Cerebrovascular Imaging: Which Test and Why? - SD

Introduction

Hello, I'm Dr. Andrea Alexandro, director of the Comprehensive Stroke Center at the University of Alabama Hospital in Birmingham, Alabama.

I would like to review with you options for cerebrovascular imaging, when, which test and why.

In the next 30 minutes or so, I will review with you which vascular and cerebral vascular imaging tests are available, when you evaluate stroke patients, which test to choose at what time and what to expect.

Standard of Care Evaluation

As you can see from this slide, the standard of care evaluation of acute stroke patient consists of history and physical, vitals, such as ECG, oxygenate blood oxygen, as well as basic labs and quick access to structural imaging such as head ct, computerized tomography, or MRI, magnetic resonance imaging.

In our center, we also prioritize these patients' access to non-invasive vascular ultrasound examinations, such as transcranial doppler, TCD and carotid duplex ultrasound.

And these non-invasive tests are used in our institution in part to select patients for invasive angiography.

And the catheter procedures are still being used in acute stroke.

And in fact, there are more technology being developed to help with invasive procedures to restore flow to the brain or to develop technologies that would allow secondary stroke prevention.

Goal of Cerebrovascular Imaging

So the goal of cerebrovascular imaging would be to select appropriate patients for appropriate therapy, and particularly those who require risky and invasive procedures.

Current Standard of Care Treatment

Next slide shows current standard of care treatment for patients who are stroke survivors, or TIA.

As you can see in the first few lines, it says Aspirin 81 3 25 milligram Plavix aox.

These are antiplatelet agents.

Obviously, these agents would be prescribed based on findings on imaging that do not indicate hemorrhage in the patient.

Other treatment options that are available mostly for secondary prevention in these patients are blood pressure control, diabetes control, statin medications to lower cholesterol.

Additionally, there are some interventions available such as carotid and ectomy, CEA, particularly for patients with symptomatic greater than 70% unilateral internal car stenosis or stenting in those patients.

And then asymptomatic patients with greater than 60 percent unilateral stenosis may be considered for muscularization if they were given a chance of best medical therapy.

And there is still disease progression and so on.

This is my more debatable area, but nevertheless, that forms the basis for screening with cerebrovascular ultrasound for the presence of significant car stenosis.

I did not put on this slide an emerging area of intracranial roose disease that is particularly prevalent in Asia, but also can be found in the United States, in particularly in southeastern population where I practice.

And screening for significant intracranial disease is becoming one of the hot areas in application of cerebrovascular imaging.

Acute Stroke Treatment

How about acute stroke treatment?

Well, for stroke treatment, we have within three hours of symptom onset.

Intravenous TPA is approved for is a frontline and the only approved therapy to reverse damage from ischemic stroke.

Trials outside three hour windows are being completed, and up to six or even eight hours.

There is option of deploying intra arterial thrombolysis or combined approaches, where first intravenous TPA is initiated and then followed by in arterial rescue.

At variable time window, one may apply newer devices for mechanical clot disruption or removal.

Again, at the variable time window, there are new devices being tested for flow augmentation to the brain.

We're still conducting some neuroprotective trials, although previous attempts were not successful.

And up to several days, one must consider hemicraniectomy to save patient's life if they're developing malignant MCA syndrome, or they develop really a large and lesions in the cerebellar tissue.

So given all these diverse treatments that one may administer, let's review indications for imaging and what imaging tests are available.

Indications for Imaging

So the main indicator for a brain and head and neck imaging is, of course stroke.

The goal here is to rule out intracerebral hemorrhage or other types of hemorrhage, establish the diagnosis of this event and give some prognosis for the patient.

And options for management.

The hot new area of imaging is transient ischemic attack, and I will mention that when we will review appropriate tests.

If you suspect arterial stenosis or occlusion, that's an indication to obtain some imaging.

Traditional indication is also carotid bru.

