How DOSE assists medical physicists in identifying and analysing significant events - part 3

August 11, 2021

By Dr. rer. nat. Hans Dieter Nagel

How DOSE supports the Medical Physics Expert in identifying and analysing significant events - Introduction
Part 1: Identification of significant events according to StrlSchV Annex 14 I and II (1)
Part 2: Identification of significant events according to StrlSchV Annex 14 I (2) a and 14 II (2+3) a

Part 3: Determining the causes of dose limit exceedances in the CT application area

The ability to identify and address significant events is one of the most important functions of a dose management system (DMS). The actual relevance of a 'significant event' is not so much the obligation to report it to the regulatory authority, but rather the associated oppor-tunity to learn from mistakes and avoid similar incidents with excessive radiation exposure in the future. To this end, determining the causes is essential.

The main reasons that lead to suspected significant events are:

  • scan protocols that are not optimized or are insufficiently optimized
  • individual case-related protocol modifications by the device operator
  • wrong protocol selection
  • foreign objects in the scan area
  • positioning and centering problems
  • excessive scan lengths
  • problems with contrast agent administration
  • extremely obese patients
  • outdated equipment
  • equipment technical deficiencies

A practical DMS is characterized by the fact that it provides the information needed to iden-tify the causes as comprehensively as possible. If this is not the case, the information must be obtained from the image and metadata. This is not only time-consuming, but also in-volves considerable delays, if the possibility of downloading the images or at least viewing the metadata is limited or non-existent.

Several causes are responsible for the majority of suspected significant event cases at the same time. Obesity is only one factor that makes the occurrence more likely. In most cases, it is operating errors and equipment problems that lead to significant overexposure. In the following paragraph, we will show how DOSE assists in identifying the causes.

1. Scan protocols not optimized or insufficiently optimized

One indicator for this is the dose level relative to the diagnostic reference level (DRL) for the parameter CTDIvol. The 'Compliance monitoring' dashboard can be used to provide the val-ues needed for evaluation (Fig. 1). Depending on whether the CT scanner has iterative re-construction (IR), the mean value for the respective protocol should be about 35% (with IR) and about 65% (without IR) (for CCT protocols: about 60% and 90%, respectively). 

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Fig. 1 Mean values and DRL for the parameter 'Weighted CTDIvol' per study group, from which the relative dose level can be derived (Note 1: DOSE is configured so that each protocol has its own study group; Note 2: 'Weighted CTDIvol' = mean of the CTDIvol values of the individual scan series - localizer scans excluded, weighted according to their scan length)

2. Case-related protocol modifications by the device operator

In most cases, these are changes in the mAs or kVp settings. Since the set values of the au-tomatic exposure control (AEC) are not documented in the RDSR, mAs modifications in AEC protocols can - if at all - only be detected by other means. The same applies to the noise pre-selection for scanners with noise-based AEC. Changes in kVp, on the other hand, can be easi-ly identified with the help of the series information, provided that the protocol setting is known (Fig. 2).

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Fig. 2 Example of a study with automatic exposure control in which the voltage was increased from 120 to 140 kV by the operator; mAs modifications from the protocol setting, on the other hand, cannot be identified.

3. Wrong protocol selection

Protocols operating with AEC are programmed with regard to anatomy and image quality requirements of the clinical task. Using them for other purposes can therefore lead to signif-icant under- or over-exposure. Such cases can usually already be identified in the study over-view by the obvious mismatch between the study description and the scan protocol used, as well as the localizer image. If the thumbnail is not sufficient, a larger image can be found in the advanced CT analysis under 'Series information -> SSDE', which provides more detailed information. The mA modulation curve is also shown there. The example in Fig. 3 shows that the modulation type of the protocol ('angular') is unsuitable for this body region and is one of the causes of the excessive dose.

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Fig. 3 Example of a study in which an abdominal scan protocol with angular dose modulation was 'misused' to image the pelvic-leg region. Wrong protocol selection and additional protocol changes by the operator resulted in an overall 8-fold excess exposure.

