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library:spine_deep_dive [2025/06/18 02:37] – scottlibrary:spine_deep_dive [2025/06/18 02:55] (current) – [I need thinner slices! What can I do?] scott
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 This page is dedicated to more technical information regarding spine imaging, to include anatomy, MRI technique, pathology, and more of the 'why' of spine MRI. This page will follow the format of a question and answer dialogue for each topic.  This page is dedicated to more technical information regarding spine imaging, to include anatomy, MRI technique, pathology, and more of the 'why' of spine MRI. This page will follow the format of a question and answer dialogue for each topic. 
  
-===Why is it important to use an odd number of slices for sagittal spine imaging?===+====Why is it important to use an odd number of slices for sagittal spine imaging?====
  
 Using an odd number of slices while planning out a mid sagittal slice ensures that the center is 'True' mid sagittal. This also makes it so that the slices on either side of the mid sag slice are a symmetrical distance away from midline. Canine and feline anatomy is so much smaller than humans that, even with a 2-3mm slice thickness, only 3'ish slices visualize the spinal cord well. Considering that each slice will have some degree of __partial volume averaging__, it is crucially important that the center slice is accurate. Consider the case of a mid-line disc extrusion in the images below; which series of slices will best image the disc? The even numbered series will place a slice on either side of the most compressive part of the extruded disc. Using an odd number of slices while planning out a mid sagittal slice ensures that the center is 'True' mid sagittal. This also makes it so that the slices on either side of the mid sag slice are a symmetrical distance away from midline. Canine and feline anatomy is so much smaller than humans that, even with a 2-3mm slice thickness, only 3'ish slices visualize the spinal cord well. Considering that each slice will have some degree of __partial volume averaging__, it is crucially important that the center slice is accurate. Consider the case of a mid-line disc extrusion in the images below; which series of slices will best image the disc? The even numbered series will place a slice on either side of the most compressive part of the extruded disc.
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 {{:library:evenslicenumber.png?600|}}  {{:library:oddslicenumber.png?600|}} {{:library:evenslicenumber.png?600|}}  {{:library:oddslicenumber.png?600|}}
  
-===How do I communicate which disc space I'm looking at?===+====How do I communicate which disc space I'm looking at?====
  
 There is some common nomenclature to be familiar with when it comes to communicating with other imagers about which bit of anatomy is being scanned. By convention, each vertebrae will be described by which section of the spine it's in and it's corresponding number in that section, ie the 4th lumbar vertebrae will be L4. The discs are described by the vertebrae on either side, ie the disc between the 4th and 5th Lumbar vertebrae is L4/5. If there are multiple discs in the area of concern, it may be described by the first and last vertebrae if it's contiguous: T12-L3 would mean each disc between T12 and L3 should be scanned. This may also be communicated like T12/13/1/2/3. Importantly, this relies on knowing that 13/1 is the junction of the thoracic and lumbar spines, and most patients will have 7 Lumbar vertebrae and 13 thoracic vertebrae.  There is some common nomenclature to be familiar with when it comes to communicating with other imagers about which bit of anatomy is being scanned. By convention, each vertebrae will be described by which section of the spine it's in and it's corresponding number in that section, ie the 4th lumbar vertebrae will be L4. The discs are described by the vertebrae on either side, ie the disc between the 4th and 5th Lumbar vertebrae is L4/5. If there are multiple discs in the area of concern, it may be described by the first and last vertebrae if it's contiguous: T12-L3 would mean each disc between T12 and L3 should be scanned. This may also be communicated like T12/13/1/2/3. Importantly, this relies on knowing that 13/1 is the junction of the thoracic and lumbar spines, and most patients will have 7 Lumbar vertebrae and 13 thoracic vertebrae. 
  
