A novel rotation method with variable-angle anterior probe for correcting the depth of the kidney to optimize renal dynamic imaging

Purpose We aimed to investigate the effect and significance of the rotation method with variable-angle anterior probe corrected for the depth of two kidneys on the determination of glomerular filtration rate (GFR) in total and single kidneys by the renal dynamic imaging Gates method. Methods Seventy-two patients who underwent dynamic renal imaging by the rotation method and abdominal CT in our hospital were collected in the present study. CT scanning, rotation method, Tonnesen's formula, and Li-Qian's formula were compared in terms of the depth of two kidneys, the depth difference between the two kidneys, and the total renal and single GFR obtained by substituting the renal depth values into Gates' formula. Results ①The depth of kidneys and GFR: Compared to CT, Tonnesen's formula significantly underestimated the depth of both kidneys and the total and single renal GFR (P < 0.05). No significant differences were found in the depth of both kidneys and the total and single renal GFR between Li-Qian's formula and the rotation method (P > 0.05), with a strong agreement and with the least bias in the values measured by the rotation method. ②Renal depth difference: Compared to CT, Tonnesen's formula and Li-Qian's formula underestimated the difference in depth between the two kidneys (P < 0.05). None of the differences were statistically significant based on the rotation method (P > 0.05). The depth difference was positively correlated with the resulting changes in single renal function (|R(CT)-R(Li-Qian)|) and (|R(Rotation)-R(Li-Qian)|) (r = 0.881, 0.641, P < 0.001). As the depth difference increased, Li-Qian's formula could not visualize changes in single renal function accurately. In contrast, the accuracy of the rotation method in assessing single renal function remains unaffected. Conclusion The rotation method obtains an accurate depth and depth difference between the two kidneys without additional CT radiation, enhancing the accuracy of the Gates method for determining total and single renal GFR. Trial registration Medical Ethics Committee of First Hospital of Shanxi Medical University, 2021BAL0146. Registered 12 January 2021. Supplementary Information The online version contains supplementary material available at 10.1186/s40658-022-00511-w.


I. INTRODUCTION
The glomerular filtration rate (GFR) is traditionally considered the best overall index of renal function in health and disease (1). Currently, the widely adopted radionuclide renal dynamic imaging in clinical practice uses Gates' method to calculate GFR. However, the accuracy for estimation of GFR depends on a number of factors, including region of interest definition, background subtraction, linear attenuation coefficient of the radionuclide in the soft tissues, net injection dose, quality of "bolus-like" injection, kidney depth and so on. Kidney depth is an important factor and variation in the skin-to-kidney center without correction for tissue attenuation has been shown to introduce errors in absolute quantification of kidney activity (2,3). Considering only the attenuation by soft tissue and assuming a linear attenuation coefficient of 0.153cm -1 for technetium-99m-( 99 Tc m ) diethylenetriamine pentaacetic acid (DTPA), a 1-cm variation in organ depth will result in a 14-percent change in external measurements (4). Many correction methods for tissue attenuation are used by the investigators, like direct measurements of kidney depth using ultrasound or Computed Tomography (CT), formulas relating kidney depth to patient height and weight (5,6) such as Tonnesen formula, Taylor formula, Inoue formula, etc. However, many researchers have proven that the estimated kidney depth has a significant difference with the values measured by CT (7), especially when the patient is too thin or obese, also is not suitable for patients with renal transplantation or ectopic kidney. NET632 SPECT, independently developed and produced by Novel Medical, is featured by two variable-angle cameras. Based on the device, a method of simultaneous kidney dynamic imaging and depth estimation is developed. Without extra procedure and time cost, relatively accurate kidney depths can be estimated at the same time of dynamic renal imaging.

II. METHOD
The principle of the method is tomosynthesis, which is to determine the target location by taking pictures in different directions.
On the transverse section of kidney hilum, kidney depth is the perpendicular distance from the dorsal skin surface to the kidney center. The projection of the kidney center is approximately the center of the projection image. Another advantage is during the first ten minutes, the radioactivity concentrates in the kidney with low background counts.
Traditionally dynamic renal imaging use only one camera, since NET632 SPECT has two cameras, one can be used to collect dynamic renal data, another can be used to collect data at different angles. The real kidney centers can be determined by calculating the cross point of the kidney center's projection lines at different angles. Besides, the patient is on supine position with his back contacting the bed surface closely; thereby the kidney depth can be determined by the coordinate position of the kidney centers and the bed. The data acquisition of the two cameras is independent and synchronous, which do not need extra procedure and time cost.
Some theoretical calculations are necessary to obtain proper position parameters of the camera at each angle. Proper position parameters can insure that kidney projections are entirely included in the useful view, and the room between the patient and the camera is suitable for patients of general body size, also make sure safe measurement without interference among patient, cameras and bed. Then, several dynamic renal imaging tests with kidney models are conducted to verify these position parameters and the data acquisition procedure.

