Visualizing the central nervous system

Before jumping into viewing images of the nervous system, the reader needs to be very familiar with the position and type of slices presented in those images. Figure 37-2 illustrates the three types of slices common to the nervous system. The horizontal slices shown in Figure 37-3 are cuts made horizontally through the brain beginning with a superior slice at the level of the beginning of the lateral ventricles. This individual would be lying supine if the brain were cut as shown. If a person were standing, the horizontal slice would be horizontal to the ground with the frontal lobes facing forward (toward the nose) and the occipital lobe in the back. The slices cut horizontally are always perpendicular to the brain stem and spinal cord or perpendicular to the upright position of the brain regardless of the position of the head in space. All right and left slices will look exactly the same as long as the nervous system has not sustained any insult. Figure 37-4 shows the position of an upright human and what the skeleton would look like in the same position. Can you visualize what the horizontal slices would look like? The horizontal section would be perpendicular to this upright position. When viewing radiological slices in horizontal, the slices will often be on a slight angle with the lower portion slightly forward. Most of these films are taken when individuals are supine, which places the brain off vertical so a correction is made. The film usually slices horizontally through the brain as if the head were slightly tucked, similar to the image of the adult in Figure 37-4, A. Also, some radiologists want a slightly different angle to the slice because they are looking for specific orientations. When viewing these films, try to look at the radiological slice pattern (scanogram), if shown. Figure 37-3 shows a progression of horizontal slices as if the individual were supine and the slices began anteriorly with emphasis on ventricular changes as part of the progression. Many of the medical images of the brain are viewed as horizontal (or axial) slices or views. The person is lying supine when undergoing computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET). Therefore the imaging process cuts 90 degrees off the horizontal plane of earth in order to achieve horizontal slices of the brain. Remember to look at the scanogram, if available, to identify the position of the head in relation to the cuts.

Coronal slices in the right hemisphere look similar to those in the left hemisphere except that the top of the slice will not look like the bottom on either side (see Figure 37-2). The two sides will reflect each other, with the tops being alike as well as the bottoms. Depending on where the cut is made, the result will be cortex on the outside, tracts (white matter) projecting downward, and gray matter again inferior and medial within the slices as the thalamus, basal ganglia, caudate, hippocampus, and so on are viewed. These slices begin at the top and cut down through both sides of the brain, with the slice ending on the inferior section. The progression of the slides can go from front (frontal lobe) to back (occipital lobe) or back to front. If visualized in a standing subject, the slice would begin in the superior frontal area and slice downward through the brain toward the feet, thus cutting equally on both sides. These slices can proceed in the posterior (backward) direction of the brain but always cutting from the top toward the bottom with equal distribution on each side. Figure 37-5 progresses from the front of the brain toward the back using the ventricles as a point of reference.

For sagittal cuts (see Figure 37-2), most cuts start by slicing down through the central fissure separating both sides of the brain. This cut is called a midsagittal section. Each sagittal slice proceeds outward toward the lateral aspect of the brain on each side respectively, depending on which side is being sliced. Anatomically, you begin as if you were slicing down through the middle of the face and the back of the head when a person is standing. Each sagittal slice moves from the inside outward toward the ear or laterally away from the midsagittal slice. Each slice cuts though the front and back of the brain from top to bottom and proceeds from medial to lateral. Sagittal images usually begin on either the right or the left side and continue slicing toward the middle to the midsagittal section separating the two sides of the brain, and then proceed toward the outside on the opposite side of the brain from the beginning slice.

Another way to conceptualize the nervous system is by visually sequencing from a view of a human in a specific position to the skeleton of a human in that position to an intact plastinated nervous system in that same position. The reader is encouraged to try to visualize what the horizontal, coronal, and sagittal slices might look like given the spatial position of the individual. Once you can easily recognize the various types of slices and where the slice was made when viewing the nervous system, you are ready to begin viewing radiological images.

Imaging of the central nervous system

It is beyond the scope of this chapter to discuss the physics behind the more common imaging modalities (see article by Kimberley and Lewis² for background). The purpose of this chapter is to provide the clinician a method of systematically evaluating medical images of the CNS, and, more important, to provide examples of how clinicians can use these images to guide practice.

CT and MRI remain the two most common modalities for imaging the CNS. CT scans of the brain are widely used in acute neurological injuries in which the speed of the examination is of primary importance. A common application of this is in acute traumatic brain injuries, when rapid assessment of information about hematoma formation and brain swelling is imperative to make appropriate decisions regarding medical management. Although there is less anatomic detail in CT than in MRI, the level of detail being generated by a CT is generally sufficient for the appropriate management of the patient with an acute injury. Because CT also administers the highest dose of radiation (as it is a series of x-ray exposures), it causes a higher risk of development of conditions associated with increased levels of radiation exposure.

Owing to its increasing availability, MRI can also be an ideal choice for imaging the brain and the spinal cord. MRI provides excellent resolution and can also be performed with contrast to enhance the detail even more. MRI, however, generally takes longer than CT to perform, and it requires the subject to avoid excessive movement while the machine is actively scanning. Also, because MRI uses powerful magnets, the use of this modality in patients with metal implants is contraindicated. With regard to cost, MRI is still more costly than CT, but this expense is rapidly coming down as more hospitals and clinics are investing in this technology.

