C.8.15.3.8 CT Exposure Macro

Table C.8-124 specifies the attributes of the CT Exposure Functional Group macro.

Table C.8-124CT EXPOSURE MACRO ATTRIBUTES

Attribute Name Tag Type Attribute Description
CT Exposure Sequence (0018,9321) 1 Contains the attributes defining exposure information. Only a single Item shall be included in this sequence.
>Exposure Time in ms (0018,9328) 1C Duration of exposure for this frame in milliseconds. If Acquisition Type (0018,9302) equals SPIRAL the duration of the exposure time for this frame shall be the Revolution Time (0018,9305) divided by the Spiral Pitch Factor (0018,9311). See C.8.15.3.8.1. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>X-Ray Tube Current in mA (0018,9330) 1C Nominal X-Ray tube current in milliamperes. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>Exposure in mAs (0018,9332) 1C The exposure expressed in milliampere seconds, for example calculated from exposure time and X-Ray tube current. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>Exposure Modulation Type (0018,9323) 1C A label describing the type of exposure modulation used for the purpose of limiting the dose. Defined Terms: NONE Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>Estimated Dose Saving (0018,9324) 2C A percent value of dose saving due to the use of Exposure Modulation Type (0018,9323). A negative percent value of dose savings reflects an increase of exposure. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL and Exposure Modulation Type (0018,9323) is not equal to NONE. Otherwise may be present if Frame Type (0008,9007) Value 1 of this frame is DERIVED and Exposure Modulation Type (0018,9323) is not equal to NONE.
>CTDIvol (0018,9345) 2C Computed Tomography Dose Index (CTDIvol), in mGy according to IEC 60601-2-44, Ed.2.1 (Clause 29.1.103.4), The Volume CTDIvol. It describes the average dose for this frame for the selected CT conditions of operation. Required if Frame Type (0008,9007) Value 1 of this frame is ORIGINAL. May be present otherwise.
>CTDI Phantom Type Code Sequence (0018,9346) 3 The type of phantom used for CTDI measurement according to IEC 60601-2-44. Only a single Item is permitted in this Sequence.
>>Include Code Sequence Macro Table 8.8-1 Defined CID 4052

Note: The dose that a patient receives in a given procedure may be reported in one or more instances of the Radiation Dose Report SOP Class using the CT Radiation Dose Template TID 10011.

C.8.15.3.8.1 Exposure Time in ms Attribute (Informative)

An example of how the value of Exposure Time in ms (0018,9328) can be calculated:

The Exposure Time T exposuretime of a volume shall be the time of the X-Ray radiation in position with this volume. It depends on the travel speed and the total collimation width.

T exposuretime of the volume represented by a distinguished frame of a Spiral Scan can be calculated as follows:

T exposuretime = N x S / V travelspeed

CT pitchfactor = DD / N x S

V travelspeed = DD / T revolutiontime

From this the exposure time can be derived:

T exposuretime = N x S x T revolutiontime / DD = T revolutiontime / CT pitchfactor

Legend:

DD = Table Feed per Rotation (0018,9310) in mm distance moved by the patient support in the z direction per 360 o revolution in Spiral scanning

S = collimation of a single slice in mm

N = number slices

N x S = Total Collimation Width (0018,9307) in mm

V travelspeed = Table Speed (0018,9309) in mm/second, travel speed of the patient table

T revolutiontime = Revolution time (0018,9305) in seconds

CT pitchfactor = Spiral Pitch Factor (0018,9311)