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Gastrointestinal Imaging |
1 From the Departments of Radiology (R.C.S., H.B.M., M.A.M.) and Surgery (W.G.C.), University of North Carolina at Chapel Hill, CB 7510, Chapel Hill, NC 27599-7510. Received September 4, 1998; revision requested October 26; final revision received January 7, 1999; accepted April 28. Address reprint requests to R.C.S. (e-mail: richsem@med.unc.edu).
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Abstract |
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 TOP Abstract IntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MATERIALS AND METHODS: Twenty patients underwent spiral CTAP and MR imaging within 1 week. Spiral CTAP and MR images were interpreted separately in blinded fashion. All patients subsequently had intraoperative confirmation. Sensitivity, specificity, and positive and negative predictive values were determined for lesion detection and segmental distribution.
RESULTS: CTAP and MR images demonstrated, respectively, 54 and 60 true-positive lesions, six and one false-positive lesions, 15 and 22 true-negative (ie, benign) lesions, and eight and two false-negative lesions. CTAP and MR images demonstrated, respectively, 57 and 62 true-positive segmental involvements, six and one false-positive segmental involvements, 89 and 95 true-negative segmental involvements, and eight and two false-negative segmental involvements. No significant difference in lesion detection was observed.
CONCLUSION: Spiral CTAP and MR imaging were approximately equivalent for lesion detection in patients who were evaluated preoperatively for resection of liver metastases. The lower cost and fewer problems with artifacts may suggest that MR imaging is the preferred modality for preoperative assessment of patients for surgical treatment of liver metastases.
Index terms: Computed tomography (CT), comparative studies • Liver neoplasms, CT, 761.12114, 761.12115 • Liver neoplasms, metastases, 761.33 • Liver neoplasms, MR, 761.121411, 761.121412, 761.121415, 761.12143 • Magnetic resonance (MR), comparative studies • Portography, 95.1242
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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With the concomitant increase in medical expenditure, analysis of the imaging modalities for the detection of liver lesions has become necessary. Clinical investigators no longer focus solely on methods for improving lesion detection; they also address justification of these modalities in light of their expense. Advances in diagnostic imaging have presumably contributed to improved survival, with the accurate identification of patients with limited disease (9,10). This requires accurate determination of the number, locations, and sizes of all primary or metastatic lesions.
Findings from prior studies (11–14) have suggested that computed tomography (CT) during arterial portography (CTAP), particularly with the spiral technique, is sensitive for the detection of liver metastases. Recent studies have also shown that current magnetic resonance (MR) imaging—with hepatic arterial–dominant phase, spoiled gradient–echo (GRE) techniques and enhancement with a gadolinium-based contrast agent (4) or another contrast agent (15) or with T2-weighted fat-saturated spin-echo techniques—may be equivalent or superior to CTAP for the detection of liver metastases.
In a prior article (4), we reported that MR imaging in the hepatic arterial–dominant phase with gadolinium enhancement has a sensitivity similar to that of spiral CTAP for lesion detection, with a greater specificity. However, this study was performed in a population of patients the majority of whom did not undergo surgery. The lack of surgical confirmation rendered the comparison to one of relative performance of the two modalities, and, therefore, it did not reflect absolute lesion detection. The purpose of the current study was to evaluate the prospective performance of spiral CTAP and MR imaging for the detection of liver metastases in patients who subsequently underwent surgery, to determine absolute lesion detection with the two imaging modalities.
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MATERIALS AND METHODS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Imaging
CTAP.—CTAP was performed with a Somatom Plus S CT scanner (Siemens Medical Systems, Iselin, NJ) by using an iodinated contrast material and a spiral imaging technique. One signal was acquired with a 22-second breath hold, an 8-mm collimation, an 8-mm/sec table speed, and an 8-mm reconstruction interval. Patients fasted for approximately 6 hours prior to imaging. Spiral imaging of the entire liver was performed during the administration of contrast material (Omnipaque 300 [iohexol]; Nycomed Amersham, Princeton, NJ) at a rate of 1.5 mL/sec into the superior mesenteric or splenic artery with a Mark IV power injector (Medrad, Indianola, Pa). The spiral acquisition was initiated 45 seconds after the start of the administration of contrast material.
MR imaging.—MR studies were performed with a 1.5-T MR imager (SP 4000 or Vision; Siemens Medical Systems). A phased-array multicoil was used in six patients. Patients fasted for approximately 6 hours prior to imaging; no other patient preparation was performed. Gadolinium-based contrast material, administered intravenously, was used in conjunction with a breath-hold spoiled GRE sequence in all patients. The spoiled GRE sequence was used to acquire images that encompassed the entire liver during a single breath–hold pass. Precontrast T1-weighted spoiled GRE images were acquired in the transverse and coronal planes, and T2-weighted fat-saturated turbo spin-echo or spin-echo images were acquired in the transverse plane. In addition, coronal T2-weighted half-Fourier rapid acquisition with relaxation enhancement (RARE [HASTE; Siemens Medical Systems]) images were acquired in three patients.
