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Original Research |
1 Department of Radiology, Mayo Clinic, 200 First St. SW, Rochester, MN 55905.
Received October 23, 2006;
accepted after revision April 19, 2007.
Address correspondence to J. F. Glockner
(glockner.james{at}mayo.edu).
Abstract
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MATERIALS AND METHODS. Reports from 380 consecutive renal MR angiography examinations performed at a single institution over a 12-month interval were examined. The presence of incidental vascular (i.e., nonrenal artery) and nonvascular findings was noted. Clinical records of patients with significant incidental findings were examined to determine whether additional imaging, biopsy, or surgery was performed.
RESULTS. Overall, 151 (40%) of 380 patients had one or more additional vascular findings not related to the renal arteries, and 221 (58%) of 380 patients had one or more additional nonvascular findings. Vascular findings included mesenteric artery stenosis or occlusion in 33% of patients, moderate to severe aortic atherosclerosis in 17%, aortic aneurysms in 7%, and aortic dissection in 2%. Incidental malignancies were detected in 10 patients (3%), and indeterminate lesions requiring follow-up imaging, biopsy, or surgery were noted in 18 patients (5%). Overall, management in 5% of patients was significantly altered (i.e., required biopsy, surgery, or other intervention) by incidental findings detected on renal MR angiography. Benign lesions not requiring additional imaging or follow-up occurred in 54% of patients and consisted predominantly of renal cysts.
CONCLUSION. Incidental findings on renal MR angiography are common. Most incidental lesions can be adequately detected and characterized with the addition of a few pulse sequences to the standard renal MR angiography protocol at a minimal cost in imaging time. The high incidence of incidental findings emphasizes the importance of performance and interpretation of these examinations by physicians with training in abdominal cross-sectional imaging.
Keywords: incidental findings • kidney • MR angiography
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The incidence and importance of incidental findings on renal MR angiography is not generally known, and the purpose of this study is to determine the frequency of these findings, categorize their importance, and assess their impact on patient treatment.
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Reports were evaluated for the following information: presence or absence of significant (> 50% diameter luminal narrowing, or moderate to severe) renal artery stenosis and significant vascular disease involving the abdominal aorta, iliac arteries, and mesenteric arteries. Incidental nonvascular findings were also noted. Patient age and sex were recorded. Patient records were assessed to determine whether subsequent intervention was performed for renal artery stenosis or other vascular findings. Records were also examined in cases in which an incidental finding of significance was noted to determine whether additional diagnostic imaging was performed and whether subsequent surgery or biopsy was performed and, if so, the pathologic diagnosis.
Patient records were evaluated to determine whether cross-sectional imaging (sonography, CT, or MRI) had been performed within 1 year before or after renal MR angiography. Reports of these examinations were then examined for discrepancies with the MR angiography reports. Vascular discrepancies in renal sonography were noted for renal arteries and the aorta; mesenteric arteries were generally not examined on these studies. These findings were correlated with the results of renal MR angiography and conventional renal angiography when available. In cases in which nonvascular discrepancies were noted, the MR angiography and additional examinations were reviewed by the author to assess which report was correct.
The final 50 renal MR angiography examinations were assessed to determine the average examination time and the percentage of the examination required to obtain the sequences not directly related to MR angiography (single-shot fast spin-echo, in-phase and out-of-phase spoiled gradient-echo, and fat-saturated contrast-enhanced spoiled gradient-echo sequences). This was done by noting the time stamp on each sequence, which occurs at the initiation of the sequence. The total examination time represented the time difference between the first and last acquisitions plus the acquisition time of the last sequence. The acquisition time for a given pulse sequence (including prescription, download, and time before scanning) was estimated by the time from the end of the previous sequence plus the acquisition time of the given sequence.
