SPECT Brain Imaging 

Updated: Mar 10, 2015
Author: Matthew Tam, MBBCh; Chief Editor: Gowthaman Gunabushanam, MD, FRCR 

Overview

Background

Brain perfusion single-photon emission computed tomography (SPECT) imaging is a functional nuclear imaging technique performed to evaluate regional cerebral perfusion.

Because cerebral blood flow is closely linked to neuronal activity, the activity distribution is presumed to reflect neuronal activity levels in different areas of the brain. A lipophilic, PH-neutral radiopharmaceutical (most commonly99m Tc-hexamethylpropyleneamine oxime [HMPAO] and99m Tc-ethylene cysteine diethylester [ECD], with a half-life of 6.02 hours) is injected into the patient, which crosses the blood-brain barrier and continues to emit gamma rays.[1, 2] A 3-dimensional representation of cerebral blood flow can be iterated using gamma detectors, allowing for interpretation.

Brain SPECT can be complemented with pharmaceutical agents that enhance regional cerebral blood flow, such as acetazolamide (carbonic anhydrase). Acetazolamide increases local pCO2 and causes arteriolar dilation, allowing for assessment of cerebrovascular reserve in transient ischemic attack, stroke, and vascular anomalies and distinguishing vascular from neuronal causes of dementia.[3]

Brain SPECT imaging has many different clinical applications.

Indications

Brain perfusion SPECT imaging can aid in the diagnosis and ongoing evaluation of many different medical conditions, as follows:

  • Detection and evaluation of cerebrovascular disease

  • Aid in the diagnosis and differential diagnoses of suspected dementia

  • Detection of seizure focus

  • Assessment of brain death

  • Evaluating suspected brain trauma

  • Neuropsychiatric disorders: Mood disorders, evaluating and subtyping attention-deficit disorder

  • Substance abuse

  • Infection/inflammation

Contraindications

Brain SPECT imaging is contraindicated in the following:

  • Pregnancy

  • Breastfeeding (this should be interrupted for 24 hours prior to imaging)

  • Lack of cooperation

Complication Prevention

Brain SPECT imaging is a safe procedure on the whole. However, care must be provided by the imaging technologist to reduce patient discomfort and minimize motion artifact. Care must also be provided to avoid tissue extravasation of radiopharmaceutical agents, as there is potential to induce tissue necrosis.

 

Periprocedural Care

Equipment

Brain SPECT imaging entails the following equipment:

  • Multiple detector SPECT cameras (the authors have a Philips BrightView XCT dual-head gamma camera equipped with low-energy, high-resolution [LEHR] collimators)

  • Brain imaging extension pallet for head positioning

Patient Preparation

For standard baseline/dementia studies, the patient should avoid caffeine, nicotine and alcohol. Sedation, if required, should be given 5 minutes post-injection of tracer. To minimize and standardize cortical activation, tracer is administered with the patient in a low-stimulation standardized environment.

For brain-death imaging, the imaging room should be prepared for a patient on life support. Any unnecessary equipment should be removed and wall oxygen extension readied. Drip stands and a table/trolley for cardiac monitoring and a ventilator should be available.

Positioning

Place the brain-imaging pallet on the bed, ensuring that the broad edge of the base does not extend past the end of the imaging table. To minimize head movement, position the patient with his or her head as far into the head holder as possible. Use the arm strap to secure the patient’s arms in place. Ensure all line access is outside the field of view. Consider a peripheral line for injection to avoid artifact from retained tracer in a central line.

For a brain-death study, the camera is positioned both anteriorly and posteriorly. The patient is positioned in the gantry so that the vertex, carotid arteries, and sternal notch are in the field of view.

Monitoring & Follow-up

Patients should be monitored for 20-30 minutes if acetazolamide is administered. Major reactions may include anaphylaxis and cerebral ischemic events. The most common reaction is a transient paresthesia of the face and digits.

 

Technique

Approach Considerations

Many protocols have been used for brain SPECT imaging, including split-dose, two-day repeat study, and dual-isotope techniques.[4]

The 2-day repeat study is usually preferred, and the challenge portion is performed first. If the initial results are normal, the baseline study can be omitted. The patient should be instructed to void at the beginning of image acquisition if acetazolamide is to be used.

Acquisition and Image Processing

Position the patient into the scanner and use the head pallet to minimize the patient’s head motion.

