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Effect of Echo Time Pair Selection on Quantitative Analysis for Adrenal Tumor Characterization with In-Phase and Opposed-Phase MR Imaging: Initial Experience¹

¹ From the Department of Radiology, Duke University Medical Center, Duke North, Room 1417, Erwin Rd, Durham, NC 27710 (S.T.S., B.J.S., D.M.D., E.M.M.); and Siemens Medical Solutions, Cary, NC (B.M.D.). Received June 18, 2007; revision requested August 27; revision received October 20; accepted January 7, 2008; final version accepted January 8. Address correspondence to E.M.M. (e-mail: elmar.merkle@duke.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

 
Purpose: To determine the effect of two pairs of echo times (TEs) for in-phase (IP) and opposed-phase (OP) 3.0-T magnetic resonance (MR) imaging on (a) quantitative analysis prospectively in a phantom study and (b) diagnostic accuracy retrospectively in a clinical study of adrenal tumors, with use of various reference standards in the clinical study.

Materials and Methods: A fat-saline phantom was used to perform IP and OP 3.0-T MR imaging for various fat fractions. The institutional review board approved this HIPAA-compliant study, with waiver of informed consent. Single-breath-hold IP and OP 3.0-T MR images in 21 patients (14 women, seven men; mean age, 63 years) with 23 adrenal tumors (16 adenomas, six metastases, one adrenocortical carcinoma) were reviewed. The MR protocol involved two acquisition schemes: In scheme A, the first OP echo (approximately 1.5-msec TE) and the second IP echo (approximately 4.9-msec TE) were acquired. In scheme B, the first IP echo (approximately 2.4-msec TE) and the third OP echo (approximately 5.8-msec TE) were acquired. Quantitative analysis was performed, and analysis of variance was used to test for differences between adenomas and nonadenomas.

Results: In the phantom study, scheme B did not enable discrimination among voxels that had small amounts of fat. In the clinical study, no overlap in signal intensity (SI) index values between adenomas and nonadenomas was seen (P < .05) with scheme A. However, with scheme B, no overlap in the adrenal gland SI–to–liver SI ratio between adenomas and nonadenomas was seen (P < .05). With scheme B, no overlap in adrenal gland SI index–to–liver SI index ratio between adenomas and nonadenomas was seen (P < .05).

Conclusion: This initial experience indicates SI index is the most reliable parameter for characterization of adrenal tumors with 3.0-T MR imaging when obtaining OP echo before IP echo. When acquiring IP echo before OP echo, however, nonadenomas can be mistaken as adenomas with use of the SI index value.

© RSNA, 2008

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

 
Precise characterization of adrenal tumors is necessary to plan an appropriate clinical approach. In-phase (IP) and opposed-phase (OP) magnetic resonance (MR) imaging has proved highly accurate in the differentiation of adenomas from nonadenomas (1–4). The diagnostic efficiency of IP and OP MR imaging is due to its high sensitivity in distinguishing tissues with different lipid contents by using the interference of fat and water signals caused by the change in chemical shift frequency shift (5). In contrast to nonadenomas, adrenal adenomas generally contain variable amounts of intracytoplasmatic lipid that result in signal intensity (SI) loss at OP imaging (6,7). To quantify this SI loss, various formulas used to calculate the relative change in SI have been published. The two most reliable formulas have proved to be those used to calculate the adrenal gland SI–to–spleen SI ratio (1,3,6,8).

At 1.5 T, the frequency shift between water and fat signals is approximately 225 Hz, resulting in IP signals at echo times (TEs) of 0.0 msec, 4.4 msec, 8.8 msec, and so on, and OP signals at TEs of 2.2 msec, 6.6 msec, 11.0 msec, and so on. At 3.0 T, the frequency shift between water and fat is doubled to approximately 450 Hz, thus halving the 1.5-T IP and OP TEs. The 1.1-msec interval between IP and OP TEs makes collecting these echoes right after the other technically challenging. Currently, acquiring the first OP echo with a TE of 1.1 msec and the first IP echo only 1.1 msec later and within the same breath hold is not feasible with standard imaging sequences. Thus, two alternate data acquisition schemes have been implemented at 3.0 T: (a) collection of the first OP echo followed by collection of the second IP echo and (b) collection of the first IP echo followed by collection of the third OP echo (9–11).

