whitepaper hitachi ct lateral shift table can improve image … · 2019. 6. 18. · in this white...

IntroductionHigh image quality, lower dose and patient comfort are key drivers in today’s imaging environment. Trying to accomplish all simultaneously can often be a challenge when imaging body parts that don’t fall nicely into the center of the CT field of view. In this whitepaper we will discuss how Hitachi’s Scenaria Computed Tomography scanner includes an automated Lateral Shift Table feature to simplify centering the patient, improving image quality and lowering radiation dose.

Hitachi CT Lateral Shift Table Can Improve Image Quality, Lower Dose, and Improve Patient Workflow

W H I T E PA P E R

Prepared by: Plexar Associates, Inc.David Rohler, Ph.D.August 2018

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2 CT Lateral Shift Table Can Improve Image Quality, Lower Dose, and improve patient Workflow

Table of Contents1 Overview...........................................................................................................................3

2 Patient Centering Review..............................................................................................4

3 Using the Lateral Shift table.........................................................................................53.1 Illustration of the lateral shift table .............................................................5 3.2 WorkflowAdvantagewiththeLateralShiftTable......................................93.3 ClinicalExperiencewiththeLateralShiftTable.........................................9

4 Spatial Resolution Advantage of the Lateral Shift Table..........................................104.1 StudyDescription........................................................................................104.2 Results...........................................................................................................11

5 Dose Reduction with the Lateral Shift table..............................................................125.1 StudyDescription........................................................................................125. 2 Results...........................................................................................................13

6 Contrast Resolution Advantage of the Lateral Shift Table.......................................146.1 StudyDescription........................................................................................146.2 Results...........................................................................................................15

7 Summary.........................................................................................................................16

8 References.....................................................................................................................17

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1. OverviewThe Hitachi Scenaria CT scanner features a lateral shifting table with a ±8cm shift range that facilitates placing the specific anatomy being studied at the isocenter of the CT scan. In this white paper we characterize specific advantages of the Lateral Shift Table for the Scenaria CT Scanner, preceded, in Section 2, with a review of the general issues regarding patient miscentering, dose and image quality.

We will review the literature to explore the general issues regarding the value of the bow tie filters (BTF) and the impact of patient miscentering on CT dose and image quality as regards (1) how correct patient centering is important for the Automatic Exposure Control feature of CT scanners and (2) how correct patient centering can often allow the small bow tie filter (BTF) to be used, thus reducing patient dose and improving image quality.

The Hitachi Lateral Shift Table is used by the CT technologist and we will demonstrate, by example, how centering the desired anatomy is automated with no requirement for the technologist to interrupt the procedure and physically move the patient. We present estimates of the procedure time lost if the technologist were to physically move the patient and repeat the Scout Scan. In routine practice without a lateral shift table, a patient is seldom moved laterally by the technologist because it is difficult to achieve an accurate shift and because the time lost is significant. Instead, the technologist will typically proceed with a suboptimal scan that uses a larger field-of-view (FOV) to encompass the off-center anatomy being studied. We have studied the impact of the suboptimal scanning on spatial resolution and on CT dose.

Regarding the dose reduction and image improvement benefits of the Lateral Shift Table;1. We will demonstrate that spatial resolution in the specific anatomy being studied will be

reduced by more than 20 % if the specific anatomy being studied is off-center. The Wire Section of the standard HMC Performance Phantom is used to establish this result.

2. We will demonstrate dose reductions up to 28% when the Lateral Shift Table and small BTF are used together. The 32 cm PMMA CT Dose Phantom is used to establish these results. We also demonstrate that use of the small BTF does not result in increased beam hardening artifact.

3. We will demonstrate that contrast resolution can be significantly improved when the Lateral Shift Table is employed and, thus, the small BTF can be used. Contrast-to-noise (CNR) improvements from 43% - 63% were demonstrated.