4. Foreign object in the scan area

Foreign bodies in the scan area (prostheses, implants, radiation protection devices) inevita-bly lead to corresponding dose increases in examinations with automatic exposure control due to the increased radiation attenuation. Unless longitudinal ('z') or 3D dose modulation ('xyz') is used, the exposure in the entire scan area is excessive. The thumbnail localizer im-age in the study overview already provides a first hint. The larger image in the advanced CT analysis under 'Series information -> SSDE' provides more detailed information as to wheth-er the foreign body is located inside or outside the scan area (Fig. 4).

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Fig. 4 Example of a study of a 22-year-old patient in which the gonadal shield located in the scan area com-bined with angular dose modulation caused an exposure increase by a factor of 4.7 throughout the scan area. In the case of foreign bodies in the scan area, the operators of this scanner were instructed to use protocols specifically designed for this purpose or to change the modulation type to 'longitudinal', which was not observed in this case.

5. Positioning and centering problems

Positioning problems such as arms in the scan area or parts of the shoulder in the scan area of head studies lead to problems similar to foreign bodies and are easily identified in DOSE using the localizer images. If the vertical centering is incorrect, the patient is shown reduced or enlarged in width on the localizer image. As a result, the patient effective diameter is miscalculated by the AEC by 0.5 cm per cm of centering error. Depending on the control characteristics of the scanner, this results in incorrect exposure by 5 to 10%. DOSE provides the corresponding information in graphical and numerical form under 'Series information -> Patient positioning' (Fig. 5).

Nagel-p3-5.png

Fig. 5 Example of a study in which the patient was centered almost 8 cm too low. With the particular scanner, the localizer image is generated in the PA projection, i.e. the patient is closer to the radiation source. As a result, the effective diameter was overestimated by 4 cm and the exposure increased by 60%.

6. Excessive scan lengths

As shown below in Figs. 7 and 8, DOSE displays the imaging area of the respective scan se-ries under both 'Series information -> SSDE' and 'Series information -> mA modulation'. This allows an assessment of whether the scan range has been selected in an anatomically ade-quate manner.

7. Problems with contrast agent administration

Often, these are problems at the injection site that cause contrast agent injection to be ex-travasated, resulting in reduced or no contrast agent flow. In the worst case, bolus monitor-ing is performed to the maximum pre-programmed time (often 60 s) and the subsequent scan series is performed on suspicion. The entire procedure must then be repeated due to this lack of success. If this happens within the same study, as is usually the case, it can be clearly traced on the basis of the information in the series information (Fig. 6). Since pre-monitoring and monitoring usually work with the same dose settings, the ratio of the CTDIvol values can be used to determine the number of rotations performed during monitoring and the total monitoring time. Alternatively, the ratio of the exposure times can be used for this purpose.

If the monitoring images are provided in the PACS or on the scanner, it can also be deter-mined under 'Series information -> Series overlap' whether the individual acquisition series overlap or are at least partially spatially separated from each other. With regard to the cu-mulative dose and the resulting reporting obligation as a significant event, this can be of decisive importance.

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Fig. 6 Example of a study in which the contrast agent was injected paravasally in the first trial. Monitoring ('tracker') was performed with the maximum pre-programmed number of rotations (30), i.e., for a total of 60 s. The planned CTA series was performed despite the absence of contrast agent enhancement and proved to be unsuccessful. The entire CTA procedure including bolus monitoring had to be repeated after a successful new injection site was established. The acquisition series of both CTA trials overlapped. The cumulative CTDIvol thus corresponds to the information given under 'CTDI (body 32 phantom)' and is above the 80 mGy- threshold.

