Back pain – CT/MRI findings
Imaging can show anatomical abnormality, but causation requires clinical–radiological concordance:
Does the scan finding match the patient’s symptoms, neurological signs, side, level, timing, and mechanism?
Routine imaging is discouraged in uncomplicated acute low back pain because many findings are incidental and may not explain the pain. RACGP notes that most acute low back pain is a clinical diagnosis, imaging is usually not indicated, and unnecessary imaging can lead to overdiagnosis, anxiety, unnecessary procedures and prolonged disability.
Imaging findings and causation
CT/MRI findings are strongest for causation when there is clinical–radiological concordance:
same side + same level + same nerve root/anatomical structure + matching symptoms/signs + plausible timing/mechanism.
Routine imaging is not recommended for uncomplicated acute low back pain without red flags because most cases are clinical diagnoses and imaging can show incidental findings that may not be the pain source.
1. Disc anatomy
| Imaging finding | Typical modality | Can cause back pain? | Can cause neurology? | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|---|
| Disc desiccation / degeneration | MRI | Yes, possible chronic axial pain | Usually no | Degenerative disc disease | Does not prove acute injury | Weak for acute causation |
| Disc height loss | X-ray/CT/MRI | Possible | Possible if foraminal narrowing | Chronic degenerative loading | Does not date injury | Weak–moderate |
| Mild broad disc bulge | MRI/CT | Possible but often incidental | Usually no unless root compromise | Degenerative disc change | Does not prove pain source | Weak |
| Focal disc protrusion | MRI | Possible | Yes, if root contact/compression | Potential radiculopathy | Does not prove timing | Moderate if concordant |
| Large disc extrusion | MRI | Possible | Yes, more likely | Clinically significant disc herniation | Does not prove exact injury date | Strong if concordant |
| Sequestered disc fragment | MRI | Possible | Yes, often significant | Free fragment causing nerve compression | Does not prove mechanism alone | Strong if concordant |
| Annular fissure / high-intensity zone | MRI | Possible discogenic pain | Usually no | Possible annular tear/irritation | Common; not necessarily traumatic | Weak–moderate |
| Acute new disc extrusion compared with prior MRI | MRI | Possible | Yes | New structural change | Still needs symptom correlation | Strong |
Stronger disc causation example
- Acute right leg-dominant pain.
- Right S1 sensory symptoms.
- Reduced right ankle reflex.
- MRI: large right paracentral L5/S1 extrusion compressing right S1 root.
Weaker disc causation example
- Central back pain only.
- MRI: multilevel mild disc bulges.
- No neurological signs.
2. Nerve root / radiculopathy anatomy
| Imaging finding | Typical modality | Relevant symptoms | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|
| Nerve root contact | MRI | Possible radicular pain | Anatomical proximity | Contact alone may be incidental | Weak–moderate |
| Nerve root displacement | MRI | Dermatomal leg pain | Mechanical irritation/compression | Does not prove symptoms if non-concordant | Moderate |
| Nerve root compression | MRI/CT | Radicular pain, numbness, weakness | Structural radiculopathy | Does not date injury | Strong if concordant |
| Nerve root swelling/oedema | MRI | Acute radicular pain | Active radiculitis/irritation | Not specific for trauma | Moderate–strong |
| Foraminal nerve root compression | MRI/CT | Exiting root symptoms | Foraminal radiculopathy | Often degenerative/chronic | Moderate–strong if concordant |
| Lateral recess compression | MRI/CT | Traversing root symptoms | Traversing root radiculopathy | Does not prove acute injury | Moderate–strong |
Root-level correlation
| Root | Typical symptoms | Motor finding | Reflex | Matching imaging level |
|---|---|---|---|---|
