I have a basic question for the DICOM protocol.
I know how I can calculate the orientation labels of every slice of a DICOM image (A,P,L,R,H,F). But when I got for example an Axial slices with numbers from 0001 to 0024 I need to know if the slice order is from Head to Feet or from Feet to Head.
How can I calculate that ?
Patient Position (0018, 5100) will tell you if the patient was scanned head-first supine, feet-first prone, head-first prone, etc. Instance Number (0020, 0013), also commonly known as slice number, contains no information about spatial location and isn't even guaranteed to be unique. Slice Location (0020, 1041) is useful, if it exists, but you can't count on it always existing because it's a Type 3 (optional) attribute. To have a robust solution, you need to use Image Position Patient (0020, 0032) together with Image Orientation Patient (0020, 0037) and Patient Position (0018, 5100) to properly order the slices in space. Image Position Patient gives you a vector from the origin to the center of the first transmitted pixel of the image. Image Orientation Patient gives you vectors for the orientation of the rows and columns of the image in space. Patient Position tells you how the patient was placed on the table relative to the coordinate system.
The origin of the patient coordinate system is arbitrary, and selected by the imaging modality. I assume that the modality is free to choose a different origin for each series of images, but the origin is fixed for a given series.
If you wish to annotate images when displayed with notations such as (L) or (R) to indicate which side of the 2D image is considered the patient's left or right side, you need the Patient Position (0018, 5100) Attribute from the General Series module.
This attribute contains the position of the patient relative to the imaging equipment space.Valid values are:
1.Head First-Prone
2.Head First-Supine
3.Head First-Decubitus Right
4.Head First-Decubitus Left
5.Feet First-Decubitus Left
6.Feet First-Decubitus Right
7.Feet First-Prone
8.Feet First-Supine.
Definitions:
Head First means the patient was laying on the imaging couch with the head facing the imaging device first.
Feet first means the patient was laying on the imaging couch with the feet facing the imaging device first.
Prone means the patient is laying on his/her stomach. (Patient's face being positioned in a downwards (gravity) direction.)
Supine means the patient is laying on his/her back. (Patient's face being in an upwards direction.)
Decubitus Right means the patient is laying with his/her right side in a downwards direction.
Decubitus Left means the patient is laying with his/her left side in a downwards direction.
NOTE -- Patient Position (0018, 5100) is a type 1 required attribute for both the CT and MR modalities. This attribute is VERY IMPORTANT for accurately interpreting the patient's orientation.
You do not need to consider the patient position tag to label dicom images orientations when presented on screen. You only need the image orientation patient. Since that's the orientation of the image in the (patient) reference coordinate system.
So, for example you will get different values for IOP from a CT scanner if the patient is prone or supine.
Related
I have a CT scan of the chest where I can't seem to figure out how to determine how to sort the aixal slices such that the first slices are the ones closet to the head.
The scan has the following dicom parameters:
Patient Position Attribute (0018,5100): FFS (Feet First Supine)
Image Position (Patient) (0020,0032): -174-184-15 (one slice)
Image Orientation (Patient)(0020,0037): 1\0\0\0\1\0
The most cranial slice (anatomically, closet to the head) has z position 13 and the most caudal (lower) -188.
However, when the Patient Position is FFS shouldn't the slice with the lowest z position (e.g. -188) be the one being the one most cranially (anatomically, i.e. closet to the head) located?
Can anyone enlighten me?
Kind regards
DICOM very clearly defines, that the x-axis has to go from the patients right to the left, the y-axis has to go from front to back, and the z-axis has to go from foot to head.
So the lower z-position of -188 has to be closer to the feet than the higher position of 13. You should always rely on this.
Patient Position Attribute is rather some informational annotation. If you do all the math yourself, then you can ignore it.
If a viewer does not do the math (there are a lot of them) and just loads the images and shows them sored by ImageNumber, then the Position Attribute is the info to annotate, if the image with ImageNumber 1 is the one closer to the head or the one closer to the feet. Meaning: when the patient went throught the ct scanner, which one was the first imate aquisitioned: the one of the head or of the feet.
By direction I mean for example from a patient's head to bottom or from his bottom to head. The CHEST CT scans I have seen so far indicates that Instance Number 1 slice is usually the first one down from the upper part of the body but I don't know whether this is part of the standard or there are some other tags that I should inspect into to determine.
There is no rule in DICOM that requires the Instance Number to be related to the slice position in a particular way. The link of Bartloimiej shows that there is a rule how the slice coordinates defined by Image Position Patient (0020,0027) and Image Orientation Patient (0020, 0037) are related to directions in the patient's body (head, feet, etc.)
So if you want to apply spatial ordering, these attributes are what you want to use. Slice Location (0020,1041) will not help you as well:
C.7.6.2.1.2 [...] This information is relative to an unspecified implementation specific reference point.
