A suggested method for setting up GSI profiles on the GE Revolution CT scanner

Abstract “GSI Assist” is the automatic exposure control (AEC) system for dual‐energy acquisitions on the GE Revolution CT scanner. This paper describes the user options of GSI Assist, and describes the method developed at UAB Medical Center to simplify the use of GSI Assist without adversely affecting the AEC Operation.


| INTRODUCTION
CT automatic exposure control (AEC) is commonly used for chest, abdomen, and pelvis CT exams. AEC systems adjust the tube current (and on some systems the tube voltage 1-3 ) to accommodate differences in patient size, both from patient to patient and within a single patient acquisition, in order to simplify the task of obtaining adequate image quality without the use of excessive radiation dose. 4,5 Dual-energy scanners acquire two independent sets of attenuation data with different energy-dependent characteristics. Dual energy General Electric Revolution CT scanners accomplish this by changing the tube voltage from one x-ray pulse to the next at a rate of more than 1000 pulses per second. 6 The tube current may vary from exam to exam, but the high-energy and low-energy tube currents are each fixed during a single exam, and the tube voltages do not vary from exam to exam.

1.A | Overview of CT automatic exposure control
Most modern scanners have AEC systems. These systems use localizer views to determine the patient size, and then adjust the tube current accordingly, using higher currents for larger patients and lower currents for smaller patients. AEC systems usually also have tube current modulation options. 7 Longitudinal modulation (or zmodulation) adjusts the tube current as the patient thickness changes along the length of the patient. Angular modulation (or rotational modulation) uses higher tube currents for lateral views and lower tube currents for anterior/posterior views; this allows for a reduction in noise without increasing patient dose. A variation on angular modulation uses a lower tube current when the tube is anterior to the patient, and a higher tube current when posterior. This lowers patient dose to the front of the body, where most radiosensitive organs are located, without increasing image noise. 8 1.B | Automatic exposure control in GE Revolution scanners 1.B.1 | Single voltage mode On the Revolution scanner, single voltage AEC is called "SmartMA." In single-voltage scans, the user chooses a setting called the "noise index," which is part of the imaging protocol but which can be overridden by the technologist at scan time. The AEC system uses the localizer images (called "scout" images on GE scanners) to determine the patient thickness, calculates the CTDI vol required to produce primary patient images whose noise is equal to the noise index, and then calculates the tube current required to produce this CTDI vol .
Since the noise is also affected by the primary image thickness and iterative reconstruction (ASIR-V) settings of the primary reconstruction series, these settings also affect the tube current. 6 However, the choice of filter kernel affects image noise but does not affect the tube current; if the primary series has a kernel other than "Standard," the noise in the primary series will not match the noise index. This is similar to other GE scanners, except that in the Discovery and Optima series the AEC takes the primary image thickness into

1.B.2 | Dual energy mode
The dual energy imaging technology used on the Revolution is called "Gemstone Spectral Imaging" (or "GSI"), and dual energy AEC on the Revolution is called "GSI Assist." The user selects the "target noise index" (or "target NI"), and the AEC system calculates the CTDI vol (the "target CTDI vol ") that would be used for a 120 kV single voltage acquisition without longitudinal modulation when the noise index is set equal to the target NI. The system then selects a combination of high-voltage and low-voltage tube currents that would produce a CTDI vol close to the target CTDI vol . According to the GSI Xtream Technology White Paper, 6 the noise in the 70 keV monochromatic image matches the image noise in a dose-matched single-energy acquisition. Therefore, the noise in the 70 keV image should be close to the target noise index.
The low-energy tube current and the high-energy tube current each remain fixed throughout the acquisition; that is, neither longitudinal nor angular modulation is used. Because longitudinal modulation is not used, the system calculates a single target CTDI vol based on the patient thickness along the entire scan length. This paper addresses the settings available to the user when setting up protocols on the Revolution, those that are available to the technologist at scan time, and the approach used at the UAB Medical Center to simplify the task of the technologist without adversely affecting image quality or radiation dose.

1.C | Details
The GSI Assist parameters are arranged in a hierarchy of groups; these groups are the "GSI Preset Family," the "GSI Recon," and the "GSI Profile"; this hierarchy is shown in Fig. 1. These groups are described in detail in Appendix C, but here we present an overview of each and how they are related.  different Preset Family. Several GE GSI Profiles are preloaded onto each scanner, but the user can define new GSI Profiles. • A GSI Protocol is simply a CT protocol that includes one or more GSI acquisitions. Each GSI Protocol must specify a Clinical Identifier, which determines which GSI Profile to use. According to GE Medical Systems, in future releases of the software, it will be possible to specify the GSI Profile for each protocol independently of the clinical identifier.