And recently, what has been scrutinized, who are the patients who do, did not have yet stroke or TIA, but requires some assessment before surgeries or high risk vascular patient who should receive cerebral vascular imaging.

Mainstay: Non-Contrast Head CT

Now let's start with the mainstay.

The mainstay of evaluation of a patient with a stroke, particularly in the hospital, is a non-contrast head ct.

The reason is, it is universally available in the emergency rooms, particularly in the United States and developed countries.

It is fast, it can be done within minutes, with minimum loss of control of the patient.

In fact, you can continue monitoring patient while in the CT scan.

There are virtually no contraindications to non-contrast CT scan.

It is considered the gold standard for ruling out hemorrhages, and the contrast injection options are available.

I'll mention some of them later during the talk.

So whenever the patient comes to the emergency room and is being taken as a first test of choice to the non-contrast ct, remember that you may also request a contrast study at the same time.

If certain parameters are met that it would be safe to administer those contrast agents.

Example: Non-Contrast Head CT Showing Intracerebral Hemorrhage

So here is an example.

This is the slide that's called non-contrast.

He CT showing intracerebral hemorrhage.

And you can see here a bright area that is asymmetric.

It's on one side of the brain.

This is one is typical hypertensive hemorrhage on the left.

And the other one is more cortical hemorrhage that represents event due to cerebral amyloid and pathy.

There are other types of bleeding.

This is beyond the scope of this talk, but the CAT scan sometimes is the first imaging test that patients with different forms of bleeding are receiving in the hospital.

Disadvantages of Head CT Scan

Disadvantages of head CT scan is mostly relates to interpretation of what's called early infarct sites signs.

These are often vaguely defined like one third of the MCA territory or certain changes in tissue besides hypo attenuation.

Besides hypodensity, that's what I would like to stress that all early signs except hypodensity very little, if any, prognostic significance for IV TPA within three hours of symptom onset.

The major finding that I'm paying attention, I'm trying to find on head CT is, is there hypo attenuation?

Because the presence of hyper continuation in a patient who is otherwise within three hours of TPA window makes me question the time of symptom onset.

Quite often history of present illness may not be ascertained in time in a very few minutes of conversations.

So finding hypo attenuation would question the time of onset because it takes longer than three hours to form a significant infarction of tissue.

CAT scan is being criticized as often being normal in acute phase of stroke and TIAs to me this is encouraging because CAT scan shows somewhat tissue window.

And if the patient has severe stroke and PA four hours elapsed and they had CAT C and had CT scan looks normal, that gives me optimism that this stroke may be as not yet as severe to produce visible damage.

And maybe this patient would benefit from some form of reperfusion procedures.

The definite disadvantage of head CT is it is insensitive to small lacunar lesions or lesions in the posterior circulation.

Multimodal MRI

Now, switching to multimodal MRI scan magnetic resonance imaging, including DWI, which stands for diffusion weighted imaging is that it is very sensitive to acute ischemia.

Over 90% of strokes can be visualized readily right on admission, and it is also positive in about half of patients with TIAs.

In fact, MRI is now recommended as the preferred imaging modality for patients with resolved symptoms such as TIA because you can prognosticate the risk of TIA recurrence based on presence or absence of lesion on DWI and presence or absence of a vascular obstruction on Mr. Angiography or MRA.

MRI is also sensitive to posterior fosse and lacunar lesions.

It is helpful to differentiate acute from subacute or chronic lesions through a DC maps, flares and other sequences.

It demonstrates topography and mechanism of cerebral ischemia, providing wealth of information of what kind of damage you will be dealing with in any particular patient.

Disadvantages of MRI

Next slide. Details, disadvantages of MRI.

It is not universally available for urgent workup.

In fact, even in large hospitals, still, MRI is not necessarily available in the emergency room or transfer from the emergency room to MRI may actually take long time.

The other issue is loss of contact with the patient because it takes longer scanning time.

Scanners physically require patient go inside the scanner, so you kind of lose direct visual contact.

