Since its clinical introduction in the mid 1970S, techniques for CT have undergone many changes that have dramatically altered how CT scans are obtained. Helical (spiral) CT allows for faster acquisition of truly volumetric CT data than is possible with conventional scanners. Routine helical CT of the abdomen is now possible because of three major technical refinements: the development of the slip-ring gantry, improved detector efficiency, and greater tube cooling capability. This article reviews the technical principles that govern helical CT, the potential advantages and disadvantages of this technique, and initial clinical experience with helical CT of the abdomen.

OBJECTIVE:Tuberculosis in the abdominal lymph nodes may be difficult to distinguish from lymphomas. This study evaluated specific CT imaging criteria for differentiating these entities.MATERIALS AND METHODS:We retrospectively reviewed the anatomic distribution and CT enhancement patterns of disease in 69 patients, 26 (38%) with tuberculosis and 43 (62%) with untreated lymphomas involving abdominal lymph nodes. Of the patients with tuberculosis, five (19%) had disseminated disease and 21 (81%) had nondisseminated disease. Of the patients with lymphomas, 16 (37%) had Hodgkin's disease and 27 (63%) had non-Hodgkin's lymphoma.RESULTS:Disseminated and nondisseminated tuberculosis involved predominantly lesser omental, mesenteric, anterior pararenal, and upper paraaortic lymph nodes. Lower paraaortic lymph nodes were involved more often in Hodgkin's disease (15 patients [94%]), non-Hodgkin's lymphoma (24 patients [89%]), and disseminated tuberculosis (five patients [100%]) than in nondisseminated tuberculosis (one patient [5%]). Mesenteric lymph nodes were involved more often in disseminated tuberculosis (four patients [80%]) and nondisseminated tuberculosis (11 patients [52%]) than in Hodgkin's disease (one patient [6%]) (p < .01). Anatomic distribution was not different between disseminated tuberculosis and non-Hodgkin's lymphoma. Tuberculous lymphadenopathy commonly showed peripheral enhancement, frequently with a multilocular appearance, whereas lymphomatous adenopathy characteristically showed homogeneous attenuation (14 patients [87.5%] with Hodgkin's disease and 19 patients [70%] with non-Hodgkin's lymphoma [p < .01]).CONCLUSION:Our findings indicate that the anatomic distribution and specific enhancement patterns of lymphadenopathy seen on contrast-enhanced CT can be useful in differentiating between tuberculosis and untreated lymphomas of the abdominal lymph nodes.

OBJECTIVE:We conducted a survey of the members of the Society of Computed Body Tomography/Magnetic Resonance to assess current techniques in liver imaging using helical CT.MATERIALS AND METHODS:The survey, which was designed to update earlier surveys from 1987 and 1993, included a questionnaire distributed to 77 members of the Society of Computed Body Tomography/Magnetic Resonance.RESULTS:Forty-nine members responded, representing 28 institutions. In 1993, 19% (5/26) of institutions used helical scanners, compared with 82% (23/28) in 1996. The group of institutions with helical CT served as the focus of this survey. In 1993, 58% of institutions used 1-cm collimation: in 1996, 78% (18/23) used thinner, 7- to 8-mm collimation. In 1987, 41% used power injectors compared with 85% in 1993 and 100% in 1996. In 1996, monophasic injections were used by 96% (22/23) of institutions. In 1993, most institutions used a contrast material injection rate of 1.5-2.0 ml/sec; in 1996, most used a 2.5-3.0 ml/sec injection rate. In 1993, 96% of institutions used 125-150 ml of contrast material; in 1996, 57% (13/23) of institutions used 125-150 ml and 30% of institutions used less than 125 ml of contrast material. A delay time of 21-45 see was used by 83% of institutions in 1993, whereas in 1996, 83% (19/23) of institutions used a longer delay time of 50-80 sec. In 1996, 13% of institutions used an individual scan delay technology and all institutions performed multiphasic scanning of hypervascular lesions.CONCLUSION:The availability of helical CT has changed radiologists' approach to liver imaging. The greatest effects of which are a more widespread use of power injectors, longer delay times, thinner collimation, increased contrast material injection rates, decreased contrast material volumes, and multiphasic scanning.