2. Patient Centering ReviewThe influence of patient centering on CT dose and image quality has been the subject of numerous studies over the last decade [6, 9, 10, 11, 12, 13, 14, 15, 18, 19]. All of the studies conclude that patient miscentering has a deleterious effect on CT dose and image quality. In the following paragraphs, we highlight the conclusions for a few of the studies.

In [6], Toth et al. observe that “Clinical image quality and dose efficiency can be improved on scanners with bowtie filters if care is exercised when positioning patients. Automatically providing patient specific centering and scan parameter selection information can help the technologist improve workflow, achieve more consistent image quality and reduce patient dose.”

In [12], Habibzadeh et al. observe that “The operation of the bowtie filter in x-ray CT is correct if the object being scanned is properly centered in the scanner’s field-of-view. Otherwise, the dose delivered to the patient and image noise will deviate from optimal setting.”

In [13], Matsubara et al. conclude that “Inappropriate patient centering causes misoperation of automatic tube current modulation systems, in which tube current is controlled with information from localizer radiographs, and thus causes increases in tube current or image noise.”

In [14], Akin-Akintayo et al. studied the prevalence and severity of vertical and horizontal miscentering in diagnostic CT and concluded that “Off-centering is common during CT imaging and has been previously demonstrated to impact dose and image quality.”In [19], McCollough et al. describe the importance of proper patient centering in determining size-specific dose estimates (SSDE) for CT.

Although most of the studies restrict their analysis to vertical miscentering because that is the most prevalent cause of miscentering [14], studies such as [15] include both vertical and horizontal miscentering, specifically concluding that “Off-center patient positions cause errors in tube current modulation that can outweigh the dose reduction gained by AEC use, and image quality is affected.”

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3. Using the Lateral Shift TableWhen a patient is scanned, the technologist often has an internal region of the anatomy (e.g. liver, kidney, lungs, shoulder) that is targeted for imaging (anatomy of interest). When the patient is initially placed on the table, the technologist may not know (a priori) exactly where that internal region of interest is located. After taking the low dose scout planning image(s), however, the technologist can typically see the anatomy of interest and assess where it is relative to the isocenter of the scanner. The technologist can easily determine (1) a quantitative vertical offset and (2) a quantitative lateral offset required to bring the anatomy of interest to the scanner isocenter.

Without a Lateral Shift Table, centering the anatomy of interest is difficult and inaccurate. Vertical centering can be accomplished using the patient table controls but horizontal centering can only be done by physically moving the patient and this manual shift of the patient is (1) inaccurate, (2) physically difficult for the technologist, (3) uncomfortable for the patient, and (4) a time-consuming interruption in the workflow. Also, if a manual shift is done, a new scout planning image must be acquired.

With the Lateral Shift Table, the technologist can now center the patient in both the vertical and lateral direction, in some cases, without leaving the scan control room.

3.1 Illustration of the Lateral Shift Table

The following sequence of images and text illustrate, with an anthropomorphic phantom, how the Lateral Shift Table can be used. The “patient” is placed on the table, somewhat off-center, as a starting position as illustrated in Figure 1 below.

Figure 1: Initial placement of “patient.”


A Scout Scan (Scanogram) is then performed using the appropriate controls on the Scan Control GUI that is shown in Figure 2 below.

The Scout Scan then appears in the viewing window of the Scan Control GUI as illustrated in Figure 3 below. The vertical blue line represents the isocenter of the scanner. In this case, even though the patient was initially positioned toward the shoulder, further shifting is desirable.

The technologist simply drags the blue line toward the desired anatomical location as illustrated in Figure 4 below and “confirms” the study.

Figure 2: Scan Control GUI

Figure 3: Scanogram of anthropomorphic phantom as initially positioned. Blue line represents the scanner isocenter.

Figure 4: Scout Scan of anthropomorphic phantom indicating desired isocenter position (blue line).

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The scanner software will then ask the technologist to press the MOVE button to activate the lateral table shift as shown in the Scan Control GUI overlay in Figure 5 below.

