{
  "id": 357509,
  "title": "Why are there negative values in imagepositionpatient_z?",
  "url": "/competitions/rsna-2022-cervical-spine-fracture-detection/discussion/357509",
  "author_name": "Akima",
  "post_date": "2022-10-04T14:14:03.808000",
  "votes": 4,
  "comment_count": 2,
  "views": 0,
  "content": "<p>My understanding of the z-axis of the imagepositionpatient is as follows.</p>\n<p>・The z-axis increases from the tail to the head<br>\n・The center of the gantry (CT system) is the origin</p>\n<p>Therefore, I believe that the z-axis value of the cervical spine area is always positive.<br>\nHowever, as far as I can see from the data, many z-axis values are negative.</p>\n<p>Why are there negative values in imagepositionpatient_z?<br>\nIf anyone is familiar with imagepositionpatient, could you please explain?</p>",
  "messages": [
    {
      "id": 1972091,
      "postDate": "2022-10-05T01:14:32.430Z",
      "content": "<p>Yes, in the DICOM coordinate system, the Z axis (axial) increases toward the head.</p>\n<p>Generally, the technologist will \"isocenter\" the patient by putting him head-first into the gantry and aligning the base of the skull, just above C1, to the center of the scanner, or Z=0. Then, the scanner pulls the patient inward as it scans head-to-feet. Slices will have negative Z values.</p>\n<p>Sometimes, the tech will isocenter the patient down below C7, in the chest, and scan feet-to-head. This causes positive Z values and is why some studies seem to be reversed.</p>",
      "rawMarkdown": "Yes, in the DICOM coordinate system, the Z axis (axial) increases toward the head.\n\nGenerally, the technologist will \"isocenter\" the patient by putting him head-first into the gantry and aligning the base of the skull, just above C1, to the center of the scanner, or Z=0. Then, the scanner pulls the patient inward as it scans head-to-feet. Slices will have negative Z values.\n\nSometimes, the tech will isocenter the patient down below C7, in the chest, and scan feet-to-head. This causes positive Z values and is why some studies seem to be reversed.",
      "votes": 3,
      "replies": [
        {
          "id": 1973067,
          "postDate": "2022-10-05T13:03:14.723Z",
          "content": "<p>Thank you for answering my question.<br>\nI understand it depends on the technologists.</p>",
          "rawMarkdown": "Thank you for answering my question.\nI understand it depends on the technologists."
        }
      ]
    },
    {
      "id": 1971299,
      "postDate": "2022-10-04T14:14:03.810Z",
      "content": "<p>My understanding of the z-axis of the imagepositionpatient is as follows.</p>\n<p>・The z-axis increases from the tail to the head<br>\n・The center of the gantry (CT system) is the origin</p>\n<p>Therefore, I believe that the z-axis value of the cervical spine area is always positive.<br>\nHowever, as far as I can see from the data, many z-axis values are negative.</p>\n<p>Why are there negative values in imagepositionpatient_z?<br>\nIf anyone is familiar with imagepositionpatient, could you please explain?</p>",
      "rawMarkdown": "My understanding of the z-axis of the imagepositionpatient is as follows.\n\n・The z-axis increases from the tail to the head\n・The center of the gantry (CT system) is the origin\n\nTherefore, I believe that the z-axis value of the cervical spine area is always positive.\nHowever, as far as I can see from the data, many z-axis values are negative.\n\nWhy are there negative values in imagepositionpatient_z?\nIf anyone is familiar with imagepositionpatient, could you please explain?",
      "votes": 4
    }
  ],
  "comments": [
    {
      "id": 1972091,
      "author_name": "David Roberts",
      "author_url": "",
      "post_date": "2022-10-05T01:14:32.430000",
      "content": "<p>Yes, in the DICOM coordinate system, the Z axis (axial) increases toward the head.</p>\n<p>Generally, the technologist will \"isocenter\" the patient by putting him head-first into the gantry and aligning the base of the skull, just above C1, to the center of the scanner, or Z=0. Then, the scanner pulls the patient inward as it scans head-to-feet. Slices will have negative Z values.</p>\n<p>Sometimes, the tech will isocenter the patient down below C7, in the chest, and scan feet-to-head. This causes positive Z values and is why some studies seem to be reversed.</p>",
      "votes": 3,
      "replies": [
        {
          "id": 1973067,
          "author_name": "Akima",
          "author_url": "",
          "post_date": "2022-10-05T13:03:14.723000",
          "content": "<p>Thank you for answering my question.<br>\nI understand it depends on the technologists.</p>",
          "votes": 0,
          "replies": []
        }
      ]
    }
  ],
  "raw_markdown_by_id": {
    "1972091": "Yes, in the DICOM coordinate system, the Z axis (axial) increases toward the head.\n\nGenerally, the technologist will \"isocenter\" the patient by putting him head-first into the gantry and aligning the base of the skull, just above C1, to the center of the scanner, or Z=0. Then, the scanner pulls the patient inward as it scans head-to-feet. Slices will have negative Z values.\n\nSometimes, the tech will isocenter the patient down below C7, in the chest, and scan feet-to-head. This causes positive Z values and is why some studies seem to be reversed.",
    "1971299": "My understanding of the z-axis of the imagepositionpatient is as follows.\n\n・The z-axis increases from the tail to the head\n・The center of the gantry (CT system) is the origin\n\nTherefore, I believe that the z-axis value of the cervical spine area is always positive.\nHowever, as far as I can see from the data, many z-axis values are negative.\n\nWhy are there negative values in imagepositionpatient_z?\nIf anyone is familiar with imagepositionpatient, could you please explain?"
  }
}