(Radiology. 1999;212:719-723.)
© RSNA, 1999
Three-dimensional US of the Prostate: Early Experience1
Ulrike M. Hamper, MD,
Victoria Trapanotto, DO, MS,
M. Robert DeJong, RDMS, RDCS, RVT,
Sheila Sheth, MD and
Cynthia I. Caskey, MD
1 From the Department of Radiology, Ultrasound Section, Johns Hopkins Hospital, 600 N Wolfe St, Baltimore, MD 21287 (U.M.H., V.T., M.R.D.J., S.S., C.I.C.); and the Department of Radiology, University of Texas at Houston, Lyndon B. Johnson General Hospital (C.I.C.). Received February 5, 1998; revision requested April 14; final revision received November 25; accepted March 29, 1999. Address reprint requests to U.M.H. (e-mail: umhamper@rad.jhu.edu).
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Abstract
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PURPOSE: To assess the feasibility of using a three-dimensional (3D) endorectal transducer at ultrasonography (US) in the prostate gland in a clinical setting.
MATERIALS AND METHODS: Sixteen patients underwent 3D imaging of the prostate gland with a 3D endorectal probe following conventional two-dimensional (2D) US and prior to prostatic biopsy. Image acquisition was performed as a volume of data with nearly immediate reconstruction and simultaneous display of sectional anatomy in three orthogonal planes—sagittal plane, transverse or coronal plane, or any arbitrary oblique plane. Images were evaluated for presence of focal lesions, glandular volume, visualization of lateral and anterior portions of the gland, and extraglandular extension of tumor.
RESULTS: Three-dimensional US allowed better visualization of the gland and focal lesions, especially on the coronally reconstructed images, which were judged superior to the sagittally or transversely reconstructed images for interpretation in 50% of the patients. Prostatic volumes obtained from 3D US were consistently smaller than volumes obtained from 2D US (20% difference, P = .006). Three-dimensional US was superior to 2D US in depicting tumor presence (nine of 10 right hemispheres, three of eight left hemispheres) and extraglandular extent of disease (three of five hemispheres).
CONCLUSION: Three-dimensional endorectal prostatic US appears to be clinically feasible and easy to perform. Added anatomic information from the coronal plane may allow better depiction of tumors and extraglandular spread than is possible with current 2D techniques.
Index terms: Prostate, neoplasms, 844.30 • Prostate, US, 844.12987, 844.12989 • Ultrasound (US), technology, 844.12987, 844.12989 • Ultrasound (US), three-dimensional, 844.12987, 844.12989
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Introduction
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Three-dimensional (3D) ultrasonography (US) has recently gained much attention in the evaluation of a variety of clinical, primarily obstetric, applications (1–7). Limited abdominal, pelvic, cardiac, ocular, or vascular applications have also been described (7–10). Reports of the use of endocavitary 3D US to image the prostate, urethra, or rectum have likewise been scant (11–20).
The purpose of this study was to assess the feasibility of using a 3D endorectal transducer in a clinical setting and to present our preliminary experience with 3D US of the prostate gland. To our knowledge, this may be the first clinical series in which 3D endorectal US was used prior to endorectal US–guided biopsy in a select patient population, with correlation at histopathologic examination.
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MATERIALS AND METHODS
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Sixteen consecutive patients scheduled for endorectal US and US-guided endorectal prostatic biopsy were examined with 3D US following conventional 2D US and prior to biopsy.
Indications for the original studies included the following: elevated prostate-specific antigen (PSA) level (n = 11), abnormal digital rectal examination findings alone (n = 2), abnormal digital rectal examination findings and elevated PSA level (n = 2), and abnormal endorectal US findings with normal digital rectal examination findings and PSA level (n = 1). The level of PSA in our population ranged from 2.9 to 40.0 ng/mL. Three patients had a PSA level less than or equal to 4.0 ng/mL, seven patients had levels between 4 and 10 ng/mL, four patients had levels between 10 and 20 ng/mL, and two patients had levels above 20 ng/mL.
All patients were examined with an experimental 7.5-MHz mechanically driven bifocal volume transducer coupled to a commercially available US unit (Combison 530; Medison America, Pleasanton, Calif) (Fig 1a). The age range of our patient population was 59–76 years, with a mean age of 66 years. Approval by the institutional review board was granted for the performance of the 3D examination. Informed consent was obtained from all patients prior to performance of the 3D endorectal US and biopsy procedure.
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Figure 1a. (a) Photograph of 3D endorectal volume transducer. (b) Schematic drawing of the acquired volume and planimetric display of orthogonal planes.