The picture in Figure 6 below shows the resulting “patient” positioning.

With this lateral shift, the technologist is also able to select a smaller FOV and, thus, the small BTF. For this specific illustration, a significant dose reduction is achieved. Figure 7, below, shows the dose display (embedded in the Scan Control GUI) before and after the Lateral Shift.

Figure 5: GUI instructions for activating the Lateral Table Shift

Figure 7: Scanner reported dose before and after lateral shift.

Figure 6: Patient position after the Lateral Shift, initiated from the scan control GUI.


Alternatively, the technologist can select the lateral shift at the scanner, using gantry buttons, as shown in Figure 8 below.

To help guide that movement, the Touch Vision Monitor will indicate the allowable range for the lateral shift movement. This range depends on the height of the table and, in this case as shown in Figure 9 below, it is 70 mm in either direction.

Figure 8: Scanner room controls for applying desired lateral shift.

Figure 9: Touch Vision Monitor on the scanner gantry showing allowable range of lateral shift.

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3.2 Workflow Advantage with the Lateral Shift Table

Without a Lateral Shift Table, if the technologist decides to manually shift the patient laterally to center the anatomy of interest, the following additional required steps can delay the completion of the scan by 3 to 5 minutes.

1. Entering the scan room and manually shifting the patient, including providing appropriate cushioning.

2. Redoing the Scout planning images and confirming that the anatomy of interest is adequately centered.

3.3 Clinical Experience with the Lateral Shift Table

The reports from clinical sites that have been using the Lateral Shift Table are very positive. Here is a representative testimonial.

“The Hitachi Scenaria 128 slice scanner has saved my back which is very important to the longevity of my career. Not having to pull patients laying on the table has been wonderful. I use the lateral table shift to put them into position prior to the Scout scan at the gantry and then make final adjustments at the console. This is particularly helpful when performing cardiac scanning.”

Marlene S. Abrams, CT TechnologistSarasota Interventional Radiology, Sarasota, FL.


4. Spatial Resolution Advantage of the Lateral Shift TableOne of the advantages asserted for the Lateral Shift Table is that the anatomy of interest will be reconstructed with the best possible spatial resolution because it is positioned closer to the scanner isocenter.

It is well known that a scanner’s best spatial resolution is achieved at or near the scanner isocenter [8]. This is true for all CT scanners. A spatial resolution study was completed for the Scenaria to determine the change in spatial resolution as a function of distance from the scanner isocenter. The specifics of that study and its results are presented in this section.

4.1 Study DescriptionEach Scenaria scanner is equipped with a set of quality assurance phantoms that is used by Hitachi service personnel to measure and track scanner performance. These phantoms have been in use for many years at many scanner installations to periodically test and confirm scanner performance. Based on Hitachi’s consistent track record with these phantoms, the Wire Section of the standard HMC Performance Phantom, shown in Figure 10 below, was selected for this spatial resolution test.

Scans were made of this phantom, manually positioned at these five (5) offsets from the isocenter: (0.0, 2.0, 4.0, 6.0, 8.0 cm). Consistent with the Hitachi High Resolution Evaluation Mode [1], the scans were performed and reconstructed at the protocol shown in Table 1 on the next page. The x, y offsets for each reconstruction were set so that the wire would be positioned approximately in the center of the reconstructed image.

Figure 10: HMC Performance Phantom, Wire Section

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Scan Time 2 Sec

kVp 120

mA 275

Focal Spot Small

Slice Thickness 10 mm

Filter 60H

Field of View 5.0 cm

For each position, the MTF (Modulation Transfer Function) was computed as the Fourier transform of the (line spread) function obtained by calculating the row average in a small image region containing the wire. From the MTF curves, the frequencies corresponding to 0%, 10% and 50% values are identified and used for the spatial resolution comparisons. This method is consistent with many MTF comparison studies [2, 3 and 4].