As mentioned earlier, contrast media can also be used in both CT and MRI to enhance the image, although that will increase the scanning time. There are also additional risks associated with contrast, such as possible allergic reactions to the medium being used to enhance the image.

Because CT scans, like conventional radiographs, are images formed by the absorption of x-rays by the body at different densities, evaluation of CT images is the same as evaluation of conventional radiographs. Structures in the body that absorb a lot of x-ray energy are radiopaque and will be white on the image. Structures that absorb less x-ray energy will be different shades of gray. Air does not absorb any x-rays and will be black on the film, and is said to be radiolucent. Bone is an example of a radiopaque structure and will be white on the image. Because the skull is composed of bones of varying thickness, portions of this structure that have more bone will be whiter than those with less bone. Contrast media can be positive (white; heavy metals such as gadolinium) or negative (black; air, however, is not used as a contrast medium in the CNS). Brain matter and spinal cord will be varying shades of gray. The sinuses are normally filled with air and will therefore show as black, whereas the ventricles are filled with cerebrospinal fluid and will be a shade of gray on a CT image.

There are a few steps to follow when evaluating neuroradiological images. These steps are presented in the following sections.

Step 2

Be familiar with background information, and orient yourself correctly to the image

After reviewing the imaging report, the clinician then examines the actual images. The clinician must be familiar with the basic background information presented on the image. Normally the film will contain information such as the patient’s name, age, and medical record number; the date of the scan; and the name of the hospital or facility that performed the procedure (Figure 37-6). The date of the scan is particularly important when attempting to establish relationships between what can be seen on the image and the patient’s presentation. There might be a mismatch between what is shown on the image and what the patient is doing because of either resolution of the condition or worsening of the pathology. Comparing scans taken at different intervals will also assist the clinician in arriving at some general impressions on the rate or recovery (or lack thereof) or prognoses following physical rehabilitation.

Markers may also be available. The use of markers significantly simplifies analysis by allowing the examiner to easily orient the film correctly. Common markers include R and L for right and left, respectively, and A and P for anterior and posterior.

Often a scale is provided for the clinician to relate the size of the structures on the image to actual size.

Next, the clinician should be familiar with some of the basic technical detail. CT and MRI can provide multiple “slices” of the brain, much like slices in a loaf of bread. Often a single large film containing all the images arranged in order can be found. Usually, the first image on the left top most corner is the “scout view” or scanogram and serves to illustrate the orientation and thickness of each slide. Depending on the suspected pathology, the slice orientation could be frontal, coronal, or sagittal, based on the specific structure in question.

Certain views of the CT image can also be produced by the radiology team by manipulating certain image parameters. These “windows” are presented in Figure 37-7. A bone window shows bone the best and is helpful when a skull fracture is suspected. A brain window shows brain matter the best and is used when the suspected pathology is caused by or affects the brain matter (tumors, atrophy of the nuclear masses, or general atrophy of the brain itself). A subdural window is beneficial when suspecting the presence of subdural hematoma or swelling of the brain. With MRI, the images can also be “weighted” (for example, T1 versus T2). T1-weighted images show better gray matter–white matter contrast, whereas T2-weighted images might show edema better. As mentioned earlier, contrast may be used with either CT or MRI to improve visualization of the structures in question.

Step 3

Do a quick scan-through, then examine thoroughly

After getting familiar with the basic information, the clinician should then make a “quick scan” of the image, noting the pathology that quickly stands out. After doing so, a more thorough analysis of each film in the series must be done. Multiple windows and/or successive slices are often helpful in visualizing the extent of the pathology. The clinician mentally constructs a three-dimensional image of the brain from the two-dimensional scan. In a CT scan, knowledge of radiodensity is helpful in making this accurate representation, as is knowledge of normal structure and function of the CNS.

Several resources recommend slightly different methods of analyzing an image. The following are suggestions to make the process more organized and efficient.

1. Examine the symmetry.⁵ Compare both sides of the brain as represented on the image. In orienting the images, the right side of the brain is usually shown on the left. Look for a midline shift by drawing an imaginary line from the anterior falx cerebri to the posterior falx cerebri. Identify the side of the greatest shift and measure it (in centimeters or millimeters). It would also be helpful to look for signs of mass effect (when structures on one side are “pushed” to the other side, or a possible atrophy on the structures on one side that pushes the structures on the other side across the midline). Additional structures to inspect for symmetry include the basal ganglia, thalamus, and corpus callosum.

2. Examine the size and shape of the structures.⁵ This is particularly helpful when looking at the integrity of the ventricles and cisterns. When filled with excessive fluid as in hydrocephalus, the ventricles enlarge disproportionately. This enlargement is seen in babies as a result of a variety of pathologies (see Chapter 15) and in children and adults after brain trauma (see Chapters 23 and 24). Likewise, in atrophy of the brain tissue the ventricles may also become enlarged; this is often seen in elderly adults as the brain loses gray matter and the ventricles enlarge to fill in the extra space (see Chapter 27). Examining the size and shape of the sulci may also be beneficial when looking at atrophy of the specific structure, thereby accounting for the abnormalities of movement being observed.

