Calculating SUV values on converted image

[aranyics]

Hi!

I would like some help with calculating SUV values from PET images, but according to the equation below, I need the time of injection (e.g. dicom tag Radiopharmaceutical Start Time) and the scan start time (probably the first Acquisition Time in the series) to calculate dT:

After conversion I found that the json file contains TimeZero and FrameDuration to calculate Acquisition Time, but the InjectionStart is always 0 in the json file, which should not be the same time as TimeZero. Is there a way to get the right timing information without reaching out for the dicom files?

The json file from dcm2niix4pet command:
{
    "Modality": "PT",
    "Manufacturer": "Philips",
    "ManufacturersModelName": "GEMINI TF TOF 64",
    "InstitutionName": "xxxx",
    "DeviceSerialNumber": "7030",
    "StationName": "r031-svr",
    "PatientPosition": "FFS",
    "SoftwareVersions": "9.5.1\PMSPT1.44",
    "SeriesDescription": "[wb_ctac] pet whole-body",
    "ProtocolName": "xxxxx",
    "ImageType": [
        "ORIGINAL",
        "PRIMARY"
    ],
    "SeriesNumber": 208110,
    "Radiopharmaceutical": "Fluorodeoxyglucose F^18^",
    "RadionuclidePositronFraction": 0.967,
    "RadionuclideTotalDose": 283000000.0,
    "RadionuclideHalfLife": 6586.2,
    "Units": "Bq/mL",
    "DecayCorrection": "START",
    "AttenuationCorrectionMethod": "CTAC-SG",
    "SliceThickness": 4,
    "ImageOrientationPatientDICOM": [
        1,
        0,
        0,
        0,
        1,
        0
    ],
    "BidsGuess": [
        "PET",
        "PET"
    ],
    "ConversionSoftware": [
        "dcm2niix",
        "pypet2bids"
    ],
    "ConversionSoftwareVersion": [
        "v1.0.20240202",
        "1.0.2"
    ],
    "FrameDuration": [
        60001
    ],
    "ReconMethodName": "BLOB-OS-TF",
    "ReconMethodParameterUnits": [
        "none",
        "none"
    ],
    "ReconMethodParameterLabels": [
        "subsets",
        "iterations"
    ],
    "ReconMethodParameterValues": [
        null,
        null
    ],
    "AttenuationCorrection": "CTAC-SG",
    "DecayCorrectionFactor": [
        1.0
    ],
    "TimeZero": "11:40:43",
    "ScanStart": 0,
    "InjectionStart": 0,
    "TracerRadionuclide": "18Fluorine"
}

Part of a DICOM header from the series that may be related:
(0028,0010) Rows [144]
(0028,0011) Columns [144]
(0028,0030) Pixel Spacing [4,4]
(0028,0051) Corrected Image [DECY,RADL,ATTN,SCAT,DTIM,RAN,NORM]
(0028,0100) Bits Allocated [16]
(0028,0101) Bits Stored [16]
(0028,0102) High Bit [15]
(0028,0103) Pixel Representation [1]
(0028,0106) Smallest Image Pixel Value [0]
(0028,0107) Largest Image Pixel Value [2373]
(0028,1050) Window Center [7849.605]
(0028,1051) Window Width [15699.21]
(0028,1052) Rescale Intercept [0]
(0028,1053) Rescale Slope [10.445709]
(0040,0000) Group 0040 Length [54]
(0040,0244) Performed Procedure Step Start Date [20180307]
(0040,0245) Performed Procedure Step Start Time [113340]
(0040,0253) Performed Procedure Step ID [5633940]
(0040,0254) Performed Procedure Step Description []
(0054,0000) Unknown Data [768]
(0054,0016) Radiopharmaceutical Information Sequence
(0018,0031) Radiopharmaceutical [Fluorodeoxyglucose F^18^]
(0018,1070) Radiopharmaceutical Route [Intravenous route]
(0018,1072) Radiopharmaceutical Start Time [104400]
(0018,1074) Radionuclide Total Dose [283000000]
(0018,1075) Radionuclide Half Life [6586.199707]
(0018,1076) Radionuclide Positron Fraction [0.967]
(0018,1078) Radiopharmaceutical Start DateTime [20180307104400]
(0054,0300) Radionuclide Code Sequence
(0008,0100) Code Value [C-111A1]
(0008,0102) Coding Scheme Designator [SNM3]
(0008,0104) Code Meaning [^18^Fluorine]
(0054,0302) Administration Route Code Sequence
(0008,0100) Code Value [G-D101]
(0008,0102) Coding Scheme Designator [SNM3]
(0008,0104) Code Meaning [Intravenous route]
(0054,0304) Radiopharmaceutical Code Sequence
(0008,0100) Code Value [C-B1031]
(0008,0102) Coding Scheme Designator [SRT]
(0008,0104) Code Meaning [Fluorodeoxyglucose F^18^]
...
(0054,1000) Series Type [WHOLE BODY,IMAGE]
(0054,1001) Units [BQML]
(0054,1002) Counts Source [EMISSION]
(0054,1100) Randoms Correction Method [DLYD]
(0054,1101) Attenuation Correction Method [CTAC-SG]
(0054,1102) Decay Correction [START]
(0054,1103) Reconstruction Method [BLOB-OS-TF]
(0054,1105) Scatter Correction Method [SS-SIMUL]
(0054,1300) Frame Reference Time [388001]
(0054,1321) Decay Factor [1]
(0054,1330) Image Index [96]

Thank you in advance if you could help, or point out what I am missing as a beginner in the field.

Best regards,
Csaba Aranyi

[bendhouseart]

Hi @aranyics to follow up on your questions:

...but the InjectionStart is always 0 in the json file, which should not be the same time as TimeZero

TimeZero and InjectionStart go hand in hand, see the helpful graphic and explanation in the spec.

Is there a way to get the right timing information without reaching out for the dicom files?

If you have the dicoms then you have access to the most accurate frame timing information possible. Temporal data might be split across mulitple files and formats dicom being one of those, but if you're using PET2BIDS then that means you have access to the dicoms.

Concerning InjectionTime it might be tempting to use Radiopharmaceutical Start Time from the dicom header, but more often than not the dicom entry for Radiopharmaceutical Start Time is inaccurate (if even noted). It's nice that the DICOM standard made provisions for capturing this information, but since there aren't auto injectors in the scanner it falls to humans to manually input this information. However, it's much easier and less error prone to record blood and radioactivity data with pen and paper while the scan is taking place. I suppose one could update the dicom headers with the injection time after the scan is done, but no one does that.

I may be showing some non-FDG bias, but for most of the scans I've encountered InjectionStart is recorded outside of the scanner console and closely corresponds to the ScanStart time. I would love to be able to rely on the dicom header but it just can't be trusted for some things.

For that reason we just use the ScanStart as a proxy for InjectionTime when provided with only dicom files.

Now, given the values in your example:

"TimeZero": "11:40:43",
"ScanStart": 0,
"InjectionStart": 0,

and

(0018,1072) Radiopharmaceutical Start Time [104400]

You have a few options with respect to TimeZero:

1. Use the Radiopharmaceutical Start Time as TimeZero by supplying pet2bids with this argument --kwargs TimeZero="10:40:00", that should set ScanStart=43.
2. Use the --kwargs arguments to set the injection time manually --kwargs InjectionStart="-43".
3. Create a spreadsheet file that contains these values and include it in the conversion, for an example see this xlsx file. This file can be dropped into the same folder as the dicoms and gathered for conversions with the --metadata-path argument or supply a full path to that spreadsheet --metadata-path /full/path/to/spreadsheet

Let me know if that was helpful.