All renal MR angiography examinations were performed on a 1.5-T TwinSpeed EXCITE system (GE Healthcare). The renal MR angiography examination consisted of a three-plane localizer acquired using either a single-shot fast spin-echo or a fast spoiled gradient-echo sequence. This was followed by coronal single-shot fast spin-echo images (TE, 80; bandwidth, 83 kHz; flip angle, 110°; matrix, 256 x 256; 0.5 excitations; 5-mm slice thickness with 1-mm gap; 40- to 44-cm field of view). Axial in-phase and out-of-phase spoiled gradient-echo images were then obtained, with coverage including the adrenals and kidneys (TR range/first-echo TE, second-echo TE, 100–200/2.1, 4.2; flip angle, 70°; bandwidth, 32 kHz; matrix, 256 x 192; 1 excitation; 6-mm slice thickness with 1-mm gap; field of view, 32–44 cm, with 0.75 partial-phase field of view). Renal MR angiography was performed using a coronal oblique 3D spoiled gradient-echo sequence with elliptic centric view ordering and the following parameters: 26- to 30-cm field of view; 0.75 phase field of view; 256 x 224 in-plane matrix; 1.6-mm section thickness; 35° flip angle; 62- to 83-kHz bandwidth; and TR/TE, 3.5/1.4. Typically 40 sections were acquired, and zero filling was performed to obtain a reconstructed 512 x 512 in-plane matrix with 50% overlapping reconstructions along the z-axis. Parameters were adjusted according to patient size and breath-hold capacity, with acquisition times generally in the range of 16–22 seconds.
A contrast dose of 0.1–0.2 mmol/L/kg (gadodiamide [Omniscan, GE Healthcare]) was injected at 3 mL/s with an automatic injector (Spectris Solaris, Medrad) and a scanning delay time was selected on the basis of a preceding test bolus acquisition using 1–2 mL of contrast material. A single arterial phase MR angiography acquisition was acquired without additional venous phase imaging. After MR angiography, an axial fat-saturated 2D spoiled gradient-echo sequence was performed (TR range/TE, 100–200/2.6; flip angle, 70°; bandwidth, 32 kHz; matrix, 256 x 192; 0.75 excitations; 32- to 44-cm field of view with 0.75 phase field of view; 6-mm slice thickness; and 1-mm gap).
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Additional Vascular Findings
Significant vascular findings outside the renal artery territory were
common (Figs. 1,
2A, and
2B). One hundred twenty-seven
patients (33%) had significant stenosis (> 50%) or occlusion of one or more
mesenteric arteries, consisting of 31 patients with celiac stenosis or
occlusion, 12 patients with superior mesenteric artery stenosis or occlusion,
39 patients with inferior mesenteric artery stenosis or occlusion, and 45
patients with stenosis or occlusion of more than one mesenteric artery. These
lesions were known in 26 patients and unknown in 101 patients. Sixty-six
patients had moderate to severe aortic arteriosclerosis (as described in MR
angiography reports), 25 had abdominal aortic aneurysms measuring greater than
3 cm in diameter (14 known and 11 unknown), 16 had penetrating atheromatous
ulcers (3 known, 13 unknown), and nine had dissections of the aorta or
mesenteric vessels (3 known, 6 unknown)
(Table 1).
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Four patients underwent subsequent surgical repair of abdominal aortic aneurysms. In two cases the aneurysm was initially detected on renal MR angiography, and in two cases the aneurysm had significantly increased in diameter since previous examinations. One patient underwent subsequent angiography and stent placement in the superior mesenteric artery, and another patient underwent angiography and stent placement in the right external iliac artery (both of these findings were initially detected on renal MR angiography).
Nonvascular Findings
Ten patients had malignancies detected
(Table 2), eight of which were
unknown before renal MR angiography. These included renal cell carcinoma in
three patients; lymphoma in two patients; and metastatic adenocarcinoma from
an unknown primary source, small cell lung carcinoma, pancreatic
adenocarcinoma, pancreatic islet cell carcinoma, and pheochromocytoma in one
patient each (Figs. 3A,
3B,
4A, and
4B).
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Indeterminate lesions requiring biopsy or follow-up imaging (as noted in the MR angiography report) were detected in 18 patients (Table 3). In five patients biopsy or surgery was performed, with a resulting benign diagnosis. These included an atypical adrenal adenoma (without signal dropout on out-of-phase images), hepatic focal nodular hyperplasia, sarcoidosis in a patient with retroperitoneal adenopathy, sclerosing mesenteritis, and ovarian serous cystadenoma (Figs. 5A and 5B). In 13 patients, lesions requiring follow-up imaging were detected, including moderately complex renal cysts in four patients (follow-up MRI performed at 6 months in three patients revealed no significant change; the fourth patient had no follow-up imaging performed at our institution), ovarian cysts in two postmenopausal patients (serial pelvic sonography was performed in one patient, no additional imaging in the second patient), small pancreatic cystic lesions in five patients (two patients had follow-up MRI without significant change), a possible pancreatic lesion in one patient (no abnormality detected on follow-up CT), and an indeterminate adrenal lesion with no follow-up imaging in one patient.