The imaging parameters for T1-weighted spoiled GRE imaging were as follows: 140–170/4.1 (repetition time msec/echo time msec), 80° flip angle, 20% gap, one signal acquired, 8–10-mm section thickness, 144–190 x 256 matrix, and 14 sections imaged during a 22-second breath-hold. The parameters for T2-weighted fat-saturated turbo spin-echo imaging were 4,000/90 and an echo train length of five. Parameters for T2-weighted half-Fourier RARE imaging were 
/103 (effective) and an echo train length of 104. Gadolinium chelates (Magnevist; Berlex Laboratories, Wayne, NJ) were injected at a dose of 0.1 mmol per kilogram of body weight, and images were acquired immediately, at 45 seconds, at 90 seconds, and at 5 minutes after administration. Image acquisition immediately after the administration of contrast material was performed in the hepatic arterial–dominant phase, which was defined as the enhancement phase in which contrast material is present in portal veins and hepatic arteries and not present in hepatic veins. Details of this imaging protocol have been described previously (4).
Image Interpretation and Data Analysis
CTAP and MR images were interpreted prospectively and separately by two experienced investigators in blinded fashion (CTAP images, by M.A.M.; MR images, by R.C.S.). Lesion detection and segmental location of liver metastases were determined. Segmental anatomy used in this study was based on the system of Couinard (16). Liver lesions that were correctly characterized as metastases with either modality were enumerated as true-positive lesions. Liver lesions that were correctly characterized as benign lesions (eg, cyst, hemangioma) with either imaging modality were enumerated as true-negative lesions. Perfusion defects were not enumerated as true-negative lesions. False-negative lesions were all lesions that were missed at examination with either modality in this study. Malignant lesions misclassified as benign lesions were enumerated as false-negative lesions.
Findings at CTAP and MR imaging were correlated separately with the surgical findings provided at consultation with the surgeon (W.G.C.) and obtained by reviewing surgery reports, which were correlated with findings from histopathologic specimens. Intraoperative ultrasonographic (US) examinations were performed in nine patients, and the results were correlated with surgical findings. Intraoperative US was performed by a radiologist at the request of the surgeons.
Sensitivity, specificity, positive and negative predictive values, and accuracy were compared on a patient-by-patient basis. Each of the measures was calculated for MR and CTAP separately for each patient. The differences between MR and CTAP were computed for each patient, and the average of the differences for each measure was tested to determine whether it was significantly different from zero. The Wilcoxon signed rank test was used since the two conditions in a paired-data structure were compared for a small population size. A significant difference was considered present when the P value was equal to .05.
In our patients, the surgical plans were designed to select the patients with suspected limited malignant liver disease with no contraindications to surgery. Evaluation at CTAP and MR imaging for exploratory laparotomy was performed to determine if patients were candidates for hepatic resection versus arterial infusion pump placement. Surgical interventions in our patient population included the following: partial hepatectomy (n = 2), right lobectomy (n = 5), exploratory laparotomy (n = 3), or exploratory laparotomy and cryotherapy (n = 2), exploratory laparotomy and cryotherapy with wedge resection (n = 3), exploratory laparotomy and wedge resection or liver biopsy (n = 5). Findings at histopathologic examination of resected specimens confirmed the diagnoses of metastatic adenocarcinomas from primary colorectal cancers (Tables 1, 2).
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Seven low-attenuation wedge defects were shown on CTAP images, and all were considered to be perfusion defects. One of these resulted in the detection of a false-negative lesion (Fig 1), and two resulted in the detection of a false-positive lesion (Fig 2). These seven perfusion abnormalities were also appreciated at MR imaging as regions of transient increased enhancement on the immediate postgadolinium spoiled GRE images. No false-positive or false-negative lesions due to perfusion abnormalities were detected at MR imaging. Five benign lesions considered to be metastases at CTAP (Fig 3) were correctly characterized at MR imaging. The two lesions missed at MR imaging and found at surgery in two patients were both superficial subcapsular lesions (<1 cm) that were detected on palpation at surgery prior to intraoperative US. These two lesions were also missed at CTAP. The other additional lesion missed at CTAP was detected at intraoperative US.
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Lesion detection was confirmed by means of surgical examination and examination of resection specimens in seven patients who underwent curative surgical procedures (Table 1) and in 13 patients who underwent palliative surgical procedures (Table 2).
In three patients, the findings at MR imaging, compared with the findings at CTAP, altered the treatment of patients. Two patients who were not considered to be candidates for curative surgery at CTAP underwent curative resection, and one patient who was considered to have a curable disease at CTAP had more extensive disease that required palliative surgery, as was shown at MR imaging. In no patient did the findings at CTAP, compared with the findings at MR imaging, alter patient treatment.
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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MR imaging, with our protocol, was technically easy to perform and suffered from fewer artifacts. Technical difficulties with CTAP included the following: (a) the need for vascular catheterization and (b) the possibility that the rate, volume, and timing of contrast medium administration and the time of image acquisition may not have been optimized.