| L3 | anterior thigh pain | hip flexion/knee extension weakness | may affect knee jerk | L2/3 or L3/4 depending anatomy |
| L4 | anterior thigh/medial shin | knee extension weakness | reduced knee jerk | L3/4 or L4/5 |
| L5 | lateral leg/dorsum foot/big toe | dorsiflexion/great toe extension weakness | no reliable reflex | L4/5 commonly |
| S1 | posterior calf/lateral foot/sole | plantarflexion weakness | reduced ankle jerk | L5/S1 commonly |
3. Vertebral body / bone marrow anatomy
| Imaging finding | Typical modality | Can cause back pain? | Can cause neurology? | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|---|
| Vertebral compression fracture with marrow oedema | MRI | Yes | Possible if retropulsion/canal compromise | Acute/subacute fracture | Mechanism still needs correlation | Strong |
| Old compression fracture without oedema | X-ray/CT/MRI | Possible chronic pain | Possible if deformity/stenosis | Chronic fracture | Does not prove acute injury | Weak for acute causation |
| Bone marrow oedema / bone contusion | MRI | Yes | Usually no | Active bone injury/stress/inflammation | Not specific for trauma | Moderate–strong if mechanism fits |
| Endplate fracture | CT/MRI | Yes | Rarely | Structural traumatic/endplate injury | May be degenerative in some settings | Moderate–strong |
| Modic type 1 endplate change | MRI | Can correlate with active axial pain | Usually no | Active inflammatory degenerative endplate change | Does not prove trauma | Moderate for pain, weak for trauma |
| Modic type 2/3 change | MRI | Possible chronic pain | Usually no | Chronic degenerative change | Does not prove acute injury | Weak |
4. Facet joint anatomy
| Imaging finding | Typical modality | Can cause back pain? | Can cause neurology? | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|---|
| Facet arthropathy | CT/MRI | Yes, axial extension-related pain | Rarely direct | Degenerative facet pain possibility | Common incidental finding | Weak–moderate |
| Facet joint effusion | MRI | Possible active facet irritation/instability | Rarely | Active facet inflammation/instability | Not necessarily traumatic | Moderate |
| Facet synovial cyst | MRI/CT | Possible | Yes if compressing root/canal | Nerve compression from cyst | Often degenerative | Moderate–strong if concordant |
| Facet hypertrophy causing foraminal/lateral recess stenosis | CT/MRI | Possible | Yes | Degenerative radiculopathy/stenosis | Does not prove acute injury | Moderate if concordant |
| Acute facet fracture | CT | Yes | Possible | Traumatic structural injury | Needs mechanism correlation | Strong |
5. Pars interarticularis anatomy
| Imaging finding | Typical modality | Can cause back pain? | Can cause neurology? | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|---|
| Pars marrow oedema | MRI | Yes | Usually no | Active pars stress reaction | Does not precisely date injury | Strong for active lesion |
| Pars defect/fracture line | CT | Yes | Usually no unless slip/stenosis | Structural spondylolysis | Could be old/chronic | Moderate |
| Sclerotic pars defect | CT | Possible chronic pain | Usually no | Chronic non-union/old defect | Does not prove acute injury | Weak for acute causation |
| Focal pars uptake | SPECT/SPECT-CT | Yes | Usually no | Active bone turnover/stress response | Does not precisely date injury | Moderate–strong |
| Pars defect without SPECT uptake | CT + SPECT | Less likely active | Usually no | Chronic inactive defect | Does not prove current pain source | Weak |
| Bilateral pars defects with spondylolisthesis | CT/MRI/X-ray | Yes | Possible via stenosis/foraminal narrowing | Structural instability/slip | May be longstanding | Moderate depending chronology |
Pars causation is stronger when:
- focal extension-related pain
- pain with sport/work involving extension-rotation
- MRI shows pars oedema or SPECT shows focal uptake
- CT shows corresponding pars defect
- symptoms are localised and temporally linked.