OBJECTIVE:The purpose of this study was to determine the accuracy of helical CT scanning in predicting the stage of carcinoma of the exocrine pancreas using TNM staging guidelines and in predicting resectability of carcinoma of the exocrine pancreas.MATERIALS AND METHODS:Twenty-six patients with proven adenocarcinoma of the pancreas underwent uniphasic or biphasic helical CT scanning. Two observers unaware of the patient's surgical stage evaluated the CT examinations using the TNM system (with specific assessment and description of disease sites). In addition, the two observers rated confidence of nonresectability using a 5-point scale (ranging from 1, definitely resectable, to 5, definitely not resectable). Observer results and preoperative interpretations were compared with surgical findings.RESULTS:Nineteen of 26 patients had nonresectable disease. The combined observer scores showed correct determination of T stage in 77% of patients, of N stage in 58%, and of M stage in 79%. The overall accuracy in determining lack of resectability was 96% and 84% for the two observers. All errors in determining resectable versus nonresectable disease occurred when the observer was not maximally confident of his or her diagnosis.CONCLUSION:Helical CT is an effective screening technique for assessing T and M stages of pancreatic carcinoma. However, helical CT is poor at detecting regional lymph node involvement. In patients with equivocal T-stage findings (such as questionable venous involvement), other studies such as endoscopic sonography may be of value.

Re: Ionic versus nonionic contrast media.P M SilvermanAudio Available | Share

OBJECTIVE:Evaluation of an approach to choosing left double-lumen tube size based on chest computed tomographic (CT) scan measurement of left bronchial diameter. DESIGN:Prospective. SETTING:The operating rooms of a university hospital. PARTICIPANTS:Patients scheduled for elective thoracic surgery. INTERVENTIONS:Patients had their left bronchial diameter measured on the preoperative chest CT scan. Left double-lumen tube size for the individual patient was chosen from a protocol based on left bronchial diameter. MEASUREMENTS AND MAIN RESULTS:The double-lumen tube size was considered appropriate for the patient if some air leak was detected when the bronchial cuff was deflated and if airtight seal of the left bronchus was obtained with a bronchial cuff volume of 2 mL or less. In 17 of 20 patients, the double-lumen tube size fulfilled both criteria. In 3 women with left bronchi measuring 10 mm or less, the bronchus was sealed without any air in the bronchial cuff of size 35 Fr left double-lumen tubes. In 1 patient, who was excluded from the study, the double-lumen tube size was chosen based on measurement of the left bronchial diameter on chest radiograph because of motion artifact on the chest CT scan. CONCLUSIONS:Chest CT scan measurement of left bronchial diameter can successfully guide the choice of left double-lumen tube size for an individual patient. In individuals with a small left bronchus measuring less than 10.0 mm in diameter, currently available adult double-lumen tube sizes will tightly wedge in their bronchus.

OBJECTIVE Routine scanning techniques used for helical CT of the abdomen result in dense cortical opacification of the kidney, whereas the medulla and collecting system are not well opacified, which potentially compromises detection of renal masses. The purpose of this retrospective study was to determine if additional delayed views (taken approximately 2-4 min after the start of injection of contrast material) are necessary for the detection and characterization of renal masses. MATERIALS AND METHODS Early (60-70 sec after the start of the injection of contrast material) and delayed scans of 40 patients with suspected renal masses were blindly evaluated by two observers. The patients harbored a total of 187 renal masses (including 62 solid masses). Each region of the kidney (upper, middle, and lower pole) was scored for the presence of a mass. Scoring was done as a binary decision and also as a five-point confidence score for receiver operating characteristic analysis. RESULTS We found 97 regions that contained renal masses and 114 regions that did not. Receiver operating characteristic analysis revealed the observers to have significantly greater confidence in detection of renal masses on the delayed scans. The binary data showed the two observers to have a sensitivity of 97% for delayed scans versus 77% (p = .0002) and 89% (p = .027), respectively, for the early scans. For the first observer, early and delayed scans were of equal specificity, but for the second observer, the delayed scans yielded greater specificity (94% versus 85%, p = .024). On the early scans, both observers were significantly more likely to miss a neoplastic lesion than a nonneoplastic lesion. The less experienced of the two observers also tended to have greater difficulty in characterizing the lesions on the early scans. CONCLUSION Because of the significant risk of missing a renal mass, especially a neoplasm, on early cortical-phase scans, additional delayed scans appear justified when a renal mass is suspected on the basis of other imaging tests or clinical history.

July 1996The misty mesentery on CT: differential diagnosis.Authors: R E Mindelzun, R B Jeffrey, Jr, M J Lane, and P M SilvermanAuthor Info & AffiliationsVolume 167, Issue 1https://doi.org/10.2214/ajr.167.1.8659422 METRICS PDFotherformats

Excellent vascular opacification, reduction in misregistration artifacts, and the option of reconstructing overlapping scans from which three-dimensional (3D) models of the abdominal vessels may be rendered are among the benefits of helical CT [1-4]. The purpose of this essay is to illustrate the findings of 3D rendering of helical CT data in patients with abdominal aortic aneurysms.

