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1 Department of Endocrinology, Odense University Hospital, DK-5000 Odense,
Denmark.
2 Department of Radiology, Odense University Hospital, DK-5000 Odense,
Denmark.
Received August 2, 2001;
accepted after revision January 3, 2002.
Supported by grants from The Agnes and Knut Mørks Foundation and The
A. P. Møller Relief Foundation.
Abstract
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SUBJECTS AND METHODS. The effect of high-dose 131I therapy on the thyroid gland and the impact on the trachea in 23 patients with a large multinodular goiter (range in volume, 100-703 mL) were monitored by observers unaware of duplicated measurements on MR imaging (n = 68) before, 1 week after, and 1 year after 131I therapy. In goiters exclusively cervically located (n = 12), cross-sectional planimetric sonographic measurements (n = 24) were performed simultaneously with MR imaging.
RESULTS. The mean intraobserver difference for the MR imaging measurements of goiter volume was 2.1 mL (1.4%, p = 0.32), and the coefficient of variation (CV) ± SD was 3.6% ± 2.6%. The mean interobserver difference was 0.4 mL (0.3%, p = 0.86), and the CV ± SD was 4.1% ± 3.5%. Compared with MR imaging, sonography underestimated goiter volume; the mean percentage difference between the volume estimates on MR imaging and those on sonography (volume estimated on MR imaging — volume estimated on sonography) was 19.5% (95% limits of agreement: -22.2% to 83.7%), and the CV ± SD was 15.0% ± 12.4%. The mean interobserver difference in the MR imaging measurement of tracheal volume along the goiter extension was 7.4% (95% confidence interval: 4.0-10.8%) and that of the smallest cross-sectional area of the trachea was 7.9% (95% confidence interval: 2.9-13.2%). The corresponding CV ± SD were 8.1% ± 6.6% and 10.3% ± 10.3%, respectively.
CONCLUSION. For the estimation of goiter volume, MR imaging has low intra- and interobserver variations. In contrast, the determination of tracheal dimensions using MR imaging has a high variability and, thus, is imprecise. Sonography significantly underestimates thyroid volume compared with MR imaging in patients with a large goiter.
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Because valid estimates of the volume of the goiter and dimensions of the involved trachea are essential in some patients, the aim of our study was to assess the inter- and intraobserver variabilities of MR measurements of these structures. Also, because sonography is the standard method by which to estimate the volume of small goiters [5], a comparison of sonographic measurements with MR imaging measurements was considered relevant.
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MR Imaging
MR imaging of the neck and thorax was performed on a superconducting system
(Gyroscan T5 II; Philips, Eindhoven, The Netherlands) operating at 0.5 T.
T1-weighted images (TR/TE, 270/15) were obtained in the axial, coronal, and
sagittal planes using a standard neck coil
(Fig. 1A). The slice thickness
was 8 mm with an interslice gap of 0.8 mm covering the entire thyroid gland.
On each axial slice, the cross-sectional area of the thyroid and that of the
trachea were measured manually by drawing a line along the contours of the
thyroid and the tracheal lumen (Figs.
1B and
1C), and the volume was
calculated by multiplying the measured areas with the slice thickness and
interslice gap. Two experienced observers (observers A and B) calculated, in a
blinded design, three variables: the total goiter volume, the smallest
cross-sectional area of the tracheal lumen, and the total tracheal volume
along the initial thyroid extension. One observer (observer A) made blinded
double calculations (first and second measurements) of the goiter volume. With
the patients in a standardized supine position, MR imaging was performed
before, 1 week after, and 1 year after 131I therapy. Thus, after
excluding one unsuccessful measurement in one patient, MR imaging data were
obtained from the 23 patients on 68 occasions.
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Sonography
We used a specially equipped compound scanner (type 1846; Brüel &
Kj
r, Copenhagen, Denmark) mounted with a 5-MHz transducer on a static
scanner arm. The principle by which the thyroid volume was estimated is
comparable to the MR imaging method just described. With the patient in the
supine position and the neck hyperextended, we obtained cross-sectional scans
covering the entire gland successively with an interslice gap of 0.5 cm. On
each image, the area outlining the thyroid gland was drawn manually on the
screen with a cursor. The volume of the thyroid was estimated using a
computerized calculation that incorporated all areas and gap distances
[6]. Because of the overlying
thoracic skeleton, a valid sonographic measurement is not feasible in goiters
with a substernal extension. In our study, 12 of the 23 patients had an
exclusively cervical goiter, thus allowing reliable sonographic measurements.
Goiter volume was assessed in these 12 patients before and 1 year after
131I therapy (24 measurements) by one experienced observer
(observer C), and these measurements were compared with the corresponding MR
imaging data.