Settings are as follows:

  • Set camera to 20% energy window centered around 140 keV

  • Set 180° step-and-shoot acquisition, noncircular motion

  • Set 1.85 zoom, 128 x 128 matrices

  • Set 20 seconds per frame and acquired at 64 steps (total scan time is 21 minutes)

Chang attenuation or CT-determined attenuation correction is used to reconstruct the SPECT data. (The smooth filter is preferable unless the total counts are high.)

The 3-dimensional acquisition is usually reoriented along a fronto-occipital axis and displayed in the transaxial, sagittal, and coronal planes.

The reconstructed data are then transferred onto NEUROSTAT, and 3D-SSP is used to generate hyperperfusion and hypoperfusion z-score images, which are then used for interpretation. (See Pearls regarding NEUROSTAT and 3D-SSP.)

Standard SPECT Brain Imaging Study

This includes baseline and dementia studies.

A cannula is inserted, and the patient is rested in a quiet darkened room for 5 minutes prior to injection. The technologist injects the tracer, trying not to rouse the patient in order to minimize cerebral activation. The patient is allowed to rest for another 5 minutes. After 5 minutes, the tracer should be well distributed. Imaging is commenced 40 minutes postinjection.

The timeline is as follows:

  1. Rest the patient for 5 minutes in a dark room

  2. Inject the tracer

  3. Rest the patient for a 5 more minutes in a dark room

  4. Perform brain SPECT imaging 40 minutes postinjection

SPECT Brain Imaging Study With Acetazolamide

This technique is used to assess the cerebrovascular reserve.[3]

The acetazolamide study is usually performed first and the baseline study performed afterward on a separate day. Performing the acetazolamide study first may preclude the need for additional baseline imaging if the findings are normal.

Prior to injection, the patient’s baseline pulse and blood pressure are measured.

A cannula is inserted and acetazolamide administered. (One gram of acetazolamide is made up with 10 mL of sterile water; this is given over 2 minutes.)

The patient is monitored until the tracer is injected, which occurs 20-30 minutes after injection of acetazolamide. Imaging may commence between 40 minutes and 4 hours postinjection.

The timeline for acetazolamide studies is as follows:

  1. Administer acetazolamide and rest the patient for 20 minutes in a dark room

  2. Inject the tracer

  3. Rest the patient for a 5 more minutes in a dark room

  4. Perform brain SPECT imaging 40 minutes postinjection

Ictal Studies

Ictal

Tracer is injected during the seizure or in the immediate postictal phase in the monitoring suite in the neurology ward. The time of the injection should be noted following seizure onset and recorded on the patient's request form.

The timeline is as follows:

  1. Tracer is injected during the ictal phase

  2. A minimum of 30 minutes is allowed to elapse

  3. Brain SPECT imaging is begun

Interictal

The interictal study is performed at some point after the ictal study.

The patient is required to be seizure-free for 24 hours prior to the interictal tracer injection.

The timeline is as follows:

  1. Rest the patient for 5 minutes in a dark room

  2. Inject the tracer

  3. Rest the patient for 5 more minutes in a dark room

  4. Perform brain SPECT imaging 40 minutes postinjection

Brain Death Studies

The timeline of a brain death study is as follows:

  1. The imaging room is prepared for a patient on life support

  2. Dynamic flow images are captured for 2 minutes

  3. Statics are captured (anterior and posterior skull view and bilateral skull view)

  4. SPECT or SPECT/CT is performed

The scan is begun and tracer is immediately injected as an intravenous bolus injection with rapid flush in proximal vein or central line. Flow images (fast 1-second frame images) are acquired for 2 minutes. The flow images should start before the arrival of the bolus in the neck and end well after the venous phase. Three-minute anterior and posterior skull view and bilateral skull views are then acquired.

In potential organ donors, additional views of the anterior and posterior kidney, lung, and liver are acquired.

SPECT or SPECT/CT can be performed as an additional view to confirm perfusion status in the brain.

Interpretation of Results

The normal brain scan result

A normal result shows symmetrical perfusion to both cerebral hemispheres. The middle cerebral arterial distributions are well defined, and the anterior cerebral arterial distributions are seen as a single midline area of activity. Symmetry is seen in the arterial-capillary phase. A large amount of activity is seen in the face and base of the skull. The sagittal and transverse sinuses are also prominent.

History of THC abuse with 2-month history of psych History of THC abuse with 2-month history of psychosis and cognitive decline. There is a normal pattern of brain perfusion throughout the neocortical structures, corpus striatum, brain stem, and cerebellum.