4.2 Results

The MTF results are shown in tabular form in Table 2 below in which the frequencies are presented in line pairs per centimeter (lp/cm).

MTF % Crossings (lp/cm)

Off-Center 0% 10% 50%

None 18.9 14.1 11.8

2 cm 17.6 14.3 11.9

4 cm 16.2 13.5 10.8

6 cm 15.7 14.2 10.4

8 cm 14.5 13.1 10.1

From these results, we can conclude that there is definite loss of spatial resolution as the object of interest is moved off-center, greater than 20% loss of resolution at the zero crossing, approximately 15% loss at the 50% crossing, and approximately 10% loss at the 10% crossing.

These same results are shown in graphical form in Figure 11 below

Table 1: Protocol for Spatial Resolution Evaluation

Table 2: Frequency at key MTF % crossings in lp/cm

Figure 11: MTF plots for offsets: [0.0, 2.0, 4.0, 6.0, 8.0 cm]


5. Dose Reduction with the Lateral Shift TableWhen the anatomy of interest for a clinical CT scan is contained within a smaller FOV (about 25 cm or smaller), the smaller BTF can be used and, thus, there is an opportunity for dose reduction, even if the anatomy of interest is internal to a larger body diameter. Typically, the dose reduction can be achieved only if the anatomy of interest is well centered in the scanner, therefore, this opportunity for dose reduction is significantly improved with the Lateral Shift Table. Dose reduction is achieved when the anatomy of interest is well-centered in the scanner and fits within a smaller FOV because the small bow tie filter (BTF) can be used.

To confirm and quantify this impact for the Scenaria scanner, a study was performed to determine the dose reduction for a given technique when the small BTF is used as compared to the dose for the same technique using the large BTF.

5.1 Study Description

To measure dose, the 32cm PMMA CT Dose Phantom was used in conjunction with the Standard Imaging Extradin A101 dosimetry system [18]. Axial scans were performed using a routine brain protocol at 120kVp, 350mAs, 0.625x32 with the following variations in scan FOV and BTF:

1. Large BTF, 350 mm FOV

2. Large BTF, 239 mm FOV

3. Small BTF, 239 mm FOV

Dose was measured at the following positions listed below.

1. Isocenter (of the 32cm PMMA phantom

2. 7 cm off center (at 3 o’clock on the 16cm insert

3. 14 cm off center (at 3 o’clock on the 32cm periphery)

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5.2 Results

At least two (2) readings were taken at each position and for each configuration and then averaged to generate the results shown below. In Table 3 below, the measurements for the Small BTF at FOV=239mm are compared with measurements for the Large BTF at both FOV=239mm and FOV=350mm.

Distance from Isocenter (cm)

Small BTF Large BTF Dosimetry Difference

(Large BTF - Small BTF)

% Dose Reduction

Benefit with Small BTFFOV (mm)

Average Dosimetry Reading

FOV (mm)Average

Dosimetry Reading

0 239 2.86 350 3.23 0.37 11.5%

0 239 2.86 239 3.22 0.36 11.3%

7 239 3.86 350 4.51 0.66 14.5%

7 239 3.86 239 4.39 0.54 12.2%

15 239 5.08 350 7.14 2.06 28.8%

15 239 5.08 239 6.98 1.91 27.3%

The results shown in Table 3 above are illustrated graphically in Figure 12 below along with a curve fit for the data.

Table 3: CT Dose Reduction with Small BTF compared to Large BTF at FOV=(239, 350 mm)

Figure 12: Dose Reduction % for Small BTF vs. Large BTF


6. Contrast Resolution Advantage of the Lateral Shift TableAs we described in Section 6 above, when the anatomy of interest for a clinical CT scan is contained within a smaller FOV (about 25 cm or smaller), the smaller BTF can be used. In Section 6, we demonstrated the dose reduction that results from the use of the smaller BTF. In this section, we demonstrate that contrast resolution is significantly improved due to use of the smaller BTF.