For original (i.e. Image Type (0008,0008) is ORIGINAL\PRIMARY...) CT slices, it is quite safe to assume that some growth in the Z-Direction is always present in a volumetric dataset. But for MRI or for reconstructed CT-slices (MPR), you may find datasets in which slices are parallel to the xz or yz plane. If your application is supposed to handle such images, make sure to avoid division by zero...
Yes, the standard defines it. DICOM PS3.3, part C.7.6.2:
The direction of the axes is defined fully by the patient's orientation.
If Anatomical Orientation Type (0010,2210) is absent or has a value of BIPED, the x-axis is increasing to the left hand side of the patient. The y-axis is increasing to the posterior side of the patient. The z-axis is increasing toward the head of the patient.
There is also a tag (0020,0037), Image Orientation (Patient), which relates actual position of the patient to the global coordinate frame. In trunk CT it is almost always 1 0 0 0 1 0 (no rotation) and you don't need to deal with it. Otherwise, see comments under the link above.
You are correct. The chest CT series are sorted from head to feet. The slice closest to the head should have the lowest Instance Number.
I don't know if this is defined by the DICOM standard or not, but I have seen a lot of DICOM images and the convention is this:
AXIAL - sorted by Z axis high to low (head to feet)
CORONAL - sorted by Y axis high to low (back to front)
SAGITTAL - sorted by X axis low to high (right to left)
Notice in all cases, the first slice in the series will be farthest from the observer.
If you need to generate Instance Number, you should sort the images by the dot product of Image Position Patient and (1,-1,-1) from low to high. In the rare degenerate case (all dot products are the same), I don't know. Pick another direction to sort, but probably (0,-1,-1) would be a good choice.
EDIT: I just discussed this with a friend who is more experienced. He said it varies. Some departments prefer back to front order, some prefer front to back. Also some DICOM viewers will give users the choice of how the slices are sorted (by Instance Number, Slice Location, IPP, Content Time, etc)
If the Image Orientation (Patient) tag (0020,0037) reads [1,0,0,0,1,0] and the Patient Position tag (0018, 5100) reads ‘HFS’, how do I interpret Slice Location tag (0020,1041), assuming that it exists?
I know that it represents the `Relative position of the image plane in millimeters', I'm just having trouble relating the end points of the range to the Z axis in the DICOM Reference Coordinates System (RCS).
Example: I have an sequence of Slice Location numbers in the range: [-1873.382, -771.782]
Since the numbers are increasing and in the DICOM RCS, the Z axis increases in the Inferior to Superior direction, can I conclude that '-1873.382' is the position of the most Inferior slice?
Also, just to note that the z coordinate of my Image Position (Patient) (0020,0032) attribute for each slice, contains the same information as my Slice Location tag.
I still advise against using the Slice Location attribute for sorting. In MR imaging, slices can have arbitrary orientation and even in CT the gantry can be tilted, so you cannot rely that all slices are parallel to the xy-plane. So you actually do not know to which axis the Slice Location refers.
What I do is to subtract ImagePositionPatient from two slices which gives me the direction in which the slices in the stack are moving. Ordering can be done by the amount of the difference vectors.
Image Position (Patient) (0020, 0032) is the x, y, and z coordinates of the upper left hand corner of the image and Image Orientation (0020, 0037) says the direction of the first row and the first column with respect patient (farther defined by patient orientation). X-axis increasing direction is towards the left hand side of the patient, y-axis increasing is towards the posterior side and z-axis increasing is toward the head of the patient.
In your case, if the Z axis is changing and increase is towards the head, I would use the Z-axis values for sorting the stack. It is more reliable than Slice Location. Yes the smallest value (e.g. value -1873.382) is the most Inferior slice.
A simple question as i am developing a java application based on dcm4che ...
I want to calculate/find the "position" of a dicom image into its sequence (series). By position i mean to find if this image is first, second etc. in its series. More specifically i would like to calculate/find:
Number of slices into a Sequence
Position of each slice (dicom image) into the Sequence
For the first question i know i can use tag 0020,1002 (however it is not always populated) ... For the second one?
If you are dealing with volumetric image series, best way to order your series is to use the Image Position (Patient) (0020, 0032). This is a required Type 1 tag (should always have value) and it is part of the image plane module. It will contain the X, Y and Z values coordinates representing the upper left corner of the image in mm. If the slices are parallel to each other, only one value should change between the slices.
Please note that the Slice Location (0020, 1041) is an optional (Type 3) element and it may not exist in the DICOM file.
We use the InstanceNumber tag (0x0020, 0x0013) as our first choice for the slice position. If there is no InstanceNumber, or if they are all the same, then we use the SliceLocation tag (0x0020, 0x1041). If neither tag is available, then we give up.
We check the InstanceNumber tag such that the Max(InstanceNumber) - Min(InstanceNumber) + 1 is equal to the number of slices we have in the sequence (just in case some manufacturers start counting at 0 or 1, or even some other number). We check the SliceLocation the same way.
