John Roach, Esq. | November 2, 2026 | Brain Injuries
Brain Imaging After a Concussion: A San Francisco Brain Injury Lawyer Explains What Each Scan Shows — and Misses
Clients ask me some version of the same question almost every week: “If my brain is really injured, why didn’t it show up on the scan?” It’s a fair question, and it deserves a real answer instead of a dismissive one. The honest answer is that most concussions and mild traumatic brain injuries were never expected to show up on the imaging your client actually got. That’s not a flaw in your case. It’s a well-documented limitation of the specific tools emergency rooms use, and understanding it is often the difference between an insurance adjuster closing your file and a lawyer building a case that holds up.
This isn’t a single-scan story. There are close to a dozen imaging technologies that get discussed in brain injury cases, each aimed at a different piece of the puzzle, each with real strengths, and each with real limits — and, increasingly, each subject to genuine scientific and legal debate about how far its findings can be pushed. Here’s what actually happens inside each of them, and why the fight over what they can prove is more nuanced than either side usually lets on.
Why Most Concussions Are Invisible on Routine Imaging
Start with the injury itself. The most common brain injury in a car crash, a fall, or a sports collision isn’t a bleed or a fracture — it’s diffuse axonal injury (DAI): microscopic tearing and stretching of the axons, the long fibers that wire different regions of the brain together. DAI happens when the brain accelerates, decelerates, or rotates inside the skull faster than the tissue itself can keep up, shearing those fibers at a cellular level.
The problem for imaging is scale. Individual axons and neurons are far too small to be seen directly by any clinical scanner. Outside of a laboratory autopsy — examining brain tissue under a microscope — there has historically been no way to directly visualize that shearing in a living patient. Every imaging technology used in a brain injury case is, in one way or another, an attempt to detect that invisible injury indirectly: through bleeding it left behind, through swelling, through changes in brain structure over time, through disrupted water movement along damaged fibers, or through altered brain activity and metabolism.
It’s also well established in the medical literature — and generally not disputed between plaintiff and defense experts — that diffuse axonal injury accompanied by a brief loss of consciousness is often not expected to appear on a routine CT or MRI at all. A scan coming back “normal” is frequently the expected result, not evidence that nothing happened. That gap between what actually occurred in the brain and what a first-line ER scan is capable of showing is where most TBI cases live or die.
CT Scans: Fast, Essential, and Not Built for This
A CT scan is the first imaging almost every head-injury patient gets, and for good reason — it’s fast, widely available, and excellent at catching the emergencies that can kill someone within hours: skull fractures, large hemorrhages, and dangerous swelling. That’s exactly what it’s designed to screen for, and it does that job well.
What a CT scan is not designed to do is detect microscopic axonal shearing. CT resolves structure at a scale far too coarse to see individual damaged fibers, and it has essentially no ability to show the kind of subtle, non-hemorrhagic injury that defines most concussions. A “normal” CT tells you there’s no surgical emergency. It tells you almost nothing about whether the brain’s wiring was damaged.
Conventional MRI: Better Detail, Same Fundamental Gap
Standard MRI sequences — T1, T2, and FLAIR — give a much more detailed anatomical picture than CT: better soft-tissue contrast, better visualization of the brain’s structure. For injuries with contusions, larger areas of bleeding, or structural abnormalities, conventional MRI often picks up findings CT misses entirely.
But for the diffuse axonal injury that defines most mild-to-moderate TBIs, conventional MRI runs into the same core problem as CT: it’s an anatomical picture, and DAI is a microstructural one. It’s a widely recognized clinical teaching point that DAI cases without an extended loss of consciousness — generally cited around the 30-minute mark — are frequently not expected to show any abnormality on routine MRI. A normal conventional MRI, like a normal CT, rules out the emergencies. It does not rule out the injury.
Susceptibility-Weighted Imaging (SWI): Catching the Microbleeds
Susceptibility-weighted imaging is a specialized MRI sequence tuned to detect tiny amounts of blood breakdown products — microhemorrhages — that are invisible on standard MRI sequences. When axonal shearing is severe enough to also tear small blood vessels, it can leave behind microscopic bleeding that SWI can pick up even when everything else looks clean.
SWI is a genuinely useful adjunct, and it’s one of the advanced sequences that professional radiology societies have specifically recognized as adding value in the evaluation of trauma. Its limitation is equally important: most diffuse axonal injury is not hemorrhagic. Axons can be torn or stretched without tearing an adjacent blood vessel, which means SWI can also come back clean in a patient with a real, significant brain injury. It closes part of the gap CT and conventional MRI leave open — not all of it.