The purpose of our study was to determine the effect of two pairs of TEs for IP and OP 3.0-T MR imaging on (a) quantitative analysis prospectively in a phantom study and (b) diagnostic accuracy retrospectively in a clinical study of adrenal tumors, with use of various reference standards in the clinical study.

	MATERIALS AND METHODS

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One author (B.M.D.) is an employee of Siemens Medical Solutions, the manufacturer of two 3.0-T MR imagers used in our study. However, authors who were not consultants for Siemens Medical Solutions had complete control of any data and information that might have presented a conflict of interest for this author.

Phantom Study
A 1.5-L cylindrical phantom was created with equal parts soybean oil (Spectrum Naturals, Petaluma, Calif) and saline 0.9% (Hospira, Lake Forest, Ill) (Fig 1) (4). Images parallel to the fat-saline boundary were acquired with a standard dual-echo two-dimensional IP and OP fast low-angle shot sequence at both 3.0 T and 1.5 T (Magnetom Trio and Avanto, respectively; Siemens, Erlangen, Germany) by using a circular polarized head coil. The 3.0-T IP and OP MR sequence included two acquisition schemes: (a) acquisition of the first OP echo (1.5 msec) and the second IP echo (4.9 msec) and (b) acquisition of the first IP echo (2.6 msec) and the third OP echo (6.2 msec). The following parameters were used for both schemes: 111-msec repetition time, 180 x 180-mm field of view, 128 x 128 matrix, 890 Hz/pixel receiver bandwidth, and anteroposterior phase-encoding direction. The 1.5-T IP and OP MR sequence was used to acquire the first OP and IP echoes (2.4 and 5.0 msec, respectively; 111-msec repetition time; 180 x 180-mm field of view; 128 x 128 matrix; 380 Hz/pixel and 420 Hz/pixel receiver bandwidths, respectively).

View larger version (15K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Schematic drawing of the fat-saline phantom consisting of 50% saline and 50% soybean oil. We acquired 41 transverse 20-mm-thick images parallel to the fat-saline boundary in 1-mm steps from –20 mm to 20 mm across the fat-saline boundary to capture images with SI variations due to the different ratios of fat and saline in the section. Data acquisition started in pure saline (–20 mm) and ended in pure fat (20 mm). The in-plane voxel dimensions were nominally 1.4 x 1.4 mm.

Single-voxel PRESS MR spectroscopy data were co-located with transverse IP and OP images to calibrate maximum water-fat content and SI variations. Acquisition parameters were as follows: 2000/30 (repetition time msec/TE msec), 1024 points, 1540-Hz sweep width, four signals acquired, and no water suppression. MR spectroscopy voxels were centered in the fat-saline phantom where the meniscus was flattest. We acquired 21 spectra, starting with the MR spectroscopy voxel entirely in the saline compartment. Because of time constraints, the voxel was moved in 2-mm increments until it was entirely in the fat compartment. For each voxel location, integrated areas under the SI index magnitude plots were used to determine the ratio of water to fat (water, 2.7–6.7 ppm; fat, 0.0–2.4 ppm). Absolute SIs from water-only voxels and from fat-only voxels were compared to estimate SI contributions from each compartment of the phantom. Water and fat areas from all voxels were normalized to these maximum values to determine if signal contributions from each compartment changed linearly. The phantom was allowed to settle in the MR imager for 20 minutes prior to spectroscopy or measurement of TE.