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Figure 1b. (a) Photograph of 3D endorectal volume transducer. (b) Schematic drawing of the acquired volume and planimetric display of orthogonal planes.
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Correlation with histopathologic results was obtained in all patients with either endorectal biopsy findings (n = 15) or radical retropubic prostatectomy findings (n = 1). Image acquisition was performed as a complete block or volume of data for a total of two to five volumes (mean, 3.8 volumes) per patient. The time to acquire a volume of data ranged from 5 to 10 seconds.
Automatic volume scanning allowed nearly immediate reconstruction and simultaneous display of sectional anatomy in three orthogonal planes—sagittal plane, transverse or coronal plane, or any arbitrary oblique plane—or in a plane parallel to the transducer surface (Figs 1b, 2). The individual planes could be manipulated in the volume in real time, which allowed measurements—dual linear, area, volume—to be performed in all planes. Total examination time was 20–30 minutes, including time for patient and machine preparation, data acquisition, and patient discharge.
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Figure 2a. Three-dimensional US planimetric images of the prostate gland. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with the patient's head to the left. (c) Coronal reconstruction image allows excellent demonstration of the lateral borders (arrows) of the gland. (d) Surface-rendered volumetric image.
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Figure 2b. Three-dimensional US planimetric images of the prostate gland. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with the patient's head to the left. (c) Coronal reconstruction image allows excellent demonstration of the lateral borders (arrows) of the gland. (d) Surface-rendered volumetric image.
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Figure 2c. Three-dimensional US planimetric images of the prostate gland. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with the patient's head to the left. (c) Coronal reconstruction image allows excellent demonstration of the lateral borders (arrows) of the gland. (d) Surface-rendered volumetric image.
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Figure 2d. Three-dimensional US planimetric images of the prostate gland. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with the patient's head to the left. (c) Coronal reconstruction image allows excellent demonstration of the lateral borders (arrows) of the gland. (d) Surface-rendered volumetric image.
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In addition, transparent volume or surface-rendered imaging was performed at a later time, which required 2–5 minutes for image reconstruction, depending on the area of interest chosen or reconstruction parameters (Fig 2d). Images were evaluated by two independent observers (U.M.H., V.T.) with special expertise in US and 3D imaging for the presence of focal lesions, size and volume of the entire gland or focal lesions, and extent of extraglandular spread. In cases of nonagreement between the observers, a consensus was reached. Assessment criteria were superior, equal, or inferior visualization of the gland or focal lesions when 3D US was compared with 2D US. In addition, a binomial (yes or no) decision as to the presence of capsular extension was made independently by the two observers.
Volume measurements were calculated on the 2D images by measuring glandular size in longitudinal, anteroposterior, and transverse dimensions. By using the formula for an ellipse, length (in centimeters) x height (in centimeters) x width (in centimeters) x 0.523, volumes were determined from the 2D images and were compared with 3D volume measurements calculated from a summation of individual areas measured on parallel sections (mean, five to six sections) and the distance between the individual sections. Data were recorded on hard-copy images or were digitally stored on removable media (cartridge; SyQuest Technology, Fremont, Calif) for later offline review. Statistical analysis was performed (by U.M.H., V.T., S.S.) by using the paired Student t test.
Endorectal US–guided core biopsy was performed (by U.M.H., S.S.) as a systematic "sextant" biopsy. If a focal hypoechoic nodule was identified at endorectal US, one to two passes through the lesions were performed in addition to the routine sextant biopsy. No staging biopsy into the neurovascular bundle or seminal vesicles was performed. Correlation of the US findings with the sextant biopsy results was performed at histopathologic examination.
The correlate for positive and negative findings at the biopsies was the hemisphere since gray-scale US alone was used, and not color Doppler US, which would warrant a correlate for each biopsy site.
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RESULTS
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Three-dimensional US was easily performed and was well tolerated by all patients. It enabled display of the prostate gland in planes usually not obtainable at conventional 2D US. The region of interest (entire gland) was seen in 16 patients (100%) on the transversely and coronally reconstructed images and in 12 patients (75%) on the sagittally reconstructed images. Visualization of focal lesions at 3D US was judged superior to that at 2D US in four patients (25%) and equal to that at 2D US in eight patients (50%). In four patients (25%), no lesions were identified at either 2D or 3D US. Coronal images were judged superior to and easier to interpret than the sagittal or transverse images in eight patients (50%) (Figs 2, 3). Prostatic volume measurements, as obtained from the 2D images by using the formula for the prostatic ellipse, yielded a mean volume of 55 mL (range, 34–148 mL).