Optimal contrast enhancement of the liver using helical (spiral) CT: value of SmartPrep.P M Silverman, B Brown, H Wray, S H Fox, C Cooper, S Roberts and R K ZemanAudio Available | Share

Helical (spiral) computed tomography (CT) is having a dramatic impact on body imaging. Unlike conventional CT, helical CT provides continued volumetric acquisition as the patient moves through the gantry. Advantages of helical CT include dramatically shortened examination times, improved visibility of vascular structures, better enhancement of parenchymal organs, the capability for retrospective imaging and three-dimensional (3D) vascular studies, and potential reduction in use of contrast material. However, helical CT requires one to be more cognizant of the relationship between contrast material administration and scanning, since the optimal temporal window for detection of disease can be missed. Factors unique to helical technology can produce artifacts, which one must be aware of when interpreting helically generated scans. Many of these artifacts relate to accentuation of vascular or parenchymal enhancement. Others occur during production of high-quality 3D images. Additional artifacts are sure to be identified with increased experience with helical CT.

OBJECTIVES:The purpose of this study was to evaluate an automated computer technique (SmartPrep) for achieving a consistently high level of contrast enhancement in the liver with helical CT. The technique compensates for variability between patients by indicating graphically the time at which scanning should be initiated to reach a desired level of hepatic enhancement.MATERIALS AND METHODS:One hundred nine consecutive patients undergoing helical CT of the abdomen were randomly evaluated, using either a standard 70-sec delay from the start of the injection of contrast material to scanning or a newly developed, commercially available automated technique, SmartPrep. A series of multiple low-dose scans was performed until an arbitrary threshold of hepatic enhancement (50 H) over baseline was achieved. Three regions of interest (ROIs) were imaged on a baseline scan and on contrast-enhanced scans at the upper, mid, and lower liver. Average hepatic enhancement and the standard deviation over baseline was calculated for each group at all anatomic levels. For the SmartPrep group, the range of time between scan initiation and onset of scanning was calculated.RESULTS:The mean hepatic enhancement for the control group (n = 56) was 59.8 +/- 20.1 H, which differed significantly (p = .0002) from that for the SmartPrep group (n = 53), which was 71.6 +/- 15.2 H. Comparison of the variability between the two groups' enhancement levels was also significant (p = .02). The range of delay times for the SmartPrep group was 48-86 sec. In two additional cases, abnormal graphically displayed enhancement curves were the first indication of an improper injection.CONCLUSION:Use of SmartPrep yields a greater and more consistent level of hepatic enhancement from patient to patient than does use of a conventional fixed delay time. The ability to scan more efficiently to achieve greater hepatic enhancement using SmartPrep has significant implications for potential contrast cost savings.

Helical scanning offers many advantages for the evaluation of benign and malignant pancreatobiliary disease. This article presents these advantages, recommended screening protocols, and guidelines for the use of three-dimensional rendering of the peripancreatic vessels and bile ducts.

OBJECTIVE. Twenty-three patients with suspected aortic dissection were evaluated in this preliminary study of helical CT to determine the usefulness of axial sections, multiplanar reformation, and three-dimensional (3D) rendering in assessing the presence of dissection and the extent of intimal flap.MATERIALS AND METHODS. Patients were referred for helical CT scanning because of chest pain or an abnormal chest radiograph. Scans were performed during bolus injection of nonionic contrast material at 2.0-2.5 ml/sec using a mean scan delay of 47 sec, Axial scans with 5-mm collimation were obtained in all patients. They extended from the great vessels to the distal thoracic aorta just above the hiatus. Delayed nonhelical sections were obtained through the upper abdomen. Multiplanar reformations and 3D models were reconstructed from the helical data in 13 patients and were compared to axial sections in 7 patients who proved to have documented dissection. The efficacy of CT was determined using surgery, angiography, or clinical outcome to establish the diagnosis.RESULTS. Of the 23 patients studied, axial sections resulted in 15 true-negative, 7 true-positive, and 1 false-positive interpretation. In three of seven patients with dissection, it was difficult to determine the extent of the intimal flap on axial sections; multiplanar reformation or 3D views clarified the relevant anatomy in all 3 cases. Among the 3D display methods, ray-sum projection views were superior to surface model or maximum-intensity-projection views.CONCLUSION. If studies of larger numbers of patients confirm our preliminary findings, multiplanar reformation and 3D rendering of helical CT scans will be a valuable addition to axial display of CT studies used to detect aortic dissection and to determine the extent of the intimal flap.