Statistical Analysis
Data are presented as means (±SD) or medians (range). The paired
Student's t test and Wilcoxon's signed rank test were used to test
for differences in the mean and median values, respectively. Simple linear
regression analysis and Spearman's rank correlation coefficient (r)
were used to test for correlation between two series of measurements. The
intraobserver, interobserver, and intermethod variabilities were determined
according to the principles of Bland and Altman
[7], and the mean difference
and 95% limits of agreement between two measurements were calculated. Because
the differences between two estimates were found at each side of zero, the
calculation of the mean percentage difference was based on log-transformed
data, thereby making a decline of a variable equivalent to an increase. In
addition, the coefficient of variation (CV) ± SD is given for each
variable. A p value of less than 0.05 was considered statistically
significant.
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Table 2 shows the variability of the duplicated measurements. The difference and correlation plots of the same variables are given in Figures 2,3,4,5 and Figures 6,7,8,9, respectively. The mean intraobserver (i.e., repeatability) and interobserver differences of the MR imaging measurements of goiter volume were 2.1 mL (1.4%) and 0.4 mL (0.3%), respectively. Because there is neither reason to believe that the two blinded measurements affected each other nor that the process of measurement changed with time, the mean intraobserver difference by definition is given as zero in the table. The limits of agreement were slightly narrower for the intraobserver observations, for which 95% of the measurements would not range below -11.3% or above 12.8% of a second measurement. However, a marked difference between the two measurements in absolute terms was occasionally apparent. Thus, the greatest intra- and interobserver differences were 44 and 57 mL, respectively.
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Although Figures 8 and 9 show a high correlation of the tracheal measurements, the difference plots (Figs. 4 and 5) reveal the high variability of these same data. The 95% confidence interval for the mean difference of the tracheal volume was 4.0-10.8% and that of the tracheal area was 2.9-13.2%, reflecting a clear bias between the two observers in their measurements of the trachea. A difference between the two observers' estimates that is less than 38.8% for tracheal volume and 58.6% for tracheal area cannot be regarded as statistically significant, as evidenced by the 95% limits of agreement (Table 2).
With a few exceptions, observers using sonography underestimated the goiter volume by a relatively large margin compared with MR imaging (Table 2 and Figs. 10 and 11). The relationship between the two methods found by simple linear regression is shown in Figure 11. The mean difference clearly expanded with an increase in mean volume (Fig. 10). The mean percentage difference between the two imaging methods (MR imaging — sonography) was 19.5%. The corresponding 95% limits of agreement ranged from -22.2% to 83.7%, meaning that a volume estimate based on sonography cannot be considered significantly dissimilar to one based on MR imaging unless the sonographic measurement ranges beyond these percentages of the MR imaging estimate.
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The advantages and limitations of MR imaging and CT for thyroid imaging have recently been reviewed [10]. MR imaging provides high-resolution three-dimensional images allowing exact identification of the boundaries of any anatomic structure. This information is useful for evaluation of a large multinodular goiter that is irregular in shape. In contrast, thyroid scintigraphy is often disabled in this setting because of the inconsistent functioning of a multinodular goiter, and the competence of sonography is reduced in cases of substernal goiter extension, which are frequently encountered. Furthermore, MR imaging has been shown to be superior to CT in assessing cases of substernal goiters when imaging findings are compared with operative findings [11]. The variability of MR imaging for volume estimates of large goiters has been evaluated previously by Huysmans et al. [12]. In 20 duplicated measurements, they found an intraobserver CV ± SD of 2.2% ± 2.0% and an interobserver CV ± SD of 4.1% ± 2.2%. In that study, a high correlation of volume estimates was obtained from coronal, sagittal, and axial measurements. The high reproducibility was confirmed in our study comprising more than three times the number of measurements with a similar interslice thickness and slice gap as in the former study. The relationship between two observations is traditionally given as rank correlation, as in the study of Huysmans et al. However, as argued by Bland and Altman [7], a correlation coefficient of two measurements of the same object is not very meaningful. A more relevant parameter of agreement is the mean difference between the two measurements. By this approach, the limits of agreement can be calculated, either in absolute terms or as a percentage. Therefore, whether a difference between two values is statistically significant can be more directly assessed. We found a mean difference of the goiter volume estimates of approximately 1% with only a marginal aberration between intra- and interobserver agreements. Thus, our data, in addition to those of Huysmans et al., lead us to conclude that MR imaging is precise in this setting. Whether MR imaging also has a high accuracy remains to be confirmed, but for now, this high-precision method should be preferred to other techniques when determining the size of a large goiter.
