Reference lines on CT and MRI: what they mean

A reference line is a thin line drawn on one image to show where another image cuts through it. If you are looking at a sagittal slice and see a line across it, that line marks the position of the axial slice you are viewing in the other series. It is a map, not a measurement, and it tells you only where you are.

The problem they solve

A CT or MRI study often has the same body region captured as several stacks, such as axial, sagittal and coronal. Each stack is a separate series. Scrolling one series tells you which slice number you are on, but not how that slice relates to the other planes.

Reference lines connect the series. They let you see, for the slice on screen, which level it corresponds to in the other view. For the study and series structure, see CT and MRI scans: studies, series and slices.

How the line is drawn

Every slice stores two pieces of geometry in its tags:

• Image position, the location of the slice in the patient’s coordinate system, in millimetres.
• Image orientation, the directions that the rows and columns of the image point in.

With those two values, software can describe each slice as a flat plane in space. A reference line is the place where the plane of one slice crosses the plane of another. Software calculates that crossing, converts it to image coordinates, and draws it as a line.

You can read the values in the metadata panel. See DICOM tags and metadata.

What the line tells you, and what it does not

The line tells you where the current slice sits relative to the other series. Move to the next slice and the line shifts. If the slices in the two series are different thicknesses, the line may seem to jump by different steps.

It does not mark an area of interest, and it does not say anything about what is in the image. It is not a finding. It is only geometry.

Why a line may not appear

Reference lines depend on the two series being in the same space. They can be missing for ordinary reasons:

• The series do not share a frame of reference. Each series has an identifier for its coordinate system. If the two differ, the positions cannot be compared reliably.
• The planes do not cross inside the image. Two parallel stacks, such as two axial series, have planes that never meet, so there is nothing to draw.
• The geometry tags are missing. Some exported or anonymised files lack position or orientation.
• The line falls outside the visible area. The other slice may be positioned beyond the edge of the current image.

None of these means the files are damaged. They mean that the information needed to draw a line is not there.

Scouts and localizers

Scanners often begin with a quick, low-detail image called a scout or localizer. It is used to plan where the detailed slices will be taken. Lines on a scout, when shown, mark the planned position of later series. The principle is the same: the line comes from position and orientation tags.

A scout is not a diagnostic image. If a disc contains one, it is normally a short series of one to a few images.

Using them on a phone

When you open a study with more than one plane, the point of reference lines is quick orientation. You scroll one series and watch the marker move on the other. That saves guessing whether you are looking at the same level in both.

Radix DICOM shows reference lines that mark where the current slice sits in the other plane. It also supports pinch to zoom and pan, and everything runs offline on the device. It is not a medical device and is not for diagnosis. To bring a disc onto the phone first, see How to open a DICOM CD on your phone.

Interpretation is for the doctor

Reference lines help you move around a scan. They do not interpret it. What the images mean, and what should be done, are questions for the patient’s doctor.