizes it for each patient. We can also see in Figure 3 that to patients with same body weight, we may use higher or lower volu mes, in co mparing LBW to TBW. When we divided the patients into four groups of BMI, we verified that hepatic enhancement means are very close, which means a low variability of values. Some authors also recommend not calculating CM volu me on basis of TBW in patients below 60 kg and over 90 kg[8]. We verify no significative statistic differences between men and wo men, unlike some prev ious studies[11,12]. The exp lanation lies in the fact that the calculation of LBW takes into account the sex of the individual. We think that previous studies observed differences between sexes because they used TBW to calculate CM volu me. On the other hand we find a statistically significant correlation between hepatic enhancement and age, wh ich is possibly related with changes in cardiac output that changes with age, and was not tested in this study. The fundamental point of this study is based on the following principle: when comparing vascularizat ion of fat tissues with other tissues, such as muscle and solid bodies, that have a greater vascularity and therefore need more volume of CM[1,2,7,13,14]. When someone gets fat, there is an increase exclusively of fat mass. Thus, if we increase the value of CM linearly with the increase in weight it will occur overdose of these individuals. This means that it’s arguable that we should increase the volume of CM in proportion to weight, making more sense to calculate based on LBW[1, 5, 13,14]. The models fo r calculating the volu me of CM based on the TBW t ry, in so me way, to correct overdose imposing a maximu m of CM volu me. In some countries such as the United States, 120 mL is reco mmended as the maximu m value. However, the relative values prevail and they aren’t adapted to the real individual constitution[4,5,15]. The division in four groups of BMI was important to demonstrate that there aren’t differences between slim and fat patients in hepatic enhancement. 5. Conclusions In clinical practice, the calculat ion of the volu me of CM on basis of LBW t ranslates into a low variability of liver enhancement – 25.50 HU. In our results 96.05% of samp le belongs to a good hepatic enhancement. If we stratify samp le by groups of BMI, we also verify no significative differences between slims and fat patients when analysed mean hepatic enhancement. We suggest that this could replace the calculation based on TBW. ACKNOWLEDGEMENTS We would like to thank to all persons of Boavista Hospital who made this study possible, special to Unidade de Imagiolog ia. 26 Rodrigues Liliana et al.: Contrast M edium Volume Optimization in Abdominal CT on Basis of Lean Body Weight REFERENCES [1] Rengo, M ., Bellini, D., De Cecco, C. N., Osimani, M ., Vecchietti, F., Caruso, D., M aceroni, M . M ., et al. (2011). The optimal contrast media policy in CT of the liver. Part I: Technical notes. Acta radiologica (Stockholm, Sweden:1987), 52(5), 467-72. doi:10.1258/ar.2011.100499 [2] Li, J., Udayasankar, U. K., Tang, X., Carew, J., Toth, T. L., & Small, W. C. (2010). 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