And if you want to obtain all sequences that are required for complete stroke workup, it'll take a considerably longer time, probably in the order of about 30 to 40 minutes to complete all sec on all sequences, putting together transport time, patient preparation time, and going through a long list of contraindications makes MRI practice in acute setting rather difficult, not impossible, but difficult.

And contraindications need to be kept in mind.

For example, presence of metal or certain devices like pacemaker, patients may be claustrophobic, patients may be unstable to go for a prolonged time laying down flat in inside the scanner.

Now, there are certain aspects of MRI that require expert interpreter.

For example, detection of hyperacute hemorrhage or old micro bleeds require some training.

It is not to say that CAT scan doesn't require that they both require that, but MRI requires training in more pathologies as it is able to visualize some conditions that CAT scan isn't sensitive to.

And we just started finally to fill the gap on objective criteria for imaging that need to be validated in randomized clinical trials.

There is an acute imaging map roadmap right now worked out.

There is several clinical trials being completed that use advanced imaging techniques, and the data will be emerging as to which parameters of MRI or advanced CT would be the most helpful in decision making.

Meanwhile, the mainstay is to identify hemorrhage and after you rolled out hemorrhage to quickly visualize tissue that's salvageable.

Examples of What MRI Can Bring

And here are some examples what MRI can bring to the table.

So here is an example of diffusion weighted imaging and acute stroke that shows on the left, like ary infarct, which is a small subcortical confined to white matter tracts area damage.

There is a scattered emboli in the middle.

This is from bilateral hemispheric embolization from a proximal cardiac source.

And on the right there is a watershed infarct between anterior cerebral and middle cerebral later territories.

Furthermore, multimodal MRI can provide interesting maps.

One is diffusion weighted imaging, which is metabolically dysfunctional tissue that is destined to infarction if a perfusion does not occur.

And around it, there could be an area highlighted here in red and green, an area of compromised perfusion in a much larger area in the middle cerebral la this could be a tissue at risk, again, if reperfusion does not occur.

And MRA on the right shows actually missing internal carotid artery and the middle cerebral artery, the reason for these profound perfusion changes.

And so, if I see a patient like this with this what's called perfusion diffusion mismatch, and the large arterial occlusion, I feel enthusiastic about trying to this patient or augment flow to try to salvage part of the brain that is hypoperfused and in hope to limit the damage.

However, the trade off obtaining these scans is time.

To get all these images may take, usually takes longer than 15 minutes, because even in the best centers, patient preparation, access, and timing of sequences altogether contribute to much longer scanning times than CAT scan.

MRA (Magnetic Resonance Angiography)

Also what you need to know about magnetic resonance an geography is that it allows you to look at extracranial and intracranial vessels.

It can provide you with three dimensional views from the arch to the circle of Willis and major branches.

And particularly with contrast, however, MRA has multiple disadvantages, flow gaps, turbulence, and flow reversal could be subtracted from the image leading to creation of the lesion where it doesn't exist or overestimating the degree of the lesion or present stenosis.

Furthermore, only with current resolution, you cannot obtain a plaque definition.

You need to apply certain coils or certain specific imaging techniques that are not yet the mainstay in, in short, we order MRA if we order MRI all the time as part of the package.

And because of the frequency of artifacts or false positive or lesion visualization on MRA, quite often magnetic resonance and geography generates another referral for ultrasound testing.

Just to double check if the flow gap or flow absence or in the certain vessel does represent a significant lesion or is it a artifact?

So in a sense, MRA and ultrasound carotid and intracranial ultrasound are very complimentary to each other.

CTA (CT Angiography)

Now, CT and geography actually has higher accuracy rates than MRA, it approaches accuracy compared to cast an geography and for certain indications, it now, it is now preferred imaging modality, particularly if it evolves around finding aneurysms or defining certain vessels and planning, surgery or other interventions.

So however, it requires injection of intravenous contrast agents.

The scanners allow very fast evaluation and may provide you a shot from the arch neck all within one injection, all the way to the brain.