A. Data acquisition procedure
Dynamic renal imaging was performed by using NET632 SPECT of Novel Medical. 99 Tc m -DTPA 5 mCi was the radionuclide used in the patient. The patient drank 500ml water before the test. A 6 seconds pre-injection count was performed by placing the syringe 30 cm from the center of the parallelhole medium-energy collimator of the device. After the patient part of the study, a similar 6 seconds post-injection syringe count was performed. The radionuclide was rapidly injected intravenously into the supine patient who was prepositioned on the bed with ensuring that the kidneys were in the projection view. Matrix size of the renal data is 64*64, that of syringe count data is 256*256. Data acquisition was initiated at the moment of injection.
The camera below the bed collects dynamic renal data by two phases, the first phase of 30 frames with 2 sec of each frame, and then the second phase of 20 frames with 1 min of each frame. Meanwhile the upper camera moves to different positions at angles of 15, 30, 45, 60 degrees respectively, and collected data in a pre-set time slot. See Fig. 1.

B. Kidney depth estimation
Perform image segment of the projection image of each angle, obtain the centroids of the segmented kidneys, and calculate the projection lines of the centroids of each angles by using the position parameters of the upper camera for each angle, then the cross point of all the angles' projection lines can be determined by optimization method, that is the real centroid of the kidney, see Fig. 2. The kidney depth can be calculated by the coordinate position of the kidney center and the bed.
Outline each kidney with its separate semilunar-shaped background region as an area of interest. The GFR estimation employs Gate's method. After the conclusion of process, the related renal parameters and the nephrogram are displayed.
The data acquisition procedure and kidney depth estimation method have been developed and integrated into the HumanSPECT software by Novel Medical. Under the options of "Kidney Depth Calculation", there is a new option of "Automatic computation by data of different angles" besides "Compute by Height and Weight" and "Manual Input".

III. RESULTS
This procedure and method has been performed on two volunteers, also CT of abdomen in the patients at Navy General Hospital were evaluated to determine kidney depths.
CT were performed on a GE PET/CT System, with the patient supine using a slice thickness of 3.8 mm. Kidney depth was determined by identifying the mid-transverse section of each kidney and measuring the perpendicular distance from the dorsal skin surface to the kidney center. Right and left kidneys were measured independently from the CT hardcopy images.
The two volunteers are all male and 35 years old. Height and weight are shown in the table. Kidney depths calculated by Tonnesen formula and SPECT were compared with the measured distances from skin-to-kidney center CT. Table 1, 2, 3 and 4 show the kidney depth results and relative error to the CT measurement results as well as GFR by Gate's method.
Compared to the kidney depth measured by CT, the result of Tonnesen formula is significantly lower; the relative error goes up to more than 10% even 20%. As a result, the GFR indirectly calculated by Tonnesen formula is lower than that by CT. While the estimated kidney depth by SPECT is larger than CT measurement with relative errors below 10%, and the sequent GFR are in a reasonable range. So compared to Tonnesen formula, the method by SPECT improves the accuracy of kidney depths and GFR estimation.

IV. DISCUSSION
In the process of dynamic 99Tcm-DTPA renal scintigraphy by SPECT, soft tissues attenuation between the camera and kidney will result in counts decrease, so it is necessary to correct the counts by the kidney depth (skin-to-kidney center distances).
Recently, Tonnesen formula is still widely used in the GFR processing software. The formula is obtained with the patients sitting and the kidney depths are measured from lateral plain by ultrasonic imaging system. While SPECT scans the patient on supine position, postural change will affect the kidney depth. Therefore, Tonnesen formula will underestimate the kidney depth which will affect the tissue attenuation correction and then affect the accuracy of GFR. Based on the Gates formula, 10 mm deviation of kidney depth will lead to 14% deviation of GFR.
The method proposed in this paper, makes full use of the two cameras without extra time cost. The kidney depth estimated by SPECT is a little larger than CT measurement. Compared to Tonnesen method, the accuracy of GFR measurement by Gates method is improved by using kidney depths estimated by SPECT. In the near future, further studies are needed to verify the accuracy and stability of kidney depth estimation by SPECT.
An invention patent about the data acquisition procedure and kidney depth estimation method has been applied and is now in the stage of examination.