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Figure 3a. Three-dimensional US planimetric images of a prostate with central benign prostatic hyperplasia. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with patient's head to the left. (c) Coronal image. Volume of the benign prostatic hyperplasia (dotted circle) was measured by summing area measurements from multiple contiguous sections.
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Figure 3b. Three-dimensional US planimetric images of a prostate with central benign prostatic hyperplasia. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with patient's head to the left. (c) Coronal image. Volume of the benign prostatic hyperplasia (dotted circle) was measured by summing area measurements from multiple contiguous sections.
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Figure 3c. Three-dimensional US planimetric images of a prostate with central benign prostatic hyperplasia. (a) Transverse image, initial acquisition. (b) Sagittal reconstruction image, with patient's head to the left. (c) Coronal image. Volume of the benign prostatic hyperplasia (dotted circle) was measured by summing area measurements from multiple contiguous sections.
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The volume obtained from summing multiple parallel planes on the 3D images yielded a consistently smaller volume, with a mean volume of 44 mL (range, 26–104 mL). This 22% difference in volume was statistically significant (P = .006, paired Student t test). The inner gland could be differentiated from the outer gland in all 16 patients in the coronal view. Volume measurements of the central gland to assess the volume of benign prostatic hyperplasia could be performed easily on the coronal 3D images (Fig 3).
Histopathologic evaluation of 32 prostatic hemispheres in 16 patients revealed tumor in the right hemisphere of the gland in 10 patients and in the left hemisphere in eight patients. Two-dimensional US and 3D US correctly depicted tumor as a hypoechoic lesion in the right hemisphere in nine of 10 patients (Figs 4, 5). Two-dimensional US images demonstrated tumor in the left hemisphere in only one of eight glands, whereas 3D US images correctly depicted three of eight of the histopathologically proved tumors in the left hemisphere (Table 1).
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Figure 4a. Three-dimensional US images of a left-sided hypoechoic nodule (arrow in a and b, arrows in c), which was proved at biopsy to be prostatic cancer. (a) Sagittal image, initial acquisition. The patient's head is visible to the left. (b) Transverse image. (c) Coronal reconstruction image. Note how the lesion is best seen on this image. (d) Same image as in c, from the same patient as in a-c, but rotated for better anatomic appreciation of the coronal plane. Lesion (dotted outline) has a volumetric assessment of 0.88 mL. Note how the lateral contour (arrow) of the gland in the region of the nodule is smooth, indicating confinement of the tumor to the gland. This was confirmed at histopathologic examination.
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Figure 4b. Three-dimensional US images of a left-sided hypoechoic nodule (arrow in a and b, arrows in c), which was proved at biopsy to be prostatic cancer. (a) Sagittal image, initial acquisition. The patient's head is visible to the left. (b) Transverse image. (c) Coronal reconstruction image. Note how the lesion is best seen on this image. (d) Same image as in c, from the same patient as in a-c, but rotated for better anatomic appreciation of the coronal plane. Lesion (dotted outline) has a volumetric assessment of 0.88 mL. Note how the lateral contour (arrow) of the gland in the region of the nodule is smooth, indicating confinement of the tumor to the gland. This was confirmed at histopathologic examination.
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Figure 4c. Three-dimensional US images of a left-sided hypoechoic nodule (arrow in a and b, arrows in c), which was proved at biopsy to be prostatic cancer. (a) Sagittal image, initial acquisition. The patient's head is visible to the left. (b) Transverse image. (c) Coronal reconstruction image. Note how the lesion is best seen on this image. (d) Same image as in c, from the same patient as in a-c, but rotated for better anatomic appreciation of the coronal plane. Lesion (dotted outline) has a volumetric assessment of 0.88 mL. Note how the lateral contour (arrow) of the gland in the region of the nodule is smooth, indicating confinement of the tumor to the gland. This was confirmed at histopathologic examination.
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Figure 4d. Three-dimensional US images of a left-sided hypoechoic nodule (arrow in a and b, arrows in c), which was proved at biopsy to be prostatic cancer. (a) Sagittal image, initial acquisition. The patient's head is visible to the left. (b) Transverse image. (c) Coronal reconstruction image. Note how the lesion is best seen on this image. (d) Same image as in c, from the same patient as in a-c, but rotated for better anatomic appreciation of the coronal plane. Lesion (dotted outline) has a volumetric assessment of 0.88 mL. Note how the lateral contour (arrow) of the gland in the region of the nodule is smooth, indicating confinement of the tumor to the gland. This was confirmed at histopathologic examination.