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Benign lesions requiring no additional investigation were detected in 207 patients, or 54% (many patients had more than one type of benign lesion). Renal cysts were by far the most common lesion, detected in 176 patients (46%). Other benign lesions included hepatic and splenic cysts, hepatic and splenic hemangiomas, adrenal adenomas, hepatic focal nodular hyperplasia, and renal angiomyolipomas (Table 4). Significant findings not involving a focal mass or lesion were detected in 32 patients, including biliary and pancreatic ductal dilatation, moderate to large pleural effusions, moderate ascites, and pulmonary atelectasis or consolidation (Table 4). Miscellaneous findings of minor importance included cholelithiasis, hepatic fatty infiltration, ventral hernia, and transplant-related lymphoceles (Table 4). Renal findings related to renovascular disease were noted in 60 patients, including renal scarring or atrophy and renal infarcts.
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Overall, 151 (40%) of 380 patients had one or more additional vascular findings not related to the renal arteries, and 221 (58%) of 380 patients had one or more additional nonvascular findings. Thirteen patients underwent biopsy or surgery for an incidental nonvascular finding not previously detected, and six patients underwent surgical or interventional treatment of nonrenal vascular lesions initially detected or worsening on renal MR angiography, resulting in 19 patients (5%) with significant changes in management after detection of incidental findings on renal MR angiography.
Additional Imaging and Discrepancies
Of the 380 renal MR angiography patients, 192 had no additional
cross-sectional imaging performed within 1 year of the MR angiography
examination. One hundred eighty-eight patients (49%) had one or more
additional cross-sectional examinations, as follows: 129 renal duplex
sonography examinations, 26 abdominal or pelvic sonography, 64 abdominal or
abdominopelvic CT (38 without IV contrast material), and five abdominal MRI.
These numbers include examinations performed for further evaluation of
findings detected on renal MR angiography.
Forty-eight nonvascular discrepancies were noted in reports of MR angiography and other cross-sectional examinations in 45 patients. Twenty-two of these discrepancies were findings missed or not visible on MR angiography. These included 10 patients with nonobstructive renal calculi seen on sonography or CT and not visualized on MR angiography, three patients with cholelithiasis present on CT or sonography and in retrospect present on MR angiography, two patients with adrenal adenomas detected on CT and in retrospect present on MR angiography, two patients with hepatic steatosis on abdominal sonography and in retrospect present on MR angiography, two patients with probable hepatic focal nodular hyperplasia on CT not detected on MR angiography, one patient with a possible pancreatic mass on MR angiography not detected on CT, one patient with a small peritransplant fluid collection on renal sonography and in retrospect present on MR angiography, and one patient with a subcentimeter renal angiomyolipoma on renal sonography and in retrospect present on MR angiography.
Eleven nonvascular discrepancies represented missed or undetected findings on CT. Eight patients had renal cysts on MR angiography that were not reported on CT (unenhanced CT in five patients), one patient had an adrenal adenoma described on MR angiography and in retrospect present on CT, one patient had a dilated common bile duct in retrospect present on CT, and one patient with a splenic hemangioma reported on CT and not detected on MR angiography that in retrospect probably represented normal late arterial phase perfusion. Eleven nonvascular discrepancies represented missed or undetected findings on sonography: seven patients with renal cysts on MR angiography not reported on sonography, two patients with minimal abdominal ascites in retrospect present on abdominal sonography, one patient with a hepatic hemangioma not detected on abdominal sonography, and one patient with a renal angiomyolipoma reported on renal sonography that was not seen on MR angiography and in retrospect likely represented fat in a cortical defect. Extrarenal lesions not noted on renal sonography were not considered missed.