Patients, Lesion Confirmation, and Reference Standards in the Clinical Study
This Health Insurance Portability and Accountability Act–compliant study was approved by our institutional review board, and the requirement for informed consent was waived. The institutional 3.0-T MR database was searched to identify abdominal MR examinations performed between March 2004 and June 2007 with MR reports that included the following keywords: adrenal adenoma, adrenal mass, adrenal tumor, and/or adrenal metastases. The review yielded 31 patients with 33 adrenal lesions (Fig 2). Adrenal lesions smaller than 1 cm in diameter were excluded because of potential difficulties with accurate region of interest placement. Lesions with insufficient diagnostic proof also were excluded. The final proof for diagnosis of adrenal adenomas was based on at least one of the following three factors: pathology data; stability of the lesion size at follow-up computed tomography (CT), MR imaging, or both within at least 6 months; or lesion attenuation of less than 10 HU on unenhanced CT images (2,3). Final diagnosis of a nonadenoma was established on the basis of pathology data; an at least 30% change in the maximal diameter of the lesion at follow-up CT, MR imaging, or both within 6 months; or both.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Flowchart provides information about the method of patient recruitment and the number of patients who underwent the two IP and OP MR acquisition schemes. Data in parentheses indicate the number of lesions.

In 15 patients, 16 tumors were diagnosed as adrenal adenomas (mean maximal diameter, 2.5 cm; range, 1.1–4.1 cm): Nine were diagnosed because they were stable in size at follow-up CT or MR imaging (mean follow-up period, 19 months; range, 6–44 months), five were diagnosed because unenhanced CT attenuation was less than 10 HU, and two were diagnosed at percutaneous biopsy. Seven tumors in six patients were diagnosed as nonadenomas (mean diameter, 4.5 cm; range, 1.9–7.5 cm): There were four metastases from clear-cell renal cell carcinoma (RCC) in three patients, one metastasis from squamous cell carcinoma of the cervix, one metastasis from colon carcinoma, and one adrenocortical carcinoma. Two metastases from RCC in one patient were diagnosed at follow-up CT within 3 months because of a marked decrease in the maximal diameters of the lesions after chemotherapy for RCC. The third metastasis from RCC was confirmed with pathologic analysis, and the fourth metastasis from RCC was confirmed on the basis of rapid enlargement at follow-up CT within 4 months in a patient with known RCC. The metastasis from squamous cell carcinoma of the cervix, the metastasis from colon carcinoma, and the adrenocortical carcinoma were confirmed with pathologic analysis.

Clinical MR Imaging Technique
Clinical MR imaging was performed with three 3.0-T MR imagers—Magnetom Trio (Siemens) (13 patients), Magnetom Tim Trio (Siemens) (five patients), and Signa Excite (GE Healthcare, Milwaukee, Wis) (three patients)—and use of dedicated eight-channel torso-array receive-only coils. The adrenal MR imaging protocol consisted of coronal half-Fourier single-shot fast spin-echo T2-weighted images or single-shot fast spin-echo T2-weighted images and transverse T1-weighted dual-echo two-dimensional gradient-echo IP and OP images. The IP and OP images were acquired in the same breath hold with various MR parameters (Table 1) and an anteroposterior phase-encoding direction. The IP and OP MR protocol included acquisition with the following schemes: In scheme A, we acquired the first OP echo with a TE of 1.5–1.6 msec and the second IP echo with a TE of 4.4–4.9 msec. In scheme B, we acquired the first IP echo with a TE of 2.2–2.5 msec and the third OP echo with a TE of 5.5–6.2 msec.

Acquisition with scheme A included 15 adenomas in 14 patients and six nonadenomas (three metastases from RCC, one metastasis from squamous cell carcinoma of the cervix, one metastasis from colon cancer, and one adrenocortical carcinoma) in five patients (Fig 2). Acquisition with scheme B included nine adenomas in eight patients and seven nonadenomas in six patients. Eight adenomas in seven patients and six nonadenomas (three metastases from RCC, one metastasis from squamous cell carcinoma of the cervix, one metastasis from colon carcinoma, and one adrenocortical carcinoma) in five patients were examined with both scheme A and scheme B.