6. Spinal canal / central stenosis anatomy
| Imaging finding | Typical modality | Symptoms it may explain | What it supports | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|
| Central canal stenosis | MRI/CT | Neurogenic claudication, bilateral leg symptoms | Anatomical stenosis | Often chronic/degenerative | Moderate–strong if symptoms match |
| Severe canal stenosis with cauda equina compression | MRI | Bladder/bowel/saddle symptoms, bilateral neurology | Emergency compression | Does not prove cause unless timing fits | Strong clinically |
| Epidural mass/abscess/haematoma | MRI | Severe pain, neurology, systemic symptoms depending cause | Serious compressive pathology | Needs clinical/lab correlation | Strong |
| Ligamentum flavum hypertrophy | MRI/CT | Claudication/radiculopathy if stenotic | Degenerative stenosis contributor | Not acute injury | Weak for acute causation |
7. Foraminal / lateral recess anatomy
| Imaging finding | Typical modality | Symptoms it may explain | What it supports | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|
| Foraminal stenosis | MRI/CT | Exiting nerve root radiculopathy | Root compression | Often chronic/degenerative | Moderate–strong if concordant |
| Lateral recess stenosis | MRI/CT | Traversing nerve root radiculopathy | Root compression | Does not date onset | Moderate–strong |
| Disc-osteophyte complex | MRI/CT | Radiculopathy if root compression | Degenerative compression | Does not prove acute injury | Moderate if concordant |
| Acute foraminal disc extrusion | MRI | Acute radiculopathy | Acute/subacute root compression possible | Exact timing not always provable | Strong if concordant |
8. Ligaments / soft tissue anatomy
| Imaging finding | Typical modality | Can cause back pain? | Can cause neurology? | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|---|
| Paraspinal muscle oedema/tear | MRI | Yes | No | Muscle strain/soft tissue injury | Usually does not explain radiculopathy | Moderate |
| Interspinous ligament oedema | MRI | Yes | No | Ligamentous sprain | Does not prove exact mechanism | Moderate |
| Posterior ligamentous complex injury | MRI | Yes | Possible if instability | Significant traumatic injury | Needs trauma context | Strong if mechanism fits |
| Haematoma | MRI/CT | Yes | Possible if compressive | Acute bleeding/trauma/anticoagulation issue | Needs clinical context | Strong if acute context |
| No soft tissue oedema | MRI | Does not exclude pain | Does not exclude radiculopathy | No visible soft tissue injury | Does not prove no injury | Neutral |
9. Sacroiliac joint / pelvis anatomy
| Imaging finding | Typical modality | Can cause back/buttock pain? | Can cause neurology? | What it may support | What it does not prove alone | Causation strength |
|---|---|---|---|---|---|---|
| SIJ degenerative change | CT/MRI/X-ray | Yes | Not usually | SIJ degeneration | Common/incidental | Weak–moderate |
| SIJ bone marrow oedema | MRI | Yes | No | Active sacroiliitis/stress/inflammation | Not specific for mechanical injury | Moderate |
| Sacral stress fracture | MRI/CT/bone scan | Yes | Rarely | Acute/subacute bony injury | Needs mechanism/risk correlation | Strong if oedema/fracture |
| Pelvic/hip pathology | X-ray/CT/MRI | Can mimic back pain | Rarely | Alternative diagnosis | Not lumbar cause | Depends on concordance |
10. Serious pathology anatomy
| Imaging finding | Typical modality | Symptoms | What it supports | Causation comment |
|---|---|---|---|---|
| Discitis/osteomyelitis | MRI | severe pain, fever, raised inflammatory markers | Spinal infection | Not work injury unless exposure/procedure context |
| Epidural abscess | MRI | severe pain, fever, neurology | Emergency compressive infection | Urgent ED/spine pathway |
| Malignancy/metastasis | MRI/CT | night/rest pain, weight loss, cancer history | Serious pathology | Not mechanical work injury unless unrelated aggravation issue |
| Inflammatory spondyloarthritis | MRI SIJ/spine | inflammatory back pain | Systemic inflammatory disease | Not traumatic causation |
| Aortic/renal/abdominal pathology | CT/US/MRI | back/flank/abdominal pain | Non-spinal mimic | Important alternative diagnosis |
11. Causation strength
| Anatomy/finding | Usually stronger for causation | Usually weaker for causation |
|---|---|---|
| Disc | large focal extrusion/sequestration compressing matching root | mild broad bulge, degeneration |
| Nerve root | clear compression + matching dermatomal/myotomal signs | root contact only, non-matching level |
| Bone | fracture with marrow oedema | old fracture without oedema |
| Pars | MRI oedema or SPECT uptake + focal extension pain | chronic sclerotic defect without uptake |
| Facet | acute fracture, synovial cyst compressing root | mild facet OA |
| Canal | severe stenosis with classic claudication/cauda equina | mild stenosis without symptoms |
| Soft tissue | MRI oedema/tear after trauma | normal MRI, non-specific spasm |
| SIJ/pelvis | sacral stress fracture with oedema | mild SIJ degeneration |
Causation strength: interpreting CT/MRI findings in back pain
Core principle
Imaging findings are strongest for causation when they are:
- anatomically significant
- on the correct side
- at the correct spinal level
- consistent with the patient’s symptoms
- consistent with objective neurological findings
- temporally linked to the reported injury
- supported by clinical progression or prior imaging.