And however, it has disadvantages, it has radiation and it requires contrast agent and some patients may be sensitive to it.

Definition of Carotid Stenosis on CTA

The next slide shows definition of carotid stenosis on CT and geography.

And as you can see here, three dimensional and two dimensional images can provide a very detailed depiction of the extent of the stenosis, the degree of stenosis, not to mention that you can measure absolute diameter of the residual lumen and make more precise estimation of the stenosis, as well as see calcified plaques and how it interplays with the overall vessel and the anatomy, including the jaw and level of vertebrae on the neck.

Role of Noninvasive Imaging

Next slide is to summarize what is the role of for noninvasive imaging as it applies to the stroke patients, and particularly to mention carotid duplex ultrasound here.

Screening requires high positive predictive value.

Well, it is true that it is better to over call the degree of stenosis than to miss a significant stenosis or occlusion.

The other thing is, however, you need to have locally validated criteria for vascular testing with ultrasound.

And that applies to carotid ultrasound as well as TCD, as well as other ultrasound practice areas.

Carotid duplex is particularly good as a complimentary test to M-R-I-M-R-A, and there is evidence that if both agree, if both show the same degree of pathology, then usually catheter angiography doesn't bring extra information or based on two non-invasive tests, M-R-I-M-R-I and carotid duplex, you can select candidates for either catheter procedures or carotid ectomy.

The other application that carotid duplex offers is find lesions other than ICA that could be suitable for treatment or carotid ectomy or stenting, for example, lesions in the common carotid artery, finding evidence of dissection.

That information can be available.

Ultrasound can also help identify mechanism of disease or complications.

And you can also monitor using ultrasound in real time.

And together with other modalities, you can show reversibility of brain damage.

Problems with Carotid Duplex Ultrasound Alone

Now the next slide identifies some of the problems with carotid stenosis detection by using just duplex ultrasound.

In fact, it goes to a broader question when you evaluate patient with a stroke or a TIA, do you order just carotid duplex or do you order carotid duplex and transcranial doppler?

In a lot of institutions, transcranial doppler is not used routinely for revelation of these patients.

And in the next few minutes, I would like to make a case why it should be the case to use both carotid and TCD, carotid duplex and TCD at the same time.

So, for example, if you look at consecutive patients with stroke, or TIA 50% stenosis is only found in about 15 to 25% of these patients.

So the rest of the strokes of stroke patients do not have their mechanism explained by this finding.

What if carotid duplex shows high bifurcation?

You simply started to see the bulb, but you never saw the exit from the bulb.

You never saw ICA on the neck.

So that may that ICA may contain a lesion that can elude detection.

What if you found proximal ICA, but high resistance in the proximal ICA and you don't see the lesion that may attribute to attributable to it or cause it?

What if you see a long, greater than two centimeter shadow from a calcified plaque?

Are you missing significant stenosis here or not?

Patients may have tandem on or elongated lesions, and the velocities and ratios during carotid duplex examination may be simply confusing or misleading.

Quite often patients have bilateral disease and one will have leading stenosis, and the other one will have compensatory velocity increase.

How do you reconcile these two?

And don't forget that some stroke patients may have not anterior, but posterior circulation symptoms.

So having someone complaining on transient weakness in arm and leg doesn't mean that this patient has carotid circulation symptoms that may come from the vertebral or basal arties.

Ischemic Stroke Mechanisms

So another issue to remember is ischemic stroke mechanisms.

And if you look at the next slide, it actually shows this mechanism.

So as you can see, large vessel thrombotic stroke accounts for average about 20% of these patients.

And there may be patients with cardio embolic mechanism like stroke mechanism, other mechanisms, and then undetermined up to 30% in early studies.

Now with more considerable workup besides admission CAT scan, besides just carotid duplex, if you deploy MRI, if you deploy carotid ultrasound TCD and cardiac workup, you may reduce the number of undetermined or crypto cryptogenic strokes down maybe 15 or even 10%.

You would never reduce it to zero, but you can substantially reduce that number of stroke patients.