Thirty-eight renal vascular discrepancies in 33 patients were noted between renal duplex sonography and renal MR angiography reports. In 15 patients, multiple renal arteries were detected on MR angiography and not described on renal sonography (angiography performed in four of these patients confirmed the MR angiography reports). In 18 patients, renal MR angiography detected moderate to severe stenosis of one or more renal arteries not described on sonography (angiography performed in seven patients confirmed the MR angiography diagnosis in six cases and sonography results in one case). In five patients, significant renal artery stenosis was detected on sonography but not on MR angiography. Renal angiography performed in one case confirmed the MR angiography findings. No CT vascular discrepancies were noted, although renal vascular findings were described in only three cases, and a significant percentage of the CT examinations were performed without contrast material.
Renal MR Angiography Examination Times
The 50 final renal MR angiography examinations evaluated had an average
total examination time of 21 ± 9 minutes, with minimal and maximal
values of 10 and 62 minutes. These numbers underestimate the true examination
times because they do not include time spent on initial patient positioning or
patient removal at the end of the examination. Estimation of the time per
sequence is also limited by the fact that some sequences were acquired in two
or more breath-holds, and therefore the true acquisition time for a particular
sequence could be longer than the calculated time. The average time spent on
non–MR angiography sequences was 10 ± 4 minutes, with maximal and
minimal values of 4 and 26 minutes. The average percentage of the examination
time occupied by non–MR angiography sequences was 57% ± 16%.
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On the other hand, incidental findings in renal MR angiography are common. Although most vascular lesions are adequately depicted on 3D contrast-enhanced renal MR angiography, nonvascular findings often require additional sequences for visualization and characterization. To some extent this is a reflection of choices made in our protocol: A small field of view with considerable wrap-around artifact is used to maximize spatial resolution, and only arterial phase images are acquired (both arterial and venous phase images are acquired in some protocols). Although most incidental nonvascular findings are benign, predominantly consisting of renal and hepatic cysts, these are often not completely characterized on the basis of scout images and the MR angiography sequence alone, and therefore might require additional imaging to classify as benign.
The additional sequences, then, are important not only to detect significant lesions but also to identify benign lesions and thereby avoid the expense of additional imaging. In-phase and out-of-phase spoiled gradient-echo sequences, for example, allow characterization of incidental adrenal adenomas without the need for follow-up imaging. The combination of T2-weighted single-shot fast spin-echo images and contrast-enhanced fat-saturated T1-weighted spoiled gradient-echo images allows confident identification of simple cysts. Hepatic hemangiomas could also be identified with confidence in most cases, on the basis of high signal intensity on T2-weighted images and uniform enhancement on contrast-enhanced spoiled gradient-echo images. Unenhanced T1-weighted spoiled gradient-echo images were not obtained in our protocol but could be helpful to assess for enhancement in complex lesions in the kidneys and elsewhere—for example, hemorrhagic cysts. T2-weighted images, in addition to identifying cysts and hemangiomas, also help to detect gallstones, dilated biliary and pancreatic ducts, and renal collecting system obstruction. All of the additional non–MR angiography sequences performed in our protocol are short acquisitions, obtained in one or two breath-holds.
The non–MR angiography pulse sequences acquired in the renal MR angiography protocol required a relatively small increment in additional imaging time (10 minutes on average); however, they constituted a significant percentage (57%) of the total examination time. This is a reflection of the short average examination time of 21 minutes (an underestimation of the true examination time, as noted previously).
Indeterminate or malignant lesions were often characterized with relatively little additional imaging. The incidentally discovered renal cell carcinomas, for example, proceeded to surgery without additional imaging (except unenhanced chest CT in one patient). This was also the case for the patient with pheochromocytoma. Imaging-guided biopsy was performed after renal MR angiography in one patient with lymphoma and in patients with metastatic pancreatic adenocarcinoma and metastatic adenocarcinoma from an unknown primary source. However, additional imaging was required in several patients, and biopsy or surgery was performed on benign lesions in five patients. Furthermore, in our practice, a staff radiologist, resident, or fellow is on-site and reviews all examinations before the patient is dismissed. If an indeterminate lesion or other finding is noted, additional imaging can be performed immediately, a luxury that is not available in many private practice groups and some academic centers.