Clinical MR Image Analysis
A 4th-year radiology resident (S.T.S.) with 2 years of experience in body MR imaging performed quantitative analysis on an MR workstation (Leonardo; Siemens). The resident was blinded to the acquisition schemes. Region of interest values for SI measurements were obtained in the same section for the adrenal mass and for three reference tissues (spleen, liver, and ipsilateral paraspinal muscle). Circular regions of interest were placed at homogeneous artifact-free areas in the center of the adrenal mass and were drawn as large as possible without including the edge of the lesion. Circular regions of interest in the spleen, liver, and paraspinal muscle were drawn as large as possible (range, 400–2000 mm²) without including blood vessels or fat tissue. By applying the copy and paste function of the workstation, identical region of interest positions could be achieved on corresponding IP and OP images. The SI value of each adrenal mass and the three reference tissues was measured three times on the IP and OP images. With use of mean SI values and the following equations, we calculated (a) SI index values, [(SI_IP – SI_OP)/(SI_IP)] x 100%; (b) the adrenal SI–to–spleen SI ratio, [(SI_OP^A/SI_OP^S)/(SI_IP^A/SI_IP^S)–1] x 100%, where SI_OP^A is the adrenal SI for OP, SI_OP^S is the spleen SI for OP, SI_IP^A is the adrenal SI for IP, and SI_IP^S is the spleen SI for IP; (c) the adrenal SI–to–liver SI ratio, [(SI_OP^A/SI_OP^L)/(SI_IP^A/SI_IP^L)– 1] x 100%, where SI_OP^L is the liver SI for OP and SI_IP^L is the liver SI for IP; and (d) the adrenal SI–to–muscle SI ratio, [(SI_OP^A/SI_OP^M)/(SI_IP^A/SI_IP^M)–1] x 100%, where SI_OP^M is the muscle SI for OP and SI_IP^M is the muscle SI for IP (1,6,8).

Clinical Data Evaluation, Statistical Analysis, and Reference Standards
The SI index and SI ratio for adenomas and nonadenomas were graphically depicted as scatterplots for the two acquisition schemes. Preliminary data for scheme A indicated mean SI index values of approximately 20% for adenomas and –10% for nonadenomas, with a standard deviation of approximately 10 both for adenomas and for nonadenomas. With use of these values, a power analysis enabled us to determine that a minimal sample size of five adenomas and two tumors would be sufficient to obtain a statistical power of .8150 for the unpaired Student t test on the mean. With each acquisition protocol and evaluation method, a two-sample t test (equal variances not assumed) was used to test for mean differences between adenomas and nonadenomas. A repeated-measures analysis of variance was used to test for mean differences between scheme A and scheme B on the subset of lesions (n = 14) that were examined by using both acquisition schemes. Additionally, a receiver operating characteristic curve was generated for each protocol and evaluation method. The area under the curve for each evaluation method, a suggested threshold, and corresponding sensitivity and specificity ratings were determined. A P value of less than .05 was considered to indicate a significant difference. The Holm-Bonferroni method was used to adjust for multiple comparisons (12). All statistical analyses were performed with SPSS 13.0 software (SPSS, Chicago, Ill).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

 
Phantom Study
As the MR section was moved through the phantom from pure fat to pure water, MR spectroscopy peak areas for the 21 spectra acquired enabled us to confirm approximately equal maximum water and fat signal contributions and a linear variation of these signals in the center of the phantom. The MR spectrum of the soybean oil had a large peak at 440 Hz from the water peak (65% total area). Smaller peaks at 490 Hz, 370 Hz, and 246 Hz away from water contributed the remaining area. Overall, the maximum signal measured from the fat region was 85% of that of the maximum water region signal.