This max - min + 1 is then the number of slices in the sequence (substitute for tag ImagesInAcquisition 0x0020, 0x1002).
Without the ImagesInAcquisition tag, we have no way of knowing in advance how many slices to expect...
I would argue that if the slice location is available, use that. It will be more consistent with the image acquisition. If it is not available, then you'll have to use or compute from the image position (patient) attribute. Part 3 section C.7.6.2.1 has details on these attributes.
The main issue comes when you have a series that is oblique. If you just use the z-value of the image position (patient), it may not change by the slice thickenss/spacing between slices attributes, while the slice location typically will. That can cause confusion to end users.
Due to showing MPR view based on Dicoms. I've made a 3D array from series of dicom files. And I show it from Coronal and Sagittal sides.
My 3D array includes:
- z = count of dicoms
- c = column value for every dicoms
- r = Row value for every dicoms
But I have a problem. When there is some space between slices, image is made by this way doesn't show a correct view. Because I can not think of simulation distance between them!
I don't know how to calculate space between slices? I want to add extra space between slices. for example, If space between slices is 4. I have to add 4 time z inner slices.
I hope to arrive my mean.
Image Position (Patient) and Image Orientation (Patient) are the two only attributes you should ever used when computing distance between slices. For more details see here or here. For an actual implementation see here, this implementation also does take into account Frame Of Reference UID, as well as Gantry/Detector Tilt.
This question is the question #1 asked on comp.protocols.dicom.
Please see ImageJ bug
I believe the answer from #Matt is erroneous, let me clarify a few things here.
No: 'DICOM does not have an attribute called Spacing Between Slices'. That is very wrong (technically it does not even mean anything).
DICOM defines IODs which define the set of required attributes available in an SOP Class Instance. Let's consider two very common cases: CT Image Storage (legacy) and MR Image Storage (legacy). So we need to compare the set of attributes in between:
CT Image IOD Modules
MR Image IOD Modules
Now let's say we want to check that MR Image Storage support Spacing Between Slices, it is easy to jump to:
MR Image Module Attributes
However it is much harder to find this attribute for CT Image Storage: simply because this attribute does not exist (per standard). So the only time you would find such attribute would be within an extended SOP Class (some vendors may decide that Spacing Between Slices attribute make sense within their extended SOP Class Instance).
Mixing in the same answer both Spacing Between Slices and Slice Thickness (0018,0050) is very confusing for new users.
I agree that Slice Thickness is perfectly defined in the standard for both CT Image Storage and MR Image Storage since they both include Image Plane Module Attributes, however let's not exchange one for the other.
I found a nice summary of Slice Thickness vs Spacing Between Slices here (if you scroll to the section, you can even play the small demo) :
CT Physics: CT Reconstruction and Helical CT
In step and shoot CT the Slice Thickness and Spacing Between Slices are identical so there is no big issue here. However for helical CT those values are not the same and can vary in any direction (they are independent).
[…] Slice Thickness is determined by the detector width and pitch,
while reconstruction interval (=Spacing Between Slices) can be chosen
arbitrarily. […]
In conclusion to compute (safely!) the Spacing Between Slices (= Reconstruction Interval), it is much safer to use Image Orientation (Patient) and Image Position (Patient) since they are available in either MR Image Storage or CT Image Storage instances.
DICOM has an attribute called Spacing Between Slices (0018, 0088) that gives the distance between two adjacent slices (perpendicular to the image plane) and it also has an attribute called Slice Thickness (0018, 0050) that gives the thickness of the imaged slice (the image plane exists at the center of the slice, with half of the volume above the plane and half below). Image Position (Patient) (0020, 0032) and Image Orientation (Patient) (0020, 0037) are also useful attributes for computing spatial relationships between slices.
For a more detailed explanation, see section C.7.6.2 of part 3 of the DICOM standard. (p. 409)
WARNING: Please be aware that different vendors use the same dicom tags for addressing different things. For instance, the attribute Spacing Between Slices (0018, 0088) means two different things depending on the vendor. See this table to have a guide, and this thread for an explanation.
As discussed in the previous answers, it is not straightforward how to calculate space between DICOM slices. Let's phrase the question differently: How to store DICOM slices in a 3D volume, i.e. a list of equally spaced slices for rendering (guess you want to upload into a 3D texture).
This is because the actual position that a CT slice is captured might not be identical to the position selected by the radiologist. A dataset might have been configured to capture 1 mm slices, but the CT returns slices at position 0.0 mm, 0.997 mm, 2.010 mm, ...
If you use an attribute such as Spacing Between Slices to calculate the size of the 3D volume, you will obtain subtle rounding errors easily. Don't go there.
Rather it is essential to use Image Position (Patient) (0020, 0032) and then perform an optimization to figure our how the slices could be fit into an grid.
Typical problems in practice to consider:
Missing slices (interpolate? Gap?)
Out of step slices (hardware defect? data defect?)