8. Extremely obese patients

Whether extremely overweight patients with correspondingly increased dose requirements are causative or contributory factors can often already be identified with the aid of the thumbnail localizer images. If provided and reliable, the patient's body dimensions are a further indication. However, the most meaningful and suitable for quantitative estimation of the dose increase is the water equivalent diameter (WED). DOSE determines the WED from the axial image data and displays the value for the centre of the scan area under 'Series in-formation -> SSDE' (Fig. 7). In addition, the course of the WED along the entire scan area is shown under 'Series information -> mA modulation' (Fig. 8). The prerequisite is that the axial images are not excessively cropped ('truncated'). In spine studies, however, truncation is the rule, so the WED of approx. 20 cm is recognizably inaccurate. In such cases, it is better to use the effective diameter obtained from the localized image, which is also provided.

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Fig. 7 Specification of the WED (here 40.9 cm) for the center of the scan area, determined from the axial image data.

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Fig. 8 Course of the WED (blue) over the entire scan area.

9. Outdated equipment

These are mainly scanners without iterative reconstruction (IR). Due to the lack of noise re-duction, the dose level can only be reduced to a limited extent during optimization and is on average twice as high as with scanners equipped with IR. Accordingly, the proportion of DRL exceedances and thus the probability of significant event occurrence is increased. Often, these scanners also lack the 'dose check' function, which indicates expected DRL exceedance and allows a justification to be given promptly. Without this function, the operator is left in the dark, which inhibits preventive measures (such as correction of the scan area).

Information on the percentage of scanners that do not comply with the state of the art is not yet available. In my field of activity, it is 2 out of 10 scanners, i.e. 20%. If a scanner has these deficits, it should be known to the medical physics expert (MPE). The role of the DMS is lim-ited to naming the scanner that was used to perform the study.

10. Equipment technical deficiencies

An example of this is the incorrect determination of the WED by the AEC of a certain manu-facturer as soon as a lateral localizer is used or co-used. In the case of extremely obese pa-tients, as well as arms and foreign bodies in the scan area, this leads to considerable WED overestimations and overexposures by up to a factor of 3. The operators of this scanner were therefore instructed to take appropriate measures in these situations until the manufacturer solves the problem. Whether such a case is causal can already be identified in DOSE from the localizer thumbnail in the study overview, in which a lateral localizer is recognizable (Fig. 9). The full localizer images with the range limits and tube current courses, which can be found under 'Series information -> SSDE', also 'reveal' that the wrong protocol with an un-suitable modulation type ('angular') was used for the task (Fig. 10). This increased further the overexposure.

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Fig. 9 Example of a study in which the scanning protocol was modified by the operator with an additional lat-eral localizer.

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Fig. 10 Lateral and PA localizer images of this study with area boundaries and tube current courses. Due to for-eign bodies in the scan area (implants), the WED was incorrectly determined by the AEC system, which always uses the lateral localizer for this purpose, resulting in significant overexposure. In addition, an in-correct scan protocol with inadequate modulation type ('angular') was used.

As these examples illustrate, DOSE by Qaelum provides the information required to identify the causes of suspected significant event cases in the CT application area to the greatest possible extent. This allows the MPE to avoid the time-consuming retrieval and analysis of image data from the PACS. In the same way, the causes of 'normal' DRL exceedances can also be determined.

The crucial insights here are provided by the advanced analysis functions ('SSDE', 'patient positioning', 'mA modulation' and 'series overlap'). The prerequisite for this is that the scan-ner generates an RDSR, the images of all acquisition series are available in the PACS or on the modality and the image data contain the required DICOM tag ('IrradiationEventUID') to be able to establish the link between the dose values and the associated acquisition series. For older scanners, as well as for scanners of a certain manufacturer, this prerequisite is not given, thus the identification of the causes has to be done in another way.

A practice-oriented DMS is decisive in determining whether the MPEs can adequately per-form their tasks within the specified time frame. This becomes particularly evident in a de-partment where the locally available DMS does not provide the desired analyses or cannot provide them due to missing information from the modality.

Disclosure statement

Dr.Nagel  Dr. Nagel worked for many years as a clinical scientist for one of the leading medical imaging manufacturers. In 2009, he founded his own company focusing on MPE services in the application areas of CT and XA. 

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