The question of whether significant findings were missed or incorrectly characterized by renal MR angiography was only partially addressed in this study. In those patients with additional cross-sectional studies performed within 1 year of renal MR angiography, a relatively small number of discrepancies were noted, and many of the findings not detected on the initial MR angiography interpretation were present in retrospect. However, renal duplex sonography constituted a significant percentage of the additional cross-sectional examinations (58%), and these studies provided no information regarding extrarenal incidental findings because only the kidneys and renal arteries were evaluated.
Several studies have examined the incidence and importance of incidental findings in other abdominopelvic imaging techniques [7–19], most notably CT colonography and CT for suspected renal colic. Yee et al. [7], for example, examined 500 men undergoing CT colonography, finding extracolonic findings in 63% of patients and clinically significant findings in 9%. Gluecker et al. [8] evaluated 681 asymptomatic patients undergoing screening colonoscopy and concurrent CT colonography and found a 10% incidence of extracolonic findings of high importance. Xiong et al. [9] reviewed 17 CT colonography studies comprising 3,488 patients, finding that 40% of patients had extracolonic findings, 14% had further investigations, 2.7% had an extracolonic neoplasm, and 0.9% had an abdominal aortic aneurysm. Katz et al. [10] examined 1,000 consecutive unenhanced helical CT examinations performed for suspected renal colic and noted significant alternative genitourinary or nongenitourinary diagnoses established or suspected in 10% of patients.
The occurrence of incidental nonvascular findings in renal MR angiography is similar to those just noted, and the percentage with additional vascular findings is higher. This is not surprising because the CT studies did not use IV contrast material, and the patient population with suspected renovascular disease is probably more likely to have concurrent aortic, mesenteric, and iliac atherosclerosis. The relatively high percentage of patients with mesenteric artery stenosis could be an overestimation because patients with celiac artery narrowing on the basis of arcuate ligament compression were not distinguished from those with atherosclerotic narrowing, and the diagnostic accuracy of MR angiography for inferior mesenteric artery stenosis might be limited on the basis of the small size of this vessel.
Renal MR angiography has both advantages and limitations with respect to CT in identifying and characterizing incidental findings. In general, MRI has superior soft-tissue contrast, and because IV contrast material is used, unenhanced and contrast-enhanced images can be obtained to evaluate the enhancement characteristics of incidental lesions. In this regard, one might expect that additional imaging to characterize indeterminate lesions is less likely to be necessary after renal MR angiography. On the other hand, anatomic coverage is usually greater in CT colonography, which generally includes the entire abdomen and pelvis, and visualization of small pulmonary nodules in the lung bases is probably superior.
This study has several limitations. The study was retrospective and relied on the initial interpretations without additional review of the images, except in cases in which discrepancies were noted between renal MR angiography and additional cross-sectional studies. The patient population was restricted to a single institution, and it is likely that the incidence and importance of incidental findings will vary depending on the characteristics of patients undergoing renal MR angiography. Long-term follow-up of patients with indeterminate lesions is relatively limited. The question of whether significant findings in the abdomen were not detected on renal MR angiography was incompletely addressed. This is most likely to be relevant with regard to the liver, because anatomic coverage of the liver is generally incomplete in our protocol, and dynamic enhancement characteristics of hepatic lesions are not obtained. Nearly all renal MR angiography examinations in this study were interpreted by abdominal radiologists, which is not typical of most practices. The success in detecting and characterizing incidental lesions may have benefited from interpretations by radiologists with extensive experience in abdominal imaging, just as it is possible that vascular radiologists would provide an added benefit in the interpretation of vascular findings. Finally, our success in characterizing incidentally detected lesions likely benefits from reviewing all studies before the patient is dismissed and performing additional sequences as necessary.
In summary, incidental vascular and nonvascular findings on renal MR angiography are common. Benign lesions, most notably renal and hepatic cysts, are by far the most common nonvascular findings; however, a small but significant percentage of patients have malignant or indeterminate lesions. The addition of a small number of additional pulse sequences to a standard renal MR angiography protocol allows detection and characterization of most incidental abdominal lesions at a relatively small cost in imaging time. The relatively high incidence of nonvascular incidental findings accentuates the importance of performance and interpretation of these examinations by physicians with training in abdominal imaging.
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