1.C.1 | Use of GSI Assist
When a scout view is taken as part of a clinical scan, the scanner will calculate the value of CTDI vol that would be produced by a 120 kV single-energy acquisition that uses the target noise index.
For each Preset Family in the GSI Profile, the scanner determines which of the available tube currents will produce the value of CTDI vol closest to the target CTDI vol . If the technologist takes no action, the primary Preset Family will be used even if use of another   The GE Reference Profiles were designed to give the technologist maximum flexibility in selecting the rotation time and helical pitch within certain constraints imposed by clinical considerations.
Because the "GE Routine Abdomen Contrast" profile includes many more Preset Families than the "UAB LIVER" profile, there are many more combinations of tube current, rotation time, and helical pitch available, and therefore the target CTDI vol can generally be more closely matched by the technologist. On the other hand, because the "UAB LIVER" profile includes a Preset Family that can produce CTDI vol values down to 4 mGy, a wider range of target CTDIs can be matched than with the reference profile. In addition, the reduced number of options available to the technologist using the "UAB LIVER" profile simplifies the task of selecting the Preset Family.

1.C.2 | User interface
After the scout view is taken, the scanner displays the scout image ( Fig. 4) and the GSI Image Quality widget dialog (Fig. 5). The IQ widget displays both the target noise index, and the "average projected noise index" (AvgPNI), which is an estimate of the image noise averaged over the scan length if the presently selected Preset Family is used. When the IQ widget first appears, the AvgPNI for the primary Preset Family is displayed.
The parts of the anatomy for which the projected noise is above the Prescribed NI are indicated by orange, as in Fig. 4. If the AvgPNI is above the Prescribed NI, it will be displayed in orange in the GSI widget as in Fig. 5.
If the projected noise is below the Prescribed NI for the entire scan, no orange sidebars are displayed (Fig. 6). Whenever the Average Projected NI is equal to or below the target Noise Index, it will be displayed in blue (Fig. 7).
As longitudinal tube current modulation is not used in GSI acquisitions, whenever the Average Projected NI is close to the Prescribed NI (Fig. 8), the projected noise is likely to be above the Prescribed NI for part of the scan. In this case, orange sidebars run along parts of the scout (Fig. 9).
The technologist has the option of changing the Preset Family selection by clicking on the "Adjust" button. The scanner will then display a Solutions dialog box ( Fig. 10) listing each of the GSI Preset Families in the GSI Profile, as well as the projected noise index (PNI) and the CTDI vol for the tube current selected for each Preset Family.
The technologist may select any one of these Preset Families, overriding the present selection.
The number of options available in this dialog box can be substantial. There will be one row of data for each GSI Preset Family in the GSI Profile; for the GE Reference Profiles, this can be as few as three rows and as many as nine rows.
If the CID Link setting is turned on, the Clinical Identifier determines which GSI profile to use. If the CID Link setting for a GSI acquisition is turned off, the GSI Profile may be selected manually.
The goal of the method presented below is to design a set of F I G . 6. The scout view of the same scan after selecting the "Large Body/80 mm/L/1.0 s/0.508:1" scan settings. Because the projected noise is below the Prescribed NI for the entire scan, no orange sidebars are present.
The GSI IQ widget for the same scan settings as Fig. 10.
Since the Average Projected NI is below the Prescribed NI, it is displayed in blue. In this case, the radiation dose is higher than desired.
F I G . 8. The GSI IQ widget for the same scan settings as Fig. 9.
At UAB Medical Center, we created User Profiles that contain only three GSI Preset Families per profile, one for small, one for medium, and one for large patients. The medium Preset Family is specified as the primary Preset Family.
For example, the Preset Families in the "UAB LIVER" profile are listed in Table 1 so this difference is not clinically significant.
If the PNI is more than 2 HU below the target noise index, the "Adjust" tool is used to select the "Small adult" Preset Family. If the PNI more than 2 HU above the target noise index, the "Adjust" tool is used to select the "Large Adult" preset.

2.A | Suggested approach to customizing GSI Protocols
The following approach can be used to customize GSI profiles to accommodate the needs of a specific facility for a specific type of exam. The goal of this procedure is to select three or four Preset Families whose CTDI vol values overlap, but with as little overlap as possible.
This allows a wide range of possible CTDI vol values with few Preset Families, and thus simplifies the selection of Preset Family.
This procedure makes use of the tables in Appendix B. Each F I G . 1 0 . GSI Image Quality widget after clicking on the "Adjust" button ("Solutions window"). This display allows the technologist to change the Preset Family ("Scan Settings"). A yellow mA (not shown here) would indicate that the maximum tube current has been reduced to avoid tube overheating; an orange PNI indicates that the projected CTDI vol is above the target CTDI vol .
T A B L E 1 "UAB LIVER" GSI protocol.  In selecting the Preset Families to include in a profile, consider the following 6 : 1. It has been observed in the GE Discovery scanners that material decomposition values are more accurate for low-current, long rotation time combinations than at high-current, short rotation time combinations. 10 This is likely because for short rotation times, the time spent switching between tube voltages is a higher fraction of the total rotation time, reducing the accuracy of the acquired data. Therefore, use of longer rotation time allows more accurate material decomposition images at the expense of increased sensitivity to patient motion. This is more likely to be useful in body studies than in chest studies.
2. The use of the Extra Large focal spot allows higher tube currents, but may degrade spatial resolution. 11 This is more likely to be a problem in MSK or chest protocols than in abdominal protocols.