Diffusion Tensor Imaging (DTI): Mapping the Wiring, Not Just the Anatomy
DTI takes a different approach entirely: instead of imaging brain structure, it tracks the movement of water molecules along the brain’s white-matter tracts — the bundles of axons that connect one brain region to another. In healthy, intact fibers, water moves in a strongly directional pattern along the fiber. When axons are damaged, that directional pattern breaks down, and DTI can measure the difference through a value called fractional anisotropy (FA).
DTI is a legitimate, decades-old, FDA-cleared imaging sequence, and it’s exactly the kind of tool that’s aimed at the specific gap CT and conventional MRI leave open. It’s also, right now, the single most contested imaging technology in TBI litigation — not because the underlying science is doubted, but because of a much narrower, much more specific question: does using an individual patient’s FA values to diagnose that one patient’s brain injury meet the legal standard for generally accepted science? Some professional radiology and neurology bodies have historically characterized that specific individual-diagnosis application as investigational, even while treating DTI itself as well-established for research. Courts applying California’s Kelly/Frye standard for novel scientific evidence have reached different conclusions on that narrower question, and the underlying science is moving faster than the case law. It’s a genuinely active, evolving fight — not a settled one in either direction.
Volumetric MRI (Structural Volumetrics): Measuring What Shrinks
Beyond function, brain trauma can cause measurable structural change over time — certain regions, like the hippocampus (central to memory), can lose volume following an injury. Volumetric MRI analysis uses computer post-processing to measure the size of specific brain structures precisely and compare left-side to right-side, or an individual patient’s measurements to population norms.
This technique faces a version of the same debate as DTI: comparing a patient to a “normative” population database runs into real confounders — age, sex, and even which scanner was used can shift the numbers — while comparing a patient’s own left hemisphere to their own right hemisphere (since the brain’s structures are normally close to symmetric) is a newer approach some radiologists argue sidesteps those confounders. Whichever method is used, volumetric findings are treated the same way responsible experts treat DTI: as one data point that has to be read alongside the clinical picture, not a stand-alone diagnosis.
Functional MRI (fMRI): Watching the Brain Work
Functional MRI measures blood-oxygen changes that occur when a brain region becomes active, effectively showing which parts of the brain “light up” while a patient performs a specific mental task. In TBI research, fMRI has been used to show that injured brains sometimes have to recruit more regions, or different regions, to accomplish the same task a healthy brain handles more efficiently — a real, measurable sign of altered function even when the anatomy looks normal.
fMRI’s practical limitation in litigation is that it’s task-dependent and effort-dependent: the results reflect what the patient was asked to do and how they performed it in that moment, which makes it harder to standardize and easier to challenge than a straightforward anatomical finding. It remains primarily a research tool, with only limited use in individual clinical or legal settings so far.
PET and SPECT: Metabolism and Blood Flow, and the Cautionary Tale
Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) both image brain function indirectly — PET by tracking glucose metabolism, SPECT by tracking blood flow — using a small amount of injected radioactive tracer. Both can show regions of the brain that appear to be working less efficiently after an injury, even when structural imaging looks completely normal.
Both also carry real drawbacks: radiation exposure, high cost, inconsistent insurance coverage, and — most importantly for how they get treated in court — a lack of specificity. Reduced metabolism or blood flow in a given brain region can result from a genuine traumatic injury, but also from aging, medication effects, sleep disruption, depression, or a range of other unrelated conditions. SPECT in particular has drawn pointed, sustained criticism from mainstream neurology and psychiatry over its use to support individual diagnoses outside a rigorous, peer-reviewed clinical framework — a useful cautionary example of what can happen when an imaging technology’s marketing gets ahead of its evidence base. It’s a pattern worth keeping in mind for any advanced imaging offered in a case: the existence of an abnormal finding is only half the story; whether that specific use of that specific technology has earned real scientific and legal acceptance is the other half.
How These Findings Actually Get Challenged
Whatever the scan, defense experts tend to attack advanced imaging findings along the same handful of lines, and it’s worth naming them plainly rather than pretending the fights don’t exist:
- Group data versus individual diagnosis. A large body of research showing that TBI patients, as a population, tend to have lower FA values or reduced brain volume doesn’t automatically mean a specific value in a specific patient proves that patient was injured. This is the core of the current DTI admissibility fight, and it applies to every quantitative imaging technique discussed here.