Imaging of the hypopharynx, larynx, and upper airway are effectively achieved with CT and MR imaging. These techniques have proved their diagnostic usefulness in assessing the deep soft tissues not visible with laryngoscopy [1]. However, with axial imaging, large numbers of images often need to be mentally stacked to envision the appearance of the airway. With helical CT, we can create high-quality three-dimensional (3D) reconstructions [2, 3]. Advantages of helical technology include rapid scanning, decreased motion artifact, and minimization of misregistration artifacts. Recent work has suggested a role for multiplanar and 3D reconstructions of helical data for assessing the tracheobronchial tree [3]. The helically derived 3D models illustrate the normal and abnormal findings affecting the airway.

A new computed tomography (CT) technology, helical (spiral) CT, allows the entire neck to be imaged in only 30 seconds. Although multiplanar and three-dimensional (3-D) imaging could be performed with conventional CT, the volumetric acquisition provided by helical (spiral) CT allows significantly improved quality and easier reconstruction for more applications. These 3-D models show an airway appearance similar to that obtained with laryngography. Independent review of the 3-D images in 12 patients with lesions by two radiologists and one otolaryngologist was performed to assess 1) image quality, 2) ability to judge lesion extent, and 3) assistance in understanding the lesion compared to that provided by routine axial scans. Rating scores of 1 to 5 were assigned, with 5 representing the best quality or greatest value. The results showed that both groups scored image quality equally: 4.7. Lesion extent for the radiologists was 2.6, while the otolaryngologist's ranking was 3.7 (p < .01). In assisting understanding of lesions versus axial scans, radiologists ranked 3-D images 2.1, while the otolaryngologist ranked them 3.1 (p < .01). In summary, 3-D models provide a complementary imaging technique in understanding upper airway disease.

Objective: Scanning protocols for conventional CT of the liver have been proposed. Current availability of helical CT with a four- to sixfold decrease in scan time requires significant adjustments in these protocols. The present study assesses the implications of time-density curves on the performance of helical liver CT.Materials and Methods: Twenty patients without liver lesions were studied for time-density analysis of the aorta, inferior vena cava (IVC), portal vein, and liver. Scans were performed at the level of the portal vein at baseline and every 15 s for 3 min following uniphasic administration of 150 ml (300 mg I/ml) nonionic contrast agent. Regions of interest were used to measure three areas in each anatomic structure over time. Median and mean peak enhancement times were calculated for all 20 patients. Cubic spline interpretation was employed to determine the point of equilibriumResults: Results demonstrated the following average maximum enhancement values and times for peak enhancement: aorta: 227 HU (75 s); liver: 123 HU (105 s); portal vein: 187 HU (90 s); IVC: 142 HU (90 s). Hepatic enhancement achieved 67 HU over baseline. Peak portal enhancement occurred 15 s prior to liver enhancement (p = 0.001). Aortic and hepatic curves became parallel (onset of equilibrium) at a median time of 120 s.Conclusion: Helical scanning requires a longer delay (70-80 s) than used for conventional CT. Upon application of these principles, scan initiation occurs higher on the liver enhancement curve, improving liver enhancement without impinging on equilibrium.

OBJECTIVE The purpose of this study was to compare two time delays between injection of contrast material and helical CT scanning to determine relative conspicuity of hepatic metastases. SUBJECTS AND METHODS Twenty-five patients with hepatic metastases were examined with helical CT. The first imaging phase was initiated at 50 sec and the second 75 sec after the start of contrast material injection (3 ml/sec, 150 ml). Differences in lesion and liver attenuation were measured quantitatively. Four radiologists used a 5-point scale to assess lesion conspicuity subjectively. RESULTS Mean differences in enhancement between liver and lesion were 41 H during the first phase and 59 H for the second phase (p = .0001). Radiologists' conspicuity score averaged 2.4 for lesions in the first phase versus 3.3 for lesions in the second phase (p = .0001). In 56 (88%) of 64 lesions, objective measurements showed greater enhancement of lesions during the later phase. Radiologists found 60 (94%) of 64 lesions to be more conspicuous on these later images. CONCLUSION Our results show that conspicuity of hepatic metastases on helical CT scans is better with a 75-sec scan delay between contrast administration and scanning than with a 50-sec scan delay. The longer delay time should be used when scanning is used to detect metastases.