Cerebrovascular diseases

Carotid stenosis: The regional cortical blood flow and clearance time are delayed compared with the opposite (normal) side.[5]

Cerebral infarction: The initial cerebral blood flow to an infarcted region is increased for 1-10 days following acute infarction. Delayed static images may return to normal by 2-3 months. There are wedge-shaped patterns of increased uptake in the infarcted vascular distribution.

Subdural hematomas: Mass effect from the hematoma can reduce peripheral activity on the dynamic flow images. Delayed images may show increased activity in the same distribution, distributed as a crescent sign.

Vascular abnormalities: There is focal area of intense blush and rapid washout on the dynamic flow phase.

Cerebrovascular reserve: Acetazolamide challenge is used to identify areas of the brain where cerebral blood-flow limitation is attributed to carotid artery disease. In areas of reduced perfusion, there has already been maximum vasodilation. Injection of acetazolamide does not further augment regional cerebral blood flow, and the affected areas are identified by relatively decreased tracer uptake on SPECT compared to the baseline study.

Three-dimensional SSP quantitative maps. Three-dimensional SSP quantitative maps.
Three-dimensional SSP quantitative maps Three-dimensional SSP quantitative maps
Patient with right hemiparesis with left carotid s Patient with right hemiparesis with left carotid stenosis and mid-basilar artery occlusion. Acetazolamide stress perfusion demonstrates mild reduction in perfusion to bilateral frontotemporal and basal ganglia regions when compared to baseline. Mild reduction in perfusion to the superior aspect of the cerebellum bilaterally

Dementia

Alzheimer disease is characterized by bilateral posterior temporal and parietal hypoperfusion; frontal hypoperfusion is also seen in severe disease. Bilateral posterior parietal-temporal perfusion is seen in more than 80% cases but is also a feature of other diseases (Parkinson disease and associated dementia). The occipital lobe, sensory motor cortex, and cerebellum are spared.[5] See images below.

Reduced activity throughout the neocortices with s Reduced activity throughout the neocortices with sparing of the occipital lobes and paracentral gyri
Three-dimensional SSP quantitative maps confirm wi Three-dimensional SSP quantitative maps confirm widespread reduction in cortical activity, most marked in the temporal lobes

Frontotemporal dementia is characterized by hypoperfusion in both frontal and temporal lobes. See images below.

Three-dimensional SSP quantitative maps Three-dimensional SSP quantitative maps
Patient presents with cognitive impairment with pr Patient presents with cognitive impairment with preservation of short-term memory. Tomographic images shows reduced tracer uptake in the frontal lobes, more prominent on the right and in the temporal lobes, more marked on the left. Tracer uptake in the parietal and occipital lobes and deep gray matter preserved. Three-dimensional SSP maps confirm this impression.
Three-dimensional SSP quantitative maps. Three-dimensional SSP quantitative maps.
Patient was investigated for memory and visual spa Patient was investigated for memory and visual spatial impairment. There are bilateral frontal lobe changes being marked on the left with lesser left temporal involvement, suggestive of frontotemporal neurodegenerative process.

Lewy-body dementia shows a distribution of hypoperfusion that is similar to that in Alzheimer disease. Occipital involvement allows differentiation. See the images below.

Three-dimensional SSP quantitative maps Three-dimensional SSP quantitative maps
History of generalized slurring, tremor, confusion History of generalized slurring, tremor, confusions and delusions. Tomographic images confirm bilateral, asymmetrical hypoperfusion involving the parietal lobes, temporal lobes, and occipital lobes. Pattern supportive of Lewy body disease

Primary progressive aphasia is characterized by asymmetric hypoperfusion in the left temporal-parietal lobe. See images below.

Three-dimensional SSP quantitative maps. Three-dimensional SSP quantitative maps.
Asymmetrical uptake in the cerebral hemispheres wi Asymmetrical uptake in the cerebral hemispheres with markedly reduced tracer uptake in the left temporal and parietal lobes. Uptake in the left sensorimotor cortex appears normal.

Pick disease is characterized by frontal lobe hypoperfusion.

Multi-infarct dementia is characterized by multiple asymmetrical perfusion defects, often involving the primary cortex and deep structures.

Brain death

The internal carotid circulation to the level of the base of the skull is visualized, but cerebral circulation and brain parenchyma are lacking. Delayed planar or SPECT/CT images should demonstrate no tracer uptake in the brain. A difference of 10% in cerebral activity between ipsilateral and contralateral regions is significant. See image below.

Delayed planar saved images demonstrate absent tra Delayed planar saved images demonstrate absent tracer uptake in the cerebral hemispheres, cerebellum, basal ganglia, and brain stem.