- Normative database problems. Comparing an individual patient’s numbers to a “normal” population database is vulnerable to real confounders: age, sex, scanner manufacturer, and field strength can all shift the baseline. Some radiologists have moved toward within-subject, left-versus-right comparisons specifically to get around this criticism.
- FDA clearance versus diagnostic validation. A sequence or software platform being FDA-cleared for imaging is a different question from whether that same tool has been validated, and accepted by the relevant professional societies, for diagnosing an individual patient’s brain injury. Defense counsel will draw that distinction sharply, and it’s a fair one.
- Lack of standardized protocol. Different imaging centers use different post-processing software, different statistical cutoffs, and different thresholds for what counts as “abnormal.” Where a field lacks one uniformly accepted protocol, that inconsistency becomes fertile ground for cross-examination.
- Specificity. An abnormal finding on any of these scans can, in some cases, be explained by something other than trauma — a preexisting condition, normal aging, or an unrelated illness. Good radiology practice rules those alternative explanations out before attributing a finding to the crash; sloppy practice doesn’t, and that gap gets exploited.
None of this means advanced imaging is worthless in a TBI case — it means it has to be used the way the most credible experts already use it: as one piece of a larger puzzle, correlated with the neurological exam, formal neuropsychological testing, and a well-documented history of the patient’s symptoms before and after the injury. A DTI or SWI finding that stands alone, disconnected from the rest of the clinical record, is exactly the kind of evidence that gets picked apart. The same finding, tied tightly to a consistent symptom history and a thorough clinical workup, is a different matter.
What This Means If You’re Building a Case
A “normal” scan from the emergency room is close to the expected result in most concussion and mild TBI cases — it is not proof that nothing happened, and it shouldn’t be treated as the end of the medical or legal inquiry. If symptoms persist after an ER visit that came back clean, the priorities are the same regardless of which advanced imaging eventually comes into play: see a doctor and go back if symptoms don’t resolve, ask about referral to a specialist who can evaluate whether more advanced imaging or neuropsychological testing is appropriate, and keep a clear, dated record of symptoms and how they’re affecting daily life and work. That record is what turns an isolated imaging finding — of any kind — into a case that actually holds together.
As your attorney, this is where I focus early: making sure my clients get to the right specialists, that the medical record actually reflects what they’re experiencing, and that any imaging in the case gets presented the way the science actually supports it — not oversold, and not dismissed. I’ve represented injured people across the Bay Area, including brain injury cases where the imaging told only part of the story, since 2009, and I bring that same preparation and extensive trial experience to every case I take on. Consultations are free, and my office handles cases in both English and Spanish — for Spanish-speaking clients and families, hable conmigo directamente en español, sin intérpretes.
Yes. Diffuse axonal injury — the microscopic axon damage behind most concussions — is often not expected to show up on routine CT or standard MRI at all, particularly without an extended loss of consciousness. A normal scan rules out the emergencies those tools are designed to catch. It doesn’t rule out a real, disabling brain injury.
CT is faster and better at catching the immediate emergencies — skull fractures, large bleeds, dangerous swelling. MRI gives more detailed soft-tissue images and can catch some things CT misses, but both are anatomical tools, and neither is designed to directly detect microscopic axonal shearing.
It depends on how it’s used. DTI itself is a well-established, FDA-cleared imaging technique with decades of research behind it. Using an individual patient’s DTI results as a stand-alone diagnostic tool for that specific patient is a narrower, still-evolving legal question, and courts applying California’s standard for novel scientific evidence have reached different conclusions depending on the specific method and how the evidence is presented.
Because each one is designed to catch a different kind of injury that ordinary imaging misses. SWI looks for microscopic bleeding; DTI looks at whether the brain’s white-matter “wiring” is functioning normally; PET and SPECT look at metabolism and blood flow. A treating physician orders these when a patient’s symptoms and clinical exam suggest a real injury that basic imaging hasn’t explained.
They frequently try to. A normal CT or MRI is one of the first things an adjuster points to when minimizing a brain injury claim. That’s exactly why a consistent, well-documented symptom history and, where appropriate, the right advanced imaging and neuropsychological testing matter so much in building the record.
Not always. Many TBI cases are proven primarily through a strong clinical record — treating physicians, neuropsychological testing, and consistent documented symptoms — without ever needing advanced imaging like DTI or PET. Advanced imaging is a tool to strengthen a case when it’s available and appropriate, not a requirement to have one.
Nothing up front. I handle these cases on contingency — no fees unless I recover compensation for you — and the initial consultation is always free, in English or Spanish.