The Catphan phantom was utilized in this study to represent the normal pancreatic parenchyma and was scanned by using various acquisition protocols, essentially varying the tube voltage between 120 kVp and 80 kVp. Computer-simulated, low attenuating lesions of various diameters, representing hypovascularised solid pancreatic tumors, were inserted into the ensuing phantom images. The rationale for using a low kilovoltage protocol was to achieve a higher attenuation difference between the tumor and the pancreatic parenchyma, in order to improve tumor conspicuity and delineation. Generally, lesions with attenuation nearly identical to that of the normal pancreatic parenchyma are difficult to visualize. By inserting 110 HU computer-simulated lesions, 20 HU less than that of the pancreatic phantom, we were able to mimic this clinical situation. When the tube voltage was decreased from 120 to 80 kVp, the mean photon energy decreased in parallel from 56.8 to 43.7
]. This lower value was closer to the K edge of iodine (33.2
keV) resulting in higher X-ray absorption and a significantly higher attenuation (67%) of the background (i.e. normal pancreatic tissue). Consequently, the attenuation difference between the digital lesions and the pancreatic background increases by 70% (from 20 HU to 34 HU).
The post-hoc analysis revealed significantly better lesion detection at 80 kVp than at 120 kVp, which means that smaller lesions and more of them are detected at 80 kVp. The main consideration for applying a lower tube voltage was the increase of image noise. In protocol C, we established the same image noise as in protocol A by using the maximum tube current possible with our 64-channel MDCT (675
mA). However, for lesions measuring ≥5
mm, the LLF at 80 kVp was not improved when the tube current was increased to 675
mA in order to establish the same image noise as in the 120 kVp protocol. In contrast, an increase of the tube current in the 80 kVp protocol (C) to achieve similar noise as in the 120 kVp protocol (A) improved the LLF for lesions with a diameter ≤4
The receiver operating characteristic (ROC) method has long been one of the standard methods in radiology to analyze and compare diagnostic accuracy [25
The ROC method is very powerful because it estimates and reports all combinations of sensitivity and specificity that a diagnostic test is able to provide [26
] and it is therefore used in this study. In the ROC paradigm, the observer is given a number of cases in some of which some kind of abnormality is present. The observer is asked to rate every case depending on how confident he or she is about whether there is an abnormality somewhere in that case.
The resulting 2 x 2 truth-response table defines correct decisions (true positives (TP) and true negatives (TN)) and incorrect decisions (false positives (FP) and false negatives (FN)) in comparison to a gold standard.
Pancreatic cancer incidence peaks between 60 and 80
years of age [27
]. The risk of developing a radiation-induced cancer is markedly age-dependent. Given an estimated less than 5% 5-year survival rate, the risk to the patient associated with an increased radiation dose in order to achieve a technically optimal MDCT is negligible. We therefore believe that for patients with a high probability of pancreatic malignancy, the examination protocol should be tailored to achieve optimal tumor conspicuity. The radiation dose must, however, be taken into account for patients with hereditary or predisposing factors for pancreatic tumors (for example familiar syndromes and chronic pancreatitis) and subjects with the multiple endocrine neoplasia syndrome Type I (MEN-I) who undergo repeated screening controls.
When designing these examination protocols, it is therefore important to remember that the radiation dose will increase when the tube voltage is decreased because at the same time the tube current needs to be adjusted to maintain similar image noise. The reason for this is that the image noise is a function of the dose to the detector and not to the patient.
This study has some limitations. In daily clinical practice we do not consider the low kilovoltage protocols suitable for large patients (> 85
kg) because of the high image noise, despite the increase in radiation dose. Real tumors are not uniformly spherical in shape and are not always located in a perfectly homogenous background. Even though the vast majority of PDACs are hypoattenuated to pancreatic parenchyma, it should be noted that 11% of solid pancreatic malignancies are isoattenuating on CT [9
]. In these cases the presence of secondary signs such as pancreatic and/or biliary duct dilatation can indicate the existence of a tumor. Also, the attenuation used as reference for pancreatic parenchyma (130HU) and the pancreatic cancers (110HU) were measured in a limited number of patients (n
15). In the experimental situation a 20 HU attenuation difference between pancreas and tumor was thus assumed wheras in the clinical situation there is a variation in this respect. Furthermore, the simulated tumors were inserted into already reconstructed images, meaning that the lesions were not affected by the modular transfer function (MTF) of the system. In future studies, the lesions may instead be convolved with the point-spread function (PSF) before inserting them into the images in order to avoid this inconsistency.
Moreover, the study was designed as a free-response ROC (FROC), but the evaluation was performed as an ROC. Because two of the readers did not generate an appreciable amount of NLs for protocols B* and C*, the statistical analysis became less reliable. An ROC analysis was therefore performed, complemented with descriptive statistics, so that the information about the location and the number of lesions was not lost.
One of the readers did not use the same type of monitor (three mega pixel grey scale) as the other two. A one mega pixel color monitor was used instead. However, all monitors were calibrated according to DICOM part 14, and since CT images do not require high-resolution monitors, this difference was considered to be of small importance.