Seizure disorders

SPECT brain imaging assists in the localization of a seizure focus, yielding a sensitivity of up to 75% in localization of interictal seizure foci and 90% ictally. For ictal identification, the radiopharmaceutical is on hand to be injected at the time of the seizure, and imaging is performed shortly thereafter.[5] See images below.

Three-dimensional SSP quantitative maps Three-dimensional SSP quantitative maps
Three-dimensional SSP quantitative maps Three-dimensional SSP quantitative maps
Left temporal ictal hyperperfusion with subtractio Left temporal ictal hyperperfusion with subtraction images showing difference between the ictal and interictal scans. There is likely left anterior temporal seizure focus. Three-dimensional SSP maps confirm this impression.

Infection

Intracerebral abscess is characterized by a is focal increased radionuclide accumulation on delayed images. In some cases, radionuclide uptake occurs with a cold center (known as a doughnut pattern)[5] .

Herpes encephalitis shows increased flow and uptake in the affected temporal lobe.

Tracer Information

Table 1. Tracer Information [6] (Open Table in a new window)

 

Tc99m-HMPAO (Ceretec®)

Tc99m-ECD (Neurolite®)

Transport mechanism

Neutral lipophilic compound

Neutral lipophilic compound

First First-pass cerebral extraction

80%

60%-70%

Peak activity

1-2 minutes postinjection

4%-7% of the dose remains within the brain

1-2 minutes postinjection

6%-7% of the dose remains within the brain

Advantage

Less radiation dose

Rapid brain uptake with excellent brain retention

Higher brain-to-background ratio

Higher gray-to-white matter ratio

Disadvantage

Poor stability

Dose needs to be used within 30 minutes postreconstitution

Higher radiation dose

Currently not available in Australia

Trapping mechanism

Trapped in all living cells; glutathione-mediate conversion to hydrophilic complex

Trapped only in metabolically intact cells and not retained during transient dysfunction; enzymatic (esterases) conversion to hydrophilic products

Radiation Dosimetry

Table 2. Radiation Dosimetry in Adults (Open Table in a new window)

Radiopharmaceutical

Administered Activity, MBq (mCi)

Organ Receiving the Largest Radiation Dose, mGy (rad)

Effective Dose, mSv (rem)

Tc-99m HMPAO

555-1110 IV

(15-30)

0.034

Kidneys

(0.126)

0.0093

(0.034)

Tc-99m ECD

555-1110 IV

(15-30)

0.073

Bladder wall

(0.27)

0.011

(0.041)

Table 3. Radiation Dosimetry in Children (Open Table in a new window)

Radiopharmaceutical

Administered Activity,

MBq/kg

(mCi/kg)

Organ Receiving the Largest Radiation Dose, mGy (rad)

Effective Dose, mSv (rem)

Tc-99m HMPAO

7.4–11.1 IV

(0.2-0.3)

0.14

Thyroid

(0.52)

0.026

(0.096)

Tc-99m ECD

7.4-11.1 IV

(0.2-0.3)

0.083

Bladder wall

(0.31)

0.023

(0.085)

Pearls

Many patients have nonspecific perfusion patterns that cannot be directly attributed to a specific disorder or causative agent.

Scan findings should be correlated with the clinical history, recent structural imaging (MRI/CT scanning of the brain), and electroencephalography results in cases of refractory epilepsy.

Color and grayscale formatted images are both examined. The transaxial and coronal planes of a section are best used for interpretation, as cerebral blood flow is normally symmetrical in healthy persons.

Sedative medications can affect tracer distribution and should be administered 5 or more minutes following tracer injection.

The NEUROSTAT software package is used to attenuate corrected transverse slices, perform nonlinear stereotactic alignment, and coregistration with an age- and sex-matched database. Activity normalization is usually global mean cortical perfusion but can be normalized to various brain areas, including the cerebellum and pons, among others. 3D-SSP is a display format in NEUROSTAT that displays maximum brain perfusion (in the cerebral cortex to a depth of 7 mm based on a perpendicular plane from the cerebral surface) as a standard deviation Z-score difference compared to the age- and sex-matched database. This is used for image interpretation, coregistration for ictal-interictal SPECT, and MRI-PET/SPECT.

The adequacy of a brain-death study requires rapid and easy injection of tracer through a patent intravenous line. In addition, cardiogenic shock precludes interpretation; the study should not be performed when the systolic blood pressure is less than 60 mm Hg.

It is recommended to use HMPAO for a brain-death study. Delayed images with HMPAO appear more sensitive for verifying the absence of posterior fossa perfusion.