There was a marked difference between the in vitro SI index plots for scheme A and scheme B acquired at 3.0 T (Fig 3). The shape of the plot for scheme A at 3.0 T was similar to that at 1.5 T.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Graph shows the SI index values acquired with the fat-saline phantom versus the fat fraction. Data were acquired at 1.5 T and 3.0 T. The plot for scheme A had a negative SI start index value (–40%), representing pure saline, whereas the plot for scheme B had a positive value (30%). The course of the plot for scheme B never reached any negative values. Since the beginning of the plot for scheme B had an almost horizontal course, small fat fractions (less than 25%) could not be distinguished from each other. The shape of the plot for scheme A at 3.0 T was similar to the shape of the plot for 1.5 T, with both having a negative initial SI index value, although the values of these two curves were quite different. For every fat fraction, the SI index value for the plot acquired at 1.5 T was greater than for the plot acquired at 3.0 T.

View larger version (9K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: Scatterplot shows the values of the (a) SI index, (b) adrenal SI–to–spleen SI ratio, (c) adrenal SI–to–liver SI ratio, and (d) adrenal SI–to–muscle SI ratio for the adenomas (A) and nonadenomas (NA) examined with TE acquisition schemes A and B. There was no overlap between SI index values of the adenomas and nonadenomas for scheme A; however, a substantial overlap was found for scheme B. When scheme B was used, no overlap occurred between the adrenal SI–to–liver SI ratio values of the adenomas and nonadenomas; however, a considerable overlap was seen for scheme A.

View larger version (9K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: Scatterplot shows the values of the (a) SI index, (b) adrenal SI–to–spleen SI ratio, (c) adrenal SI–to–liver SI ratio, and (d) adrenal SI–to–muscle SI ratio for the adenomas (A) and nonadenomas (NA) examined with TE acquisition schemes A and B. There was no overlap between SI index values of the adenomas and nonadenomas for scheme A; however, a substantial overlap was found for scheme B. When scheme B was used, no overlap occurred between the adrenal SI–to–liver SI ratio values of the adenomas and nonadenomas; however, a considerable overlap was seen for scheme A.

View larger version (9K): [in this window] [in a new window] [Download PPT slide]   Figure 4c: Scatterplot shows the values of the (a) SI index, (b) adrenal SI–to–spleen SI ratio, (c) adrenal SI–to–liver SI ratio, and (d) adrenal SI–to–muscle SI ratio for the adenomas (A) and nonadenomas (NA) examined with TE acquisition schemes A and B. There was no overlap between SI index values of the adenomas and nonadenomas for scheme A; however, a substantial overlap was found for scheme B. When scheme B was used, no overlap occurred between the adrenal SI–to–liver SI ratio values of the adenomas and nonadenomas; however, a considerable overlap was seen for scheme A.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 4d: Scatterplot shows the values of the (a) SI index, (b) adrenal SI–to–spleen SI ratio, (c) adrenal SI–to–liver SI ratio, and (d) adrenal SI–to–muscle SI ratio for the adenomas (A) and nonadenomas (NA) examined with TE acquisition schemes A and B. There was no overlap between SI index values of the adenomas and nonadenomas for scheme A; however, a substantial overlap was found for scheme B. When scheme B was used, no overlap occurred between the adrenal SI–to–liver SI ratio values of the adenomas and nonadenomas; however, a considerable overlap was seen for scheme A.

View larger version (104K): [in this window] [in a new window] [Download PPT slide]   Figure 5a: Right adrenal metastasis from RCC (5.6 x 5.1 cm), which demonstrated a marked change in size at follow-up imaging. Both image acquisition schemes were used. In scheme A, transverse (a) IP (193/4.9) and (b) OP (193/1.6) MR images show a right adrenal mass, with a higher mean SI value of the mass on b (268) than on a (234). The SI index value measured –13.2%. In scheme B, transverse (c) IP (230/2.2) and (d) OP (230/5.7) MR images show the right adrenal mass, with a lower mean SI value of the mass on d (256) than on c (315). The SI index value was 18.9%. The positive SI index value indicating an SI loss on d could have been caused by intracytoplasmatic lipid in the adrenal lesion or by T2* decay.