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Figure 5a. (a-c, e) Three-dimensional US images of a right-sided lobulated hypoechoic tumor. (a) Transverse image of the tumor (arrows). (b) Sagittal image. (c) Coronal image. Contour of the gland is irregular and bulging, indicating extraglandular spread of the tumor (arrowheads), which is best seen in the coronal plane. This spread was confirmed at histopathologic examination. (d) Schematic representation of the imaging plane in a in the 3D volume. (e) Image from the same patient as in a-d in coronal view for better visualization of the irregular contour of the gland (arrowhead).
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Figure 5b. (a-c, e) Three-dimensional US images of a right-sided lobulated hypoechoic tumor. (a) Transverse image of the tumor (arrows). (b) Sagittal image. (c) Coronal image. Contour of the gland is irregular and bulging, indicating extraglandular spread of the tumor (arrowheads), which is best seen in the coronal plane. This spread was confirmed at histopathologic examination. (d) Schematic representation of the imaging plane in a in the 3D volume. (e) Image from the same patient as in a-d in coronal view for better visualization of the irregular contour of the gland (arrowhead).
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Figure 5c. (a-c, e) Three-dimensional US images of a right-sided lobulated hypoechoic tumor. (a) Transverse image of the tumor (arrows). (b) Sagittal image. (c) Coronal image. Contour of the gland is irregular and bulging, indicating extraglandular spread of the tumor (arrowheads), which is best seen in the coronal plane. This spread was confirmed at histopathologic examination. (d) Schematic representation of the imaging plane in a in the 3D volume. (e) Image from the same patient as in a-d in coronal view for better visualization of the irregular contour of the gland (arrowhead).
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Figure 5d. (a-c, e) Three-dimensional US images of a right-sided lobulated hypoechoic tumor. (a) Transverse image of the tumor (arrows). (b) Sagittal image. (c) Coronal image. Contour of the gland is irregular and bulging, indicating extraglandular spread of the tumor (arrowheads), which is best seen in the coronal plane. This spread was confirmed at histopathologic examination. (d) Schematic representation of the imaging plane in a in the 3D volume. (e) Image from the same patient as in a-d in coronal view for better visualization of the irregular contour of the gland (arrowhead).
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Figure 5e. (a-c, e) Three-dimensional US images of a right-sided lobulated hypoechoic tumor. (a) Transverse image of the tumor (arrows). (b) Sagittal image. (c) Coronal image. Contour of the gland is irregular and bulging, indicating extraglandular spread of the tumor (arrowheads), which is best seen in the coronal plane. This spread was confirmed at histopathologic examination. (d) Schematic representation of the imaging plane in a in the 3D volume. (e) Image from the same patient as in a-d in coronal view for better visualization of the irregular contour of the gland (arrowhead).
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Extraglandular extent of disease was present at histopathologic evaluation in five hemispheres in four patients. Two-dimensional US images depicted extraglandular extent of disease in only one hemisphere in one patient, whereas extraglandular spread was depicted on 3D US images in three hemispheres in two patients (Table 2), with histopathologically proved extracapsular disease best seen on the coronal 3D images (Figs 4, 5).
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DISCUSSION
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Endorectal US, introduced by Watanabe et al (21) in 1971, has become the dominant imaging modality for the evaluation of prostatic disease, especially as the guidance modality for prostatic biopsy. Two-dimensional endorectal US examinations, however, are limited primarily by the subjectivity and expertise of the examiner. In addition, the examiner must interpret multiple 2D images and must mentally integrate this information to develop a 3D impression of anatomic or pathologic structures. With a 3D US system, transducer motion is mechanically controlled and standardized, and the 3D integration is achieved by means of a computer system (8,19). Although each examiner must gain experience with the operation of the system and with the interpretation of 3D volumes, the computer-generated image acquisition and display aids the examiner by decreasing subjectivity and by giving the examiner an unlimited opportunity to view findings in multiple planes, thereby increasing his or her certainty of findings or diagnosis.
Another advantage of 3D US is the data are stored on a hard drive for subsequent retrieval, which keeps examination times short and keeps patient discomfort at a minimum. In addition, it allows for later detection of lesions or findings that may have been missed on the original 2D scans.
Volume determination of the prostate gland or focal lesions is clinically important. It is used in conjunction with interpretations of the PSA level to identify patients at risk for prostatic cancer through calculation of PSA density (PSA level/volume), to help with treatment planning, or to evaluate response to therapy for benign prostatic hypertrophy (22–24). Volume determinations of the prostate gland or individual lesions at 3D US may in the future be performed with greater accuracy than is currently possible at 2D US.