-===How many axial slices should I use for each disc space?===+====How many axial slices should I use for each disc space?====
  
 There is some variation depending on the slice thickness and the size of the animal, but a general guideline would be to use 3-7, erring on the side of 7 if time allows. For most canine and feline patients, 7 slices at 3mm thick, centered on the disc, will cover from mid-body to mid-body on the vertebrae to either side of the disc. This extra coverage is good to have, as extruded disc material is not always completely visualized on sagittal images, and not always confined to just the space above the disc. Often, disc material can be found cranially or caudally to the origin, and even into the foramina laterally. Additionally, there are veins in the middle of the spinal cord on the ventral side along it's entire length; these can be damaged and result in hemorrhage that may also travel some distance from the disc. There is some variation depending on the slice thickness and the size of the animal, but a general guideline would be to use 3-7, erring on the side of 7 if time allows. For most canine and feline patients, 7 slices at 3mm thick, centered on the disc, will cover from mid-body to mid-body on the vertebrae to either side of the disc. This extra coverage is good to have, as extruded disc material is not always completely visualized on sagittal images, and not always confined to just the space above the disc. Often, disc material can be found cranially or caudally to the origin, and even into the foramina laterally. Additionally, there are veins in the middle of the spinal cord on the ventral side along it's entire length; these can be damaged and result in hemorrhage that may also travel some distance from the disc.
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 {{ :library:axslicecoverage.png?600 |}} {{ :library:axslicecoverage.png?600 |}}
  
-===There's a microchip causing artifact on my image, is it safe and what can I do?===+====There's a microchip causing artifact on my image, is it safe and what can I do?====
  
 Most RFID tracking chips used in pets contain a small iron core which can cause substantial artifact, especially in small animals. While there isn't much literature, it is good to note that these implants do appear safe and are well encapsulated in fibrous tissue, so the risk of movement and heating are quite low. To manage the artifact, there are several things to do if there isn't a metal suppression sequence: Most RFID tracking chips used in pets contain a small iron core which can cause substantial artifact, especially in small animals. While there isn't much literature, it is good to note that these implants do appear safe and are well encapsulated in fibrous tissue, so the risk of movement and heating are quite low. To manage the artifact, there are several things to do if there isn't a metal suppression sequence:
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 The susceptibility artifact evolves over time throughout the frequency encoding period. Increasing the bandwidth helps by reducing the echo spacing so that the time between data points is shorter. This also reduces the time between strong refocusing pulses in the FSE sequence, further suppressing the artifact. In addition, the amplitude of the frequency encoding gradient is increased, which also helps to reduce the severity of the artifact. Similar ends are achieved by increasing the imaging matrix and reducing the slice thickness; it will impose stronger gradients onto the microchip. Swapping the frequency encoding direction won't suppress the artifact, but it will redirect it or change the shape of it, depending on the orientation of the microchip. All of these parameter changes are likely to increase scan time, but it is best to avoid using parallel imaging if the artifact is significant, as it may cause it to show up centrally in the image. The susceptibility artifact evolves over time throughout the frequency encoding period. Increasing the bandwidth helps by reducing the echo spacing so that the time between data points is shorter. This also reduces the time between strong refocusing pulses in the FSE sequence, further suppressing the artifact. In addition, the amplitude of the frequency encoding gradient is increased, which also helps to reduce the severity of the artifact. Similar ends are achieved by increasing the imaging matrix and reducing the slice thickness; it will impose stronger gradients onto the microchip. Swapping the frequency encoding direction won't suppress the artifact, but it will redirect it or change the shape of it, depending on the orientation of the microchip. All of these parameter changes are likely to increase scan time, but it is best to avoid using parallel imaging if the artifact is significant, as it may cause it to show up centrally in the image.
  
-===I have to cover a large area with an axial slices, what is the best way to do it?===+====I have to cover a large area with an axial slices, what is the best way to do it?====
  
 Imaging of disc disease can be complicated! There is no one 'best' way to do it. You may find that frequently large areas of coverage will be needed for a few reasons: Imaging of disc disease can be complicated! There is no one 'best' way to do it. You may find that frequently large areas of coverage will be needed for a few reasons:
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 {{:library:axialbugstackplan.jpg?600|}} {{:library:axialbugstackplan.jpg?600|}}
  
-===I need thinner slices! What can I do?===+====I need thinner slices! What can I do?====
  