But again, the message is do not look just at the neck for this proximal ICA as the source of all the answers in stroke patient workup.

Transcranial Doppler (TCD)

The next slide shows actually a diagram of the circle of Willis with projection of the spectral waveforms out of the circle of Willis.

And this is a regional approach with transcranial doppler as defined by Rooney athlete.

In fact, it's his drawing right there.

And for a long time we had simple single gate spectral waveform analysis that was very un obvious to sonographers because there was no image attached.

And it required a long time to acquire skill and ability to navigate through the circle of Willis by just switching the depths and putting together in your head these spectral waveforms.

As technology evolves, now more centers have transcranial duplex ultrasound, and whenever I say TCD, same comment applies to transcranial duplex because you can combine carotid duplex examination with transcranial duplex examination and do a considerable imaging study of the cerebral circulation.

The bottom right part of the image is the power motion doppler that expands capabilities of TCD in detecting and localizing the SIG signals.

Normal Waveforms in MCA and ACA

If you look at the next slide, it shows normal waveforms in the middle, cerebral and anterior cerebral arteries in the asymptomatic suit, 3-year-old man with normal blood pressure values.

As you can see, there is sharp systolic tro.

There is good diastolic runoff, and there is a bidirectional signal where both vessels have low resistance flows.

Abnormal Waveforms in Severe ICA Stenosis

Now, if you advance to the next slide, you will see a 67-year-old man with a recent TIA and severe ICA stenosis.

And you see how this wave forms obtained that similar location change, middle cerebral artery distal to the stenosis becomes what's called blunted or delayed STO flow acceleration flattened waveform because of campaign story vasodilation.

And because the flow had to pass through narrow collateral channels that are not yet developed or pass through a significant narrowing to get to the point, and the waveform below shows a very high velocity jet with diastolic velocity approaching 190 centimeters per second, this is a part of a partial arterial cross filling in that particular patient.

So as you can see, looking at downstream changes to the ICA on transcranial doppler can reveal some of the striking findings that will help grade severity of the internal carotid artery involvement.

Downstream TCD Findings

So next slide shows one of the papers on this subject, downstream TCD findings.

Our group published a what was an improved broad diagnostic battery from previous works, where we put together typical findings that one can anticipate on transcranial doppler if there is a major carotid stenosis or occlusion.

So the next slide details major criteria such as collaterals, abnormal MCA waveforms, delayed historic acceleration, and minor criteria such as decreased MCA ity, some presence of flow diversion or campaign story velocity increases.

If you apply these criteria like we did.

And the next slide shows the table where percent stenosis was graded by NASA criteria from normal to occlusion and percentage of abnormal TCD findings.

As you can see, when the stenosis becomes severe 70 to percent to occlusion, the percentage of abnormal TCD findings raises dramatically.

And that information is useful to actually confirm or rule out hemodynamic presen presence of hemodynamic significance of the ICA lesion.

That's number one. Number two, it helps refine some of the carotid duplex examinations.

For example, if one presents with false positive carotid duplex study doing TCD and finding that carotid duplex said 90%, but TCD is normal, surely will raise the question, how accurate was that carotid duplex examination?

So that's number one. Number two, if someone presents with normal carotid duplex examination, seeing abnormal TCD with these findings would question was the a lesion missed or is the a lesion at the entrance to the skull, for example?

And that's how quite often we find intracranial lesions or dissections and so on.

Identifying High Risk Plaque

Next slide is called How to identify a high risk plaque.

Well, high risk plaque may be identified as hypoechoic, predominantly eent on B mode, or the one with irregular surface on B mode and possible ulceration as a combination of color flow and b mode on carotid ultrasound.

But it may also be identified through additional test on TCD.

And the three parameters that I look for is either emboli, spontaneous emboli on transcranial doppler distal to the plaque.

Is there delayed systolic flow acceleration or the blunted signal that would imply incompleteness of circle of Willis and suboptimal collaterals.