Findings from a previous article (13) have shown that ellipsoid volume calculations in the prostate gland had errors of about 20%. Recent study findings by Elliot et al (19) showed 3D US images accurately reflect true prostatic volumes measured in vitro. Our in vivo results seem to agree with findings in this study.
Although no exact volume correlation could be performed with prostatectomy specimens in our study, the measurement of prostatic volumes as obtained at 3D US, when compared with 2D US, showed a statistically significant difference of 20%. Although it is generally accepted that an inaccuracy rate of 20% is acceptable for the evaluation of conservative therapy for prostatic cancer or hypertrophy, this degree of volume difference is not acceptable for calculating PSA densities and for assessing the size of prostatic cancer (13). Therefore, more accurate means of determining prostatic volumes at 3D US may be important for a more accurate determination of the PSA density measurement and of the selection criteria for patients to undergo a prostatic biopsy, especially when PSA elevations are in the "gray zone" between 4 and 10 mg/mL.
In our initial experience, 3D US of the prostate gland appears to be a clinically feasible and useful adjunct to 2D US. The short time needed for the initial acquisition and immediate multidimensional reconstruction, with unlimited off-site review of images, may, in the future, allow for increased efficiency and may maximize the patient's, the sonographer's, and the sonologist's time. Added information from the coronal or oblique planes enables better evaluation of the lateral, anterior, superior, and inferior aspects of the gland. This, in turn, allows better assessment of the extraglandular spread of disease, as seen in our small series. Future developments of 3D color flow imaging may allow targeting of isoechoic yet hypervascular lesions with greater accuracy than is possible with 2D US, thus further increasing diagnostic yield.
Limitations of our study include the small number of patients examined. In addition, although the patients represented a consecutive group of men, they were part of a select group of patients, of whom a relatively high proportion had abnormal results at digital rectal examination and had high PSA levels. This selection bias may also partially explain the greater number of hemispheres with positive findings and hypoechoic lesions in our series compared with the number in most populations of men who undergo prostatic biopsy.
Likewise, although the study is based on findings in 16 consecutive patients and therefore does not demonstrate an exclusion bias, the fact that the 3D examination was always performed after the 2D examination may lead to other biases. The patient may be more amenable to one rather than two endorectal examinations, therefore limiting the results of one technique. Patient cooperation theoretically could decrease for the 3D examination because of fatigue and discomfort, which may hamper the results of the 3D examinations.
On the other hand, since the investigators were not blinded to the findings on 2D scans, they may have been biased by the 2D results, thus improving the results of the 3D study. Some of these biases, however, could not be eliminated since the clinically requested 2D endorectal US examination had to be performed initially for patient care and management decisions prior to any experimental 3D examinations.
Transparent rendered images were subjectively judged difficult to interpret and were not found to be useful for lesion detection. Last, histopathologic correlation was performed in most patients with only biopsy specimens and not with prostatectomy specimens.
Further large-scale studies are needed, preferably studies in which 3D US findings are correlated with findings from histopathologic specimens obtained from radical retropubic prostatectomy for exact histopathologic correlation, and further clinical experience is needed to realize the full clinical potential of 3D US of the prostate.
In summary, our preliminary data suggest 3D US has the potential to become a practical imaging technique for the evaluation of the prostate gland in a busy clinical environment. Three-dimensional US may become a valuable clinical tool and an adjunct to 2D US, allowing depiction of normal and abnormal structures in previously unattainable planes. This facilitates clinical diagnoses, increases the operator's diagnostic confidence, and yields increased efficiency by allowing unlimited off-site review of previously acquired volume data. Further work, however, is needed before the use of this technique can become routine practice and accepted not only as a research tool but as a clinically valuable and cost-effective measure for the evaluation of men with prostatic problems.
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Footnotes
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Abbreviations: PSA = prostate-specific antigen
2D = two-dimensional
3D = three-dimensional
Author contributions: Guarantor of integrity of entire study, U.M.H.; study concepts and design, U.M.H., V.T., C.I.C.; definition of intellectual content, U.M.H., V.T., C.I.C.; literature research, U.M.H., V.T.; clinical studies, U.M.H., V.T., M.R.D.J.; data acquisition, U.M.H., V.T., M.R.D.J.; data analysis, U.M.H., V.T., S.S.; statistical analysis, U.M.H., V.T., S.S.; manuscript preparation, U.M.H., V.T., M.R.D.J., C.I.C.; manuscript editing and review, all authors.
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Abstract
Introduction