Imaging is weakest for causation when findings are mild, degenerative, common in asymptomatic people, non-concordant with symptoms, or cannot be dated.
A. Causation strength by anatomical finding
| Anatomy / finding | Usually stronger evidence for causation | Usually weaker evidence for causation |
|---|---|---|
| Disc | Large focal disc extrusion or sequestration compressing a matching nerve root | Mild broad-based disc bulge, disc desiccation, disc degeneration, multilevel degenerative change |
| Nerve root | Clear nerve root compression with matching dermatomal pain, sensory loss, myotomal weakness or reflex change | Nerve root “contact” only, no compression, non-matching side or level |
| Bone / vertebra | Acute fracture or bone marrow oedema at the symptomatic level | Old fracture without marrow oedema, chronic degenerative sclerosis |
| Pars interarticularis | MRI pars oedema or focal SPECT uptake with focal extension-related pain | Chronic sclerotic pars defect with no SPECT uptake |
| Facet joint | Acute facet fracture, facet synovial cyst compressing a concordant nerve root, active facet inflammation | Mild facet osteoarthritis, age-related facet arthropathy without concordant symptoms |
| Spinal canal | Severe canal stenosis with classic neurogenic claudication, cauda equina features or objective neurological deficit | Mild canal stenosis without symptoms or neurological signs |
| Foramen / lateral recess | Severe foraminal or lateral recess stenosis compressing the clinically affected nerve root | Mild narrowing without concordant radiculopathy |
| Soft tissue / ligament | MRI oedema, tear or haematoma after a plausible acute mechanism | Normal MRI, non-specific muscle spasm, vague soft-tissue symptoms without imaging correlate |
| SIJ / pelvis | Sacral stress fracture or SIJ/pelvic marrow oedema with matching pain pattern | Mild SIJ degeneration or pelvic degenerative change without clinical correlation |
B. Strong vs weak imaging evidence for causation
| Imaging situation | Causation strength | Why |
|---|---|---|
| Large right L5/S1 disc extrusion compressing the right S1 nerve root, with acute right S1 radicular pain and reduced right ankle reflex | Strong | Imaging, symptoms, side, level and neurological signs are concordant |
| MRI shows multilevel mild disc bulges in a patient with central low back pain only | Weak | Mild disc bulges are common incidental findings; no neurological concordance |
| CT shows chronic bilateral pars defects with sclerosis and no SPECT uptake | Weak for acute injury | More consistent with old inactive pars defects rather than active stress injury |
| MRI shows pars marrow oedema or SPECT shows focal pars uptake after a recent extension/rotation injury | Moderate to strong | Supports active pars stress response, but still requires clinical chronology |
| Acute vertebral compression fracture with marrow oedema after a fall | Strong | Marrow oedema supports acute/subacute fracture if mechanism and symptoms match |
| Degenerative foraminal stenosis with new radicular symptoms after lifting | Moderate | May represent symptomatic aggravation of pre-existing degenerative stenosis rather than a new structural injury |
| MRI months after injury shows disc degeneration only | Weak for proving original injury | Degeneration may be pre-existing and cannot usually be dated |
| Prior MRI normal, later MRI shows new disc extrusion matching acute symptoms after injury | Strong | Prior comparison helps establish new pathology and improves temporal linkage |
| MRI shows nerve root compression on the opposite side to symptoms | Weak | Imaging does not match the clinical presentation |
| MRI shows severe canal stenosis with classic walking-induced leg symptoms relieved by flexion | Moderate to strong | Imaging and neurogenic claudication pattern are concordant |
| Normal MRI in a patient with acute mechanical low back pain | Does not exclude injury | Soft-tissue strain and non-specific low back pain may not be visible on imaging |
C. What imaging can support reasonably well
| Imaging can support | Examples |
|---|---|
| Presence of structural pathology | disc extrusion, fracture, stenosis, pars defect, tumour, infection |
| Level and side of abnormality | right L5/S1 disc extrusion, left L4/5 foraminal stenosis |