Cervical compression and dyspnea are common symptoms among patients with a goiter. However, the subjective feeling of respiratory distress correlates poorly with the impact of the goiter on the upper airways as verified by a lung function test [4]. On CT scans, the tracheal cross-sectional areas also show a poor relationship with lung function [13]. However in an earlier study [2], we found that MR imaging estimates of the tracheal area correlate significantly with inspiratory capacity in patients with a large goiter. In particular, if 131I therapy is being considered, one should pay attention to the involved section of trachea because this type of therapy occasionally causes the thyroid to enlarge by 10-25% during the early postradiation period [2, 14]. Although other authors [3] have used MR imaging for tracheal imaging in patients with a large goiter, this application has not previously, to our knowledge, been the object of a closer evaluation. Our data show that the interobserver agreement was poor with regard to tracheal area as well as tracheal volume. Furthermore, a systematic measurement bias between the two observers was noticed. Thus, despite the high resolution of the MR images, the poor precision of the tracheal measurements probably reflects the small dimensions of the structure being examined. In consequence, if tracheal dimensions are monitored on MR imaging, these dimensions should be based on repeated measurements (as was the case in our former study [2]) to increase the accuracy of the measurement; however, even with repeated measurements, the reproducibility is still low. We suggest that in evaluating a goiter's impact on the trachea imaging findings cannot be used alone and that a lung function test should be included.
Although not evaluated in this study, volume calculations made from thyroid scintigrams, which have been in use for many years because of the lack of other techniques, will be discussed. The applied principles and validation are based on older studies [15,16,17], most of which included patients with exclusively diffuse goiters. Compared with goiter volumes calculated using surgical or postmortem specimens, scintigraphic volume estimates varied considerably [15,16,17]. In the more recent study by Huysmans et al. [12] of large multinodular goiters, thyroid volume estimates from 131I scintigraphy were compared with those from MR imaging. The intra- and interobserver CVs of the scintigraphic measurements ranged from 5% to 10%. Correlation with MR imaging was poor (CV = 17%, r = 0.67), and scintigraphy underestimated the volume by up to 80%. In patients with a moderately enlarged goiter, Wesche et al. [18] estimated thyroid volume using 131I scintigraphy based on two different formulas and compared these estimates with sonographic calculations, using the same technique as we used in our study. Correlation of each of the formulas with sonography relied on whether a diffuse or nodular goiter was being measured. Furthermore, considerable disagreement—differences of up to nearly 200%—was seen between the two methods. These results indicate that thyroid volume measurements on scintigraphy are unreliable and should not be used.
Sonography has been used to estimate thyroid volume for several years [1, 5]. As is the case with scintigraphy, different methods of thyroid measurement on sonography have been adopted. The ellipsoid method evaluated by Brunn et al. [19] and Knudsen et al. [20] over- and underestimated, respectively, thyroid volume when compared with autopsy specimens as the reference. The CV of this method was 16% [20].
The method used in our study requires special equipment and is based on computerized calculations from cross-sectional areas covering the entire gland [6]. The problem with estimating the volume of an irregularly shaped goiter, for which the ellipsoid method is invalid, can thereby be overcome. Evaluation in several studies of this method has shown a high accuracy when compared with findings from surgical or postmortem specimens [6, 21, 22]. In our hands, the precision of this method is high: The CV was 5% for intraobserver duplicated measurements [6]. However, validation has until now been based on studies that included only small or moderately enlarged goiters. Our present data on more voluminous goiters further clarify the precision of the method. Compared with MR imaging, the correlation was poor. If we consider MR imaging to be the most definite method available, then sonography clearly leads observers to underestimate goiter volume in most cases. This result cannot be explained by inadequate sonographic measurement due to a substernal extension because these goiters were excluded from this part of the study. Large multinodular goiters, irregular in shape and structure, often profoundly expand widely and laterally in the neck, so the divergence most likely is caused by difficulties in defining the goiter contours by the sonographic method. Increasing the transducer frequency will not solve this problem because imaging of some parts of the thyroid will be more detailed at the expense of poorer penetration into the deep part of the neck.
We did not perform duplicated sonographic measurements, which would have enabled us to calculate observer variation. However, because of the marked inaccuracy of sonography compared with MR imaging and because the differences between the two methods extended on both sides of zero, the variability of the sonographic method is probably high. In goiters less than 150-200 mL in volume, the divergence between MR imaging and sonography was less pronounced, and in these cases we believe that MR imaging can be omitted to reduce cost.
Although sonography is still useful for imaging-guided fine-needle biopsy, MR imaging should be preferred in patients with large goiters if a precise volume estimate is essential—for example, in patients for whom 131I therapy is being considered.
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