View larger version (105K): [in this window] [in a new window] [Download PPT slide]   Figure 5b: Right adrenal metastasis from RCC (5.6 x 5.1 cm), which demonstrated a marked change in size at follow-up imaging. Both image acquisition schemes were used. In scheme A, transverse (a) IP (193/4.9) and (b) OP (193/1.6) MR images show a right adrenal mass, with a higher mean SI value of the mass on b (268) than on a (234). The SI index value measured –13.2%. In scheme B, transverse (c) IP (230/2.2) and (d) OP (230/5.7) MR images show the right adrenal mass, with a lower mean SI value of the mass on d (256) than on c (315). The SI index value was 18.9%. The positive SI index value indicating an SI loss on d could have been caused by intracytoplasmatic lipid in the adrenal lesion or by T2* decay.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 5c: Right adrenal metastasis from RCC (5.6 x 5.1 cm), which demonstrated a marked change in size at follow-up imaging. Both image acquisition schemes were used. In scheme A, transverse (a) IP (193/4.9) and (b) OP (193/1.6) MR images show a right adrenal mass, with a higher mean SI value of the mass on b (268) than on a (234). The SI index value measured –13.2%. In scheme B, transverse (c) IP (230/2.2) and (d) OP (230/5.7) MR images show the right adrenal mass, with a lower mean SI value of the mass on d (256) than on c (315). The SI index value was 18.9%. The positive SI index value indicating an SI loss on d could have been caused by intracytoplasmatic lipid in the adrenal lesion or by T2* decay.

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 5d: Right adrenal metastasis from RCC (5.6 x 5.1 cm), which demonstrated a marked change in size at follow-up imaging. Both image acquisition schemes were used. In scheme A, transverse (a) IP (193/4.9) and (b) OP (193/1.6) MR images show a right adrenal mass, with a higher mean SI value of the mass on b (268) than on a (234). The SI index value measured –13.2%. In scheme B, transverse (c) IP (230/2.2) and (d) OP (230/5.7) MR images show the right adrenal mass, with a lower mean SI value of the mass on d (256) than on c (315). The SI index value was 18.9%. The positive SI index value indicating an SI loss on d could have been caused by intracytoplasmatic lipid in the adrenal lesion or by T2* decay.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

When we obtained the first OP echo before the second IP echo, we did not observe any overlap of SI index values between the 15 adenomas and the six nonadenomas. In contrast, acquisition of the first IP image before the third OP image resulted in a substantial overlap of SI index values between the adenomas and nonadenomas. In two studies in which a shorter TE was used for the IP echo compared with that used for the OP echo at magnetic field strengths lower than 3.0 T, researchers found similar overlaps of SI index values between adenomas and malignant lesions (14,15). In these studies, several malignant tumors had a misleading SI loss on the OP images, as did seven nonadenomas in our study.

At IP and OP MR imaging, two factors influence SI loss on OP images: chemical shift effect and T2* decay. The latter is the most plausible explanation for the misleading SI loss associated with nonadenomas with little to no lipid content on OP images when the IP echo is acquired before the OP echo. This scenario should yield a positive SI index value, whereas reverse TE selection is most likely to yield a negative SI index value. This pattern was demonstrated with our fat-saline phantom, in which the 3.0-T plot for the SI index of scheme B never reached negative values. In our clinical study, we found five nonadenomas that had been examined with both scheme A and scheme B, which demonstrated a negative SI index value and a positive SI index value, respectively. Thus, radiologists selecting a TE that is shorter for the IP echo than for the OP echo are confronted with a diagnostic dilemma since they are unable to decide whether the SI loss in an adrenal lesion on an OP image is caused by chemical shift effect in a fat-containing adenoma or by T2^* decay in a malignant lesion with little to no fat. In 1995, Tsushima and Dean (16) noted the influence of the selection of TEs on the SI index for IP and OP MR imaging when characterizing adrenal tumors. Despite this alert, until 2006, various investigators still acquired the IP echo before the OP echo for both 1.5- and 3.0-T chemical shift MR imaging (7,10,11,15).