The major image artifact with CTAP is the presence of perfusion defects, which may interfere with lesion detection (4). The major problem with MR imaging is the need for patients to suspend respiration for 22 seconds, which is also required for spiral CTAP. A correct timing for contrast medium administration is also essential to capture the hepatic arterial–dominant phase in a reproducible fashion.
In our previous article (4), we reported that the charges for MR imaging ($1,274 [1993 billing codes]) are substantially less than the charges for CTAP ($3,499 [1997 billing codes]), while at the same time, MR imaging demonstrates a greater effect on patient treatment.
A critical aspect of the high diagnostic accuracy of MR imaging is the use of current technique. We consider it essential to use a breath-hold spoiled GRE technique, with full liver coverage in one pass, in conjunction with dynamic administration of the gadolinium-based contrast material and image acquisition in the hepatic arterial–dominant phase of enhancement. Optimization of the technique is important when comparison studies with other imaging modalities are evaluated.
In our current practice, we also routinely use a phased-array multicoil, which we consider to be important to achieve high-quality diagnostic images. Occasional mirror artifacts of the aorta are transposed over the left lobe of the liver. The periodicity of this artifact is lessened by using a relatively long repetition time of 150 msec, as we routinely do.
In addition, we use the precontrast coronal spoiled GRE images to evaluate the regions of the left lobe that may be obscured by flow artifact. We have found that the addition of this coronal plane sequence provides more information than does an additional spoiled GRE sequence in which the phase and frequency have been swapped. The major strength of MR imaging over CTAP was higher specificity in that perfusion abnormalities were consistently recognized as perfusion defects and not lesions, and lesions approximately 1 cm in diameter were clearly characterized on MR images as benign or malignant.
One rationale for the continued performance of CTAP is that it may be performed in the same setting with conventional angiography to delineate the hepatic arterial anatomy. However, at our institution, CTAP is now rarely performed because of the results of our institutional comparison between MR imaging and CTAP. Many surgeons believe that the knowledge of hepatic arterial anatomy is important preoperative information, particularly if they are planning to place an intraarterial pump for the delivery of chemotherapy or if they are planning to perform a segmental lobectomy. However, the need for catheter angiography may not be absolute.
Other alternatives include combining a conventional angiogram with MR arterial portography or performing MR angiography as part of the MR examination by using an intravenous injection of gadolinium-based contrast material. Dravid et al (17) have shown that MR arterial portography with an injection of a gadolinium-based contrast material is more sensitive for the detection of liver lesions than is CTAP with an injection of an iodinated contrast material. The substitution of MR arterial portography for CTAP may be reasonable, as is the use of MR arterial portography in conjunction with conventional angiography. MR angiography has also advanced dramatically in the past 2 years; major branches of the abdominal aorta may be demonstrated virtually as well with MR angiography as with conventional angiography (18). MR angiography may not be at the point at which it can reproducibly demonstrate medium-to-small hepatic arterial branches.
Advances in organ-specific contrast agents, such as ferumoxides (15), and newer sequence designs (19) suggest that further improvement in the performance of MR imaging is anticipated in the near future. Ferumoxides is a tissue-specific contrast agent for MR imaging that has been used for the detection of hepatic metastases. Findings from one prior study (15) showed that ferumoxides-enhanced MR imaging may be as sensitive as CTAP for the detection of hepatic metastases. Therefore, we would expect even greater advantages with MR imaging than with CTAP over the next few years.
In three patients, findings at MR imaging altered patient treatment, compared with the findings at CTAP, whereas in no patient was the reverse observed. This corresponded to the findings from our previous study (4).
The current study was limited by the relatively small number of patients included. A larger multiinstitutional study is required to definitively confirm the greater accuracy of MR imaging over spiral CTAP. Another limitation of the study was that standardized comparison with intraoperative US was not performed. This reflected the fact that intraoperative US was performed in only nine of 20 patients and that interpretation of findings from intraoperative US was not blinded.
In conclusion, we have shown that MR imaging and CTAP were both sensitive and approximately equivalent in the detection of liver metastases. MR imaging appeared to be more accurate as it characterized lesions well and did not suffer from problems with perfusion defects. The previously reported lesser charges and greater effect on patient treatment are further compelling reasons to adopt MR imaging over CTAP (4). Also, further advances in contrast agents and imaging techniques are ongoing with MR imaging.
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Footnotes |
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Abbreviations: CTAP = CT during arterial portography GRE = gradient echo RARE = rapid acquisition with relaxation enhancement
Author contributions: Guarantor of integrity of entire study, R.C.S.; study concepts, R.C.S., H.B.M.; study design, H.B.M.; definition of intellectual content, H.B.M., R.C.S.; literature research, H.B.M.; clinical studies, W.G.C., R.C.S., H.B.M.; data acquisition and analysis, R.C.S., H.B.M.; statistical analysis, H.B.M.; manuscript preparation and editing, R.C.S., H.B.M.; manuscript review, all authors.
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