And what is the vasomotor activity on TCD? Is it impaired?

Is are the vessels already maximally dilated and they cannot accommodate for any more narrowing of that particular vessel?

Example: Duplex Image with Ulcerated Plaque

And here is an example.

Next slide shows a duplex image with a color coded insert of a mild plaque.

It basically caused no more than 30% stenosis, no velocity elevation, but it had an ulcerated crater.

In fact, the image belongs to Dr. Charles Staler and d Oror out of wake Forest University laboratory in Winston-Salem.

And the lesion like this can confuse everyone because by percent diameter reduction, it doesn't reach severity of NASA trial infected.

It may not be even on the scale, but in a young person, a plaque like this, particularly with recurrent symptoms while on best antiplatelet medication, may resuscitate some intervention in surgery if one can demonstrate the presence of emboli.

And that indeed was the case in this particular situation.

Embolic Black Plaque Burden

So here is an example of data next slide on embolic black plaque burden.

This is an example of data that was obtained prospectively by hug Marcus Group out of London in England.

And here is 111 patients with greater than 60% stenosis.

69 of them were symptomatic.

42 asymptomatic embolic signals were detected in 37% of these.

And if TCD study on top of carotid ultrasound, if TCD study was showing bolli for the same percent stenosis, the risk was eight fold higher for first ever or stroke or second stroke occurrence.

Vasomotor Activity Data

Next slide is also showing some vamo activity data.

This is on asymptomatic greater than 70% stenosis.

This is the study done by Italian group of maril, published in jama.

In fact, one of the papers that actually led to acceptance of vasomotor activity testing as part of routine and reimbursable practices in the United States.

Here is the overall risk of any ischemic stroke, 7.9% per observation year in the study.

If breath holding index was normal, risk recurrence was lower than average.

And if breath holding index showed impaired response to holding breath, then a risk of stroke was increasing more than threefold for the same degree of asymptomatic stenosis.

And the formula, how to calculate the test is there, I don't have time to go into details.

You can look it up. The methodology of this has been published, even though it's not the best test to assess vasomotor activity, if you don't have any dedicated or specifically validated equipment, I think a majority of patients can do reasonable job tolerating and cooperating during the test, and you may get some useful information out of it.

And the same breath holding index value has been replicated with patients with carotid occlusion where if you have two collaterals, your risk of stroke is least as defined by TCD one collateral.

You go high on risk of stroke.

But if you have impaired vasomotor activity on top of that, that's the highest risk of stroke recurrence distal to carotid artery occlusion.

Combined Use of Carotid Duplex Ultrasound and TCD

From my final slide shows combined use of carotid duplex ultrasound NTCD as it's shown here.

When do we use it?

Well, in our institution, when one evaluates acute a stroke patient, acute, subacute, chronic, or a TIA patient, both tests are ordered at the same time.

And in fact, majority of our patients have CAT scan on admission, CT scan on admission, non-contrast, followed by MRI within 24 48 hours of admission and carotid duplex and TCD done in the emergency room if the patients were acute or on the floor, if they were subacute.

But all of these tests would be front loaded and done on the first day or two of patients stay.

So we can get to the pathogenic mechanism, we can identify the offending mechanism of stroke and tailor made our secondary prevention strategies and treatments.

When to Order Both Carotid and TCD

So particularly, remember when to order both carotid and TCD.

If you don't do it routinely, if you didn't see the distal end of the ICA lesion on duplex ultrasound, you didn't see the end, you may be missing the most significant portion of the stenosis.

So ICA lesions extending beyond direct intonation field, if you have tandem or bilateral lesions, if you want to identify high risk plaque or occlusions at high risk of stroke recurrence, remember that I mentioned only screening for intracranial disease and other stroke mechanisms.

This is becoming more and more important field.

And the last but not the least, you can use transcranial doppler for monitoring thrombolysis.

This is a rapidly developing area in the stroke treatment arena.

And I'm looking forward to update you on the progress on this in the future.

Thank you.