3.
As each Preset Family imposes a lower limit on the tube current, to achieve appropriately low radiation dose for smaller patients it may be necessary to include Preset Families with higher helical pitches. This is unlike single-energy AEC systems, which will compensate for higher helical pitches by raising the tube current to maintain the same CTDI vol . 12 However, the use of high helical pitch may introduce image artifacts.

5.
The range of available values of CTDI vol will depend on the selected constraints. For example, allowing helical pitch values ranging from 0.508 to 1.531 will allow the use of both higher F I G . 1 1 . The range of CTDI vol values that can be produced by various user GSI profiles produced with the procedure described in this paper, and by various profiles supplied by the vendor.
and lower values of CTDI vol than available if the helical pitch is constrained to always be 0.992.

2.B | Example
Suppose that you wish to build a new GSI Profile based on the "GE Routine Abdomen Contrast" profile, using the large body field of view, 80 mm collimation, and helical pitches of no more than 1.375, and which may use the extra large focal spot.
1. See Table B1 in Appendix B; this lists the Preset Families for the large body SFOV and 80 mm collimation.

2.
Find the first Preset Family with a helical pitch of no more than 1.375; the CTDI vol for this Preset Family ranges from 3.4 to 8.8 mGy. This is the "Small Patient" Preset Family.
3. Find the last Preset Family that uses the large focal spot, has a helical pitch of no more than 1.375, and has a minimum CTDI vol of less than 8.8 mGy; the CTDI vol for this combination ranges from 7.1 to 18.9 mGy. This is the "Medium Patient" Preset Family.

Find the last Preset Family that uses the large focal spot with a
helical pitch of no more than 1.375 and a minimum CTDI vol of less than 18.9 mGy. The CTDI vol ranges from 17.2 to 45.5 mGy.
This is the "Large Patient" Preset Family.

Find the Preset Family with the highest maximum CTDI vol and a
helical pitch of no more than 1.375. The CTDI vol ranges from 49.7 to 53.7 mGy. This is the "Extra Large Patient" Preset Family.

| DISCUSSION AN D CONCLUSION
The vendor supplied GSI profiles allow the scanner to match the PNI to the target noise index to within about 1%. However, the range of values of CTDI vol available to the scanner when using some profiles is relatively limited, resulting in excess radiation dose for small patients and excess noise for large patients. In addition, to achieve the full precision of the PNI, the technologist must always select the right preset profile in the Image Quality Adjust tool. Thus it may be difficult in practice to achieve this 1% match in target noise index. This profile is designed for abdomen and pelvis protocols, where patient motion is not an issue but there is concern about artifacts from helical pitches larger than 1.375. This profile is designed for abdomen and pelvis protocols, where patient motion is not an issue but there is concern about artifacts from helical pitches larger than 1. By developing and using custom GSI Profiles using the technique described here, we have managed to both simplify the technologist's task in selecting presets and increase the range of patient sizes that can be accommodated. However, these goals are achieved by allowing larger noise index error (up to 5%) than the reference GSI profiles for some imaging conditions. We believe that deviations at this level are clinically insignificant. Over the length of an abdomen/pelvis exam, the patient size may easily change by 5 cm; this corresponds to a change in patient transmission of about a factor of 2, or a 40% change in the image noise. An error of 5% in the average noise may be considered insignificant compared to the 40% variation in image noise along the length of the scan.

ACKNOWLEDG MENTS
The author would like to thank Desiree Morgan, M.D, and Susan Fletcher, ASRT, of UAB Medical Center, and Scott Slavic, Holly McDaniel, Priti Madhav, and Dominic Croty of GE Medical Systems for reviewing this paper and providing useful suggestions.

CONFLI CTS OF INTEREST
No conflict of interest. This profile is designed for abdomen protocols for which priority is given to quantitative pixel value accuracy.

APPEN DIX A ADD ITIONAL GSI PROFILES
The following tables describe GSI profiles for 40 mm detector coverage, and for the medium body field of view. This information is based on information in Table 11  Parameters specified by a GSI Preset Family; this is a summary from Chapter 12, Section 6, Table 11 of the Revolution User's Manual 13 . According to GE Medical Systems, Preset Families using the Adult Head SFOV will be added to a version of the software released late in 2018.
T A B L E C 3 List of the parameters included in a GSI Profile. Any GSI acquisition needs to specify the GSI Profile to use. The user can define custom GSI Profiles.

Primary Preset Family
The GSI Preset Family that will be used unless the user overrides this choice Description Text description of the profile GSI Recons A list of the GSI Recon series that will be produced GSI Preset Families A list of the GSI Preset Families that the user can select using the "GSI Assist Tool" Parameters specified by a GSI Profile.