6.1 Study Description

For this study, we used the Low Contrast Detectability (LCD) section of the Catphan phantom [17]. We performed Scenaria scans with this phantom at 120 kVp, 10 mm slice thickness and with the standard soft tissue kernel (32). Scans were taken at all combinations of the following:

1. mA: 125, 250

2. BTF: small, large

3. Lateral Shift Table Position (cm): 0.0, 2.0, 4.0, 6.0, 8.0

For each of the scans, we measured the mean and standard deviation in the center of the largest (15 mm) contrast pins in the 1.0% and 0.5% contrast sections and the mean and standard deviation in background regions near the 1.0% and 0.5% pins. These measurements are illustrated in Figure 13 below.

Figure 13: Contrast measurement illustration

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6.2 Results

As expected, the CNR values correlated with the mA used and we found that the CNR values correlated with the choice of BTF, that is, the CNR was significantly improved with the small BTF as compared with the large BTF. We did not, however, find any significant correlation of CNR with the amount of lateral shift. For that reason, the results presented in Table 4 below use only the 0.0 cm, 4.0 cm and 8.0 cm lateral shift values.

1.0% Pin 0.5% Pin

Bow Tie mA Lateral Position

Pin Mean

Back Mean Back SD Contrast CNR Pin

MeanBack Mean Back SD Contrast CNR

Small 250 0.0 cm 66.10 55.00 5.50 11.10 2.02 57.70 51.00 5.00 6.70 1.34

Large 250 8.0 cm 62.50 54.70 7.30 7.80 1.07 59.90 54.40 7.10 5.50 0.77

Large 250 4.0 cm 63.10 54.50 5.20 8.60 1.65 58.80 54.10 5.40 4.70 0.87

Large 250 0.0 cm 62.90 54.10 5.80 8.80 1.52 56.40 51.80 5.60 4.60 0.82

Small 125 0.0 cm 66.60 55.10 7.50 11.50 1.53 57.50 50.90 7.10 6.60 0.93

Large 125 8.0 cm 64.50 54.60 9.90 9.90 1.00 60.20 54.90 9.80 5.30 0.54

Large 125 4.0 cm 62.90 54.80 8.20 8.10 0.99 58.10 53.30 7.00 4.80 0.69

Large 125 0.0 cm 63.20 53.50 8.60 9.70 1.13 56.20 52.60 7.30 3.60 0.49

In Table 4, we highlight (in green) the CNR values for the small BTF with 0.0 lateral shift and the corresponding CNR values for the large BTF at the selected lateral shifts (orange). We then determined an average CNR percentage improvement for the small BTF relative to the large BTF as follows:

1. 1.0% pin at 250 mA: 43%

2. 1.0% pin at 125 mA: 48%

3. 0. 5% pin at 250 mA: 63

4. 0.5% pin at 125 mA: 62%

Table 4: Contrast and CNR measurements for selected scans


7. SummaryThe Lateral Shift Table can result in scans with (1) up to 20% better spatial resolution because the anatomy of interest is centered in the scanner, (2) up to 30% lower dose because when the anatomy of interest is centered in the scanner, the small BTF may be used instead of the large BTF and (3) more than 60% improvement in contrast resolution.

The Lateral Shift Table can improve throughput, saving 3 to 5 minutes in the scan procedure because manual patient re-positioning is not required, thereby also reducing the possibility of physical strain on the technologist.

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8. References1. “SCENARIA Ph3.0 Detailed Specification.” March 16, 2015.

2. Richard S., Husarik D., Yadava G., Murphy S., and Samei E., “Towards task-based assessment of CT performance: System and object MTF across different reconstruction algorithms.” Med. Phys. 39, 4115–4122 (2012).10.1118/1.4725171.

3. 4C. J. Bischof and J. C. Ehrhardt, “Modulation transfer function of the EMI CT head scanner.” Med. Phys. 4(2), 163–167 (1977).