| Nerve root compression | L5 or S1 nerve root compression, displacement or swelling |
| Acute/subacute bony injury | marrow oedema, acute compression fracture, bone contusion |
| Active bone stress | SPECT uptake, MRI pars oedema, stress reaction |
| Chronicity in selected cases | sclerosis, old non-united pars defect, old fracture without oedema |
| Serious pathology | infection, malignancy, epidural abscess, cauda equina compression |
| Change over time | new disc extrusion compared with previous imaging |
D. What imaging cannot reliably prove by itself
| Imaging cannot reliably prove alone | Why |
|---|---|
| That the imaging finding is definitely the pain source | Many spinal findings are asymptomatic |
| That pain is real or not real | Pain can be severe despite normal imaging |
| The exact date of injury | Most disc and degenerative findings cannot be precisely timed |
| That work, sport or a specific event caused the abnormality | Requires mechanism, chronology, examination findings and clinical correlation |
| That degeneration is symptomatic | Degenerative changes are common with age and may be incidental |
| That a disc bulge equals acute injury | Disc bulges are often chronic or incidental |
| That normal imaging means no injury | Soft-tissue strain, non-specific low back pain and early nerve irritation may not show |
| That later imaging proves the original injury | Pathology may be pre-existing, progressive, degenerative or due to a subsequent event |
| That a pars defect is acute | A pars defect may be chronic unless there is MRI oedema or SPECT uptake |
| That stenosis is newly caused by an incident | Stenosis is often degenerative, though an incident may aggravate symptoms |
E. Stronger causation language
Use when imaging, symptoms and examination are concordant.
Imaging demonstrates a large right paracentral L5/S1 disc extrusion compressing the right S1 nerve root. This is concordant with the patient’s acute right S1 radicular pain, reduced right ankle reflex and plantarflexion weakness. The clinical-radiological findings support right S1 radiculopathy.
For injury causation:
The imaging abnormality is anatomically capable of explaining the patient’s symptoms. The temporal relationship between the reported incident, onset of symptoms, examination findings and subsequent clinical course supports the incident as a significant contributing factor.
F. More cautious causation language
Use when imaging shows degenerative or non-specific findings.
Imaging demonstrates multilevel degenerative changes, including disc bulges and facet arthropathy. These findings are common and may be pre-existing. Imaging alone does not prove that these findings are the cause of the current symptoms. Clinical correlation is required.
For delayed imaging:
The later MRI demonstrates degenerative/disc pathology. Imaging alone cannot determine whether this was caused by the original incident, was pre-existing, or developed subsequently. Causation should be assessed using the documented mechanism of injury, timing of symptom onset, examination findings, continuity of symptoms and any prior imaging.
G. Practical hierarchy of causation strength
| Causation level | Features |
|---|---|
| Strong | Clear structural lesion, correct side/level, objective neurological signs, acute chronology, plausible mechanism, prior imaging supports new change |
| Moderate | Imaging could explain symptoms, but findings may be degenerative or timing is uncertain; clinical chronology supports aggravation |
| Weak | Mild degenerative findings, no objective neurology, non-concordant symptoms, delayed imaging only, no prior comparison |
| Not established by imaging | Imaging abnormality is incidental, opposite side/wrong level, or unrelated to symptoms |
Practical takeaway
The most defensible causation opinion comes from combining:
- Mechanism — what happened?
- Timing — when did symptoms start?
- Symptoms — back only vs radicular pattern?
- Examination — objective neurological signs?
- Imaging — does level/side/pathology match?
- Course — continuous symptoms vs recovery/new injury?
- Prior history/imaging — pre-existing condition or new change?
Imaging is strongest when it shows a lesion that is specific, severe, anatomically concordant, and temporally supported by the clinical history.