Some authorities have suggested applying an internal reference tissue to reduce the effect of T2^* decay (1,7,17,18). Reference tissues used in the past have included the spleen, liver, and paraspinal muscle. In several investigations, researchers have recommended the spleen as the most suitable reference tissue since fat infiltration of the liver or paraspinal muscle may substantially influence any quantitative evaluation (1,7,17). However, the SI ratio of the spleen can also be subject to alteration, namely in the case of abnormal iron deposition (eg, hemosiderosis). In our study, we found the adrenal SI–to–liver SI ratio obtained by using scheme B to be an excellent discriminator between adrenal adenomas and nonadenomas, whereas the adrenal-to-spleen ratio demonstrated some overlap. Thus, radiologists who acquire the IP echo before the OP echo and who use the liver or spleen as an internal reference tissue need to be cautious and aware of hepatic steatosis and splenic iron deposition.

A limitation of our investigation was the small number of adrenal tumors that were included. However, despite the small number of lesions, our study had sufficient statistical power, as indicated by both the power analysis and the significant differences for most combinations of our evaluation methods and acquisition schemes. To further address this limitation, we added a phantom study that clearly demonstrated the effect of T2^* decay on the quantitative analysis of IP and OP MR images at 3.0 T. On the basis of our findings and the fact that, to our knowledge, no scientific report had been published on IP and OP 3.0-T MR imaging for characterization of adrenal lesions, we believe it is appropriate to inform the radiologic community of the effect of T2^* decay on IP and OP MR imaging and to provide preliminary guidelines for IP and OP 3.0-T MR imaging in the characterization of adrenal lesions. Another limitation of our study was that four metastases were from clear-cell RCC. It is well known that this tumor type may contain small amounts of intracytoplasmatic fat (19,20). However, in our study, the metastases from clear-cell RCC did not indicate an SI loss on OP images obtained with scheme A. Finally, only two echo pairs for IP and OP MR imaging were evaluated in our study. In future studies, a systematic comparison between multiple echo pairs, including a sequence with shortest IP and OP echo, should be performed to determine the optimal echo pairing.

In conclusion, selection of the TE scheme for IP and OP 3.0-T MR imaging may greatly affect the quantitative analysis of adrenal tumors. To avoid misclassification of adrenal tumors on IP and OP MR images, the SI index should be applied when the TE of the IP echo is longer than the TE of the OP echo. If the IP echo is collected before the OP echo, however, we recommend the use of an internal reference tissue, such as the liver or spleen, to correct for T2^* decay effects.

	ADVANCES IN KNOWLEDGE

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

 

• At 3.0 T, the selection of echo pairs for in-phase (IP) and opposed-phase (OP) MR imaging greatly affects the quantitative analysis used to differentiate adrenal adenomas from nonadenomas.
  
• Signal intensity index is the most accurate measurement for quantitative analysis when the OP echo is acquired before the IP echo.
  

	IMPLICATION FOR PATIENT CARE

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

 

• For IP and OP MR imaging at 3.0 T, selecting the correct quantitative analysis of the echo time pair can enable one to avoid misclassification of nonadenomas as adrenal adenomas.
  

	ACKNOWLEDGMENTS

 
We thank Richard Youngblood, MA, for editing the manuscript and Lisa K. Wall, BSRT, for searching the institutional 3.0-T MR database.

	FOOTNOTES

 

Abbreviations: IP = in phase • OP = opposed phase • RCC = renal cell carcinoma • SI = signal intensity • TE = echo time

See Material and Methods for pertinent disclosures.

Author contributions: Guarantors of integrity of entire study, S.T.S., E.M.M.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, S.T.S.; clinical studies, B.M.D., E.M.M.; experimental studies, S.T.S., B.J.S., B.M.D., E.M.M.; statistical analysis, D.M.D.; and manuscript editing, all authors

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ADVANCES IN KNOWLEDGE IMPLICATION FOR PATIENT CARE References

 

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