4. “MTF Measurement from CATphan.” ImageOwl Wiki, Catphan®QA MTF, http://help.imageowl.com/index.php/Catphan%C2%AEQA_MTF.

5. N. Mail, D. J. Moseley, J. H. Siewerdsen, and D. A. Jaffray, “The influence of bowtie filtration on cone-beam CT image quality.” Med. Phys. 36(1), 22–32 (2009).

6. Toth, Thomas & Ge, Zhanyu & Daly, Michael. (2007). “The influence of patient centering on CT dose and image noise.” Medical physics. 34. 3093-101. 10.1118/1.2748113.

7. 10J. Li, M. Kalra, T. Toth, U. Udayasanka, J. Seamans, and W. Small, “Automatic patient centering for multisection CT: Effect on radiation dose and image quality.” AJR, Am. J. Roentgenol. 188, 547–552, (2007).

8. S. Kam, H. Youn, H.K. Kim, H.Jeon, “An experimental study on the shift-variant MTF of CT systems using a simple cylindrical phantom.” Medical Imaging 2013, 1605-7422/13.

9. Filev PD, Mittal PK, Tang X, et al. “Increased computed tomography dose due to miscentering with use of automated tube voltage selection: Phantom and patient study.” Curr Probl Diagn Radiol 2016; 45:265–70.

10. Tsalafoutas IA, Georgolopoulou P, Abatzoglou I. CT dosimetry considerations in nontypical conditions: The effect of scan field of view and table height selection. Phys Med 2012; 28:83–90.

11. Kaasalainen T, Palmu K, Reijonen V, et al. “Effect of patient centering on patient dose and image noise in chest CT.” Am J Roentgenol 2014; 203:123–30.

12. Habibzadeh MA, Ay MR, Asl AR, et al. “Impact of miscentering on patient dose and image noise in x-ray CT imaging: Phantom and clinical studies.” Phys Med 2012; 28:191–9.

13. Matsubara K, Koshida K, Ichikawa K, et al. “Misoperation of CT automatic tube current modulation systems with inappropriate patient centering: phantom studies.” Am J Roentgenol 2009; 192:862–5.

14. Akin-Akintayo OO, Alexander LF, Neill R, Krupinksi EA, Tang X, Mittal PK, Small WC, Moreno CC, “Prevalence and Severity of Off-Centering During Diagnostic CT: Observations From 57,621 CT scans of the Chest, Abdomen, and/or Pelvis, Current Problems in Diagnostic Radiology.” 2018.

15. Gudjonsdottir J. “Efficient Use of Automatic Exposure Control Systems in Computed Tomography Requires Correct Patient Positioning.” Acta Radiologica, 8, 2009.

16. Catphan 500 and 600 Manual.” The Phantom Laboratory, PO Box 511, Salem, NY 12865-0511, 2006.

17. https://www.standardimaging.com/exradin/ct-ion-chambers.

18. Posniak M. “CT scan optimization- Patient Centering: a pitfall to avoid. 16th Annual International Symposium on Multi-detector-Row CT.” International Society of Computed Tomography. June 9-12, 2014.

19. McCollough C, Bakalyar DM, Bostani M, Brady S, Boedeker K, Boone JM, Chen-Mayer H, Christianson OI, Leng S, Li B, McNitt-Gray MF, Nilsen RA, Supanich MP, Wang J. “Use of Water Equivalent Diameter for Calculating Patient Size and Size-Specific Dose Estimates (SSDE) in CT: The Report of AAPM Task Group 220.” AAPM Rep. 2014 September; 2014: 6–23.

Reprinted by: Hitachi Healthcare Americas1959 Summit Commerce ParkTwinsburg, Ohio 44087 USATel: 330.425.1313 800.800.3106Fax: 330. 425.1410

0818/1000/DM#127718 © 2018 Hitachi Healthcare Americas

whitepaper hitachi ct lateral shift table can improve image … · 2019. 6. 18. · in this white...

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