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Technical Innovation |
1 Department of Radiology, AZ-Vrije Universiteit Brussels, Brussels,
Belgium.
2 Department of Radiology, University of Michigan Medical Center, 1500 E Medical
Center Dr., TC-2910G, Ann Arbor, MI 48109-0326.
Received May 24, 2004;
accepted after revision March 9, 2005.
Address correspondence to M. De Maeseneer.
Abstract
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CONCLUSION. Acceptable correction factors are situated in the range of 0.494-0.554. We propose a correction factor of 0.529 when using the ellipsoid formula.
Keywords: correction factor • MDCT screening • thyroid volume
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Volumetric evaluation of the thyroid gland is based on the use of an
ellipsoid model. Hence, a value is obtained that replaces clinical evaluation
of volume. With the ellipsoid model, the height, width, and depth of each lobe
are measured and multiplied. The obtained result is then multiplied by a
correction factor, which is 
/ 6, or 0.524
[5].
The work of Brunn et al. [6]
in 1981 was based on volume measurement of cadaver glands subsequently
immersed in water. Brunn et al. concluded that a modified correction factor of
0.479 resulted in a more accurate assessment of thyroid volume compared with
the previously accepted correction factor of /6, or 0.524. Based on these
findings, the WHO has used 0.479 as the correction factor in the assessment of
thyroid volume.
With the advent of MDCT, volume scanning became possible [7, 8]. In this study, we used reconstructed CT images of the thyroid gland to calculate thyroid volume, and we compared that volume with the volume calculated by using the ellipsoid formula with different correction factors. Our purpose was to propose an optimal correction factor.
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CT showed normal thyroid glands in all these patients. The ethics committee waived informed consent because our analysis consisted of postprocessing of existing images, and identification of the patients was not possible from the images analyzed. Images of the neck were reconstructed with a slice thickness of 3 mm and a collimation of 0.1 mm. Each lobe was evaluated separately. Thyroid gland volume was automatically calculated using software to obtain volume measurements (Wizard, Siemens) (Fig. 1). The height, width, and depth of each lobe were measured on the same workstation by the principal investigator. On these data, we applied the volumetric ellipsoid method (height x width x depth x correction factor 0.524) (Fig. 2). Volumetry was also performed using the correction factor 0.479. Other correction factors were used for statistical analysis with SPSS software (SPSS, Inc.). A paired Student's t test was used for evaluating statistically significant differences between the volumes calculated with different correction factors versus the volumes calculated automatically using CT.
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In recent decades, the WHO has changed the diagnostic criteria for goiter. The diagnosis of goiter used to be based on palpation, but now it is based on volume measurement using sonography. Volume measurement of the thyroid gland is especially easy to obtain because the gland has a different echogenicity compared with adjacent soft tissues [6]. Because of its conical morphology, a thyroid lobe is assumed to resemble an ellipsoid, and its volume is approximated using height x width x depth x a correction factor. Other methods such as 3D sonography and the automated transverse surface area method have been proposed to evaluate thyroid volume [10, 11].
Thyroid lobes, however, show variations in shape as is evident in anatomic and imaging studies [12, 13]. The study of Brunn et al. [6] correlated results of volume of the thyroid calculated by sonography with the volume of the thyroid assessed after dissection of the gland and immersion in water. This methodology, however, does not take into account incomplete dissection, dissection of connective tissue not part of the thyroid gland, and drying of tissue after dissection.
With the advent of MDCT, volume measurement has become routine in CT body imaging and has been shown to be highly accurate [14]. MDCT enables us to postprocess the scanned body part in different planes and obtain accurate volume measurements [15].
Brunn et al. [6] in 1981 suggested the use of a correction factor of 0.479 instead of the accepted 0.524. According to the results of our analysis, estimated thyroid lobe volume is not statistically significantly different from the CT volume measurement with the latter correction factor (0.524). Our results, however, indicate that there is a statistically significant difference of thyroid lobe volume assessment compared with CT volume measurement when a correction factor of 0.479 is used.
The strengths of our analysis include automated measurement using CT, which
corresponds to a calibrated measurement, and the use of the parametric
Student's t test compared with the kappa test used in the Brunn et
al. [6] study. Given our
results, we suggest the use of a mean correction factor between 0.494 and
0.554, which corresponds to 0.529. The correction factor 0.524 ( / 6) also
is acceptable because it lies in this range. A centrally located correction
factor (Fig. 3) may best take
into account anatomic variability. Indeed, when using lower or higher
correction factors, the chance of obtaining erroneous measurements seems
increased. Our results indicate that the correction factor 0.479 should be
avoided for measurement of thyroid volume using the ellipsoid model.
Our investigation has some limitations. Our study was based on volume measurement by MDCT. However, given the difference in density between the thyroid gland and adjacent soft tissue, tracing of the thyroid is easy on CT images. Also, previous studies have shown that CT volume measurement is highly accurate [15]. At sonography, the thyroid gland has an echo pattern different from adjacent soft tissues, simplifying measurements with this technique. Our study focuses, however, on the optimal correction factor, and since MDCT is a calibrated method, we considered this the most optimal approach. We acknowledge that the shape of the thyroid may vary in ways such as the nodular glands, and the overall shape may then be different from an ellipsoid.
In conclusion, we compared different correction factors for calculating thyroid gland volume using the ellipsoid method with volume measurements using MDCT images. The recently proposed correction factor of 0.479 leads to statistically significantly different measurements compared with CT volume measurements. This latter correction factor, used by the WHO, may not be optimal for volume assessment of the thyroid. Acceptable correction factors are situated in the 0.494-0.554 range. We suggest the use of a mean value of 0.529 to calculate the volume of the thyroid lobe when using the ellipsoid formula.
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