 When imaging very small anatomy with 2D FSE sequences, it can be difficult to reduce slice thickness to much less than 2mm without sacrificing too much scan time or SNR. If less than 2mm imaging is required, it is necessary to move to a 3D sequence. Any time this is done, it is a good idea to discuss with the clinician the caveats for whichever sequence is being substituted. Before substituting, consider the following questions: When imaging very small anatomy with 2D FSE sequences, it can be difficult to reduce slice thickness to much less than 2mm without sacrificing too much scan time or SNR. If less than 2mm imaging is required, it is necessary to move to a 3D sequence. Any time this is done, it is a good idea to discuss with the clinician the caveats for whichever sequence is being substituted. Before substituting, consider the following questions:
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 Below are some commonly available sequences, with their vendor specific names, where to find them, caveats, and advantages: Below are some commonly available sequences, with their vendor specific names, where to find them, caveats, and advantages:
  
-==Balanced Steady State Free Precession with Phase Cycling (FIESTA-C, CISS)==+===Balanced Steady State Free Precession with Phase Cycling (FIESTA-C, CISS)===
  
 Where to find: Most vendors have this saved in an IAC protocol, occasionally spine. Where to find: Most vendors have this saved in an IAC protocol, occasionally spine.
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 Advantages: Very high inherent SNR; it's possible to get really high resolution or very thin slices. It can be run isotropically for reformats. Bright fluid contrast can help with anatomic information Advantages: Very high inherent SNR; it's possible to get really high resolution or very thin slices. It can be run isotropically for reformats. Bright fluid contrast can help with anatomic information
  
-==RF Spoiled Fast Gradient Echo (FLASH, FSPGR)==+===RF Spoiled Fast Gradient Echo (FLASH, FSPGR)===
  
 Where to find: Most vendors have this saved in a brain protocol Where to find: Most vendors have this saved in a brain protocol
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 Advantages: Pretty robust sequences with relatively high inherent SNR. There are usually different fat saturation options available such as SPECIAL/SPAIR which can be more homogenous and less time intensive that regular FatSat options. The grey/white matter contrast is superior to FSE Advantages: Pretty robust sequences with relatively high inherent SNR. There are usually different fat saturation options available such as SPECIAL/SPAIR which can be more homogenous and less time intensive that regular FatSat options. The grey/white matter contrast is superior to FSE
  
-==RF Spoiled Fast Gradient Echo with k space modifications (LAVA, VIBE)==+===RF Spoiled Fast Gradient Echo with k space modifications (LAVA, VIBE)===
  
 Where to find: Most vendors have this saved in a routine liver protocol Where to find: Most vendors have this saved in a routine liver protocol
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 Advantages: These are really designed to run for speed more than anything else, so they'll be faster the FSPGR/FLASH. Reserve this for some thing incidental in the soft tissues outside the spine. Advantages: These are really designed to run for speed more than anything else, so they'll be faster the FSPGR/FLASH. Reserve this for some thing incidental in the soft tissues outside the spine.
  
-==3D Fast Spin Echo (CUBE, SPACE)==+===3D Fast Spin Echo (CUBE, SPACE)===
  
 Where to find: Most vendors have this saved in a spine or brain protocol Where to find: Most vendors have this saved in a spine or brain protocol
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 Advantages: Very high SNR, these can be run isotropically or with very thin slices. Due to the long ETL and varying flip angle intravascular signal will be dark, which is excellent for vessel wall imaging  ruling out vessel vs. lesion. True T2 weighting can be achieved with a higher TE ~120, though keep the ETL a bit shorter than the out of the box parameters. If using a STIR, consider running it post contrast! This will suppress intravascular signal and help visualize nerves. Advantages: Very high SNR, these can be run isotropically or with very thin slices. Due to the long ETL and varying flip angle intravascular signal will be dark, which is excellent for vessel wall imaging  ruling out vessel vs. lesion. True T2 weighting can be achieved with a higher TE ~120, though keep the ETL a bit shorter than the out of the box parameters. If using a STIR, consider running it post contrast! This will suppress intravascular signal and help visualize nerves.
  
-==Multi Echo GRE (MERGE, MEDIC)==+===Multi Echo GRE (MERGE, MEDIC)===
  
 Where to find: Most vendors have this saved in a cervical spine protocol Where to find: Most vendors have this saved in a cervical spine protocol