3. Look for lesions in the brain. Space-occupying lesions such as brain tumors or hematoma can be visualized (Figure 37-8). Because the brain has such a finite size to fit snugly within the cranial vault, any lesion in the brain has the potential to push brain structures to the midline or to displace the structure and create more serious pathology. Plaque formations secondary to multiple sclerosis can also be visualized as bright white spots in the gray matter (Figure 37-9).

4. Examine densities.⁵ As mentioned previously, different tissues will have different densities, and these differences are what forms the generated images. Hyperdensities may include blood, tumors, or enhancing lesions, whereas common hypodensities include air, non-enhancing tumors, and chronic hematoma.

Step 4

Establish relationships between the structures involved and the presenting impairments, movement dysfunctions, and activity limitations

There are two possible clinical scenarios in which the medical imaging information is reviewed. First, the therapist may be reviewing the medical imaging information before seeing the patient for the initial examination or evaluation. If this is the case, the therapist is expected to use this information in relation to other medical information in the chart (from physician’s history and physical examination, nursing notes, and so on) to get an initial sense of what the patient may be able or unable to do. This will allow the therapist to plan for the appropriate tests and measures to perform during the examination (Case Study 37-1).

CASE STUDY 37-1    A PATIENT WITH A BRAIN TUMOR*

The patient was a 71-year-old woman with history of glioblastoma multiforme, the most aggressive form of brain tumor (refer to Chapter 25). It has been reported that a second recurrence of this condition is associated with a less favorable prognosis.⁶ The patient had previously undergone tumor resection, chemotherapy, and radiation. She had then been found to have another recurrence of the left frontal glioblastoma multiforme of 4 cm (Figure 37-10), for which she had undergone a stereotactic resection (Figure 37-11) of the tumor. After this surgery she underwent intensive inpatient rehabilitation.

Her past medical history included history of radiation therapy, seizure disorder, diabetes mellitus, hypertension, anemia, and leukopenia. Medications include Dilantin, Lipitor, dexamethasone, fosinopril, and metformin.

The patient’s primary caregiver was her husband. They were both retired, living in a single-story home with two steps to enter. She had two supportive daughters who lived nearby and were available for support as needed. Before the most recent hospitalization, the patient had been at a supervised level of assistance with ambulation outdoors and independence for household ambulation, with no assistive device required.

The medical images revealed a resected parietal lobe tumor in the left side of her brain. In the imaging report it was noted that there was significant edema in the surrounding area and that mainly white matter tracts were affected. However, no midline shift or mass effect was noted. In addition, the tumor was reported to be located near the Broca and Wernicke areas but superficial to the ventricles.

Based on the information provided by the imaging reports and the associated medical images, the clinician developed several initial impressions about the patient’s presentation and care. First, the note about significant edema indicated more diffuse, global effects on movement and function; associated structural impairments rather than only dysfunctions specific to parietal lobe damage would be expected. The lack of midline shift or mass effect indicated a more favorable prognosis for survival for the patient. The information about the tumor being located near the speech areas indicated the possibility of deficits in expressive and receptive communication. Finally, damage to the parietal lobe indicated the potential for agraphia, aphasia, and agnosia. Because the parietal lobe is also largely important for perception and interpretation of somatosensory information, the formation of the idea of a complex purposeful motor act may also have been impaired.

The initial examination confirmed and supported all the expected movement deficits and clinical presentation of the patient. She underwent intensive occupational, physical, and speech therapy to improve her functional mobility and ambulation, self-care and activities of daily living, and also speech, swallowing, and communication. There was a concerted effort among the rehabilitation team and the patient’s family to optimize communication strategies while minimizing patient frustration. For example, more complex tasks such as transfers were broken down into smaller components and then practiced extensively as components and as whole functions. The transfer task was broken down into three steps: (1) locking the brakes, (2) removing the legrests, and (3) standing and turning to sit on the destination surface. These step-by-step instructions were written on the patient’s whiteboard; practiced during speech, physical, and occupational therapy; and communicated to the family and rehab team. Hand-over-hand guidance and facilitation were also included during instruction, as was use of mental rehearsal to remediate executive and visuomotor deficits to improve motor sequencing and problem solving while decreasing perseveration and frustration.⁷ The rehabilitation team worked closely together to standardize treatment techniques, increase opportunity for task carryover, and decrease the patient’s frustration with learning. Speech therapy started to integrate pictures and words representing tasks learned in physical and occupational therapies. In addition, therapy became more focused on repeated task training of three or four essential skills versus multiple activities, games, and skills.

This case example demonstrated how medical imaging information not only confirmed the expected presenting deficits in movement, function, communication, and learning of the patient but also included information that guided the rehabilitation team in selecting interventions that optimized function for this patient.

*Case adapted from Parikh M: The use of medical imaging in neuromuscular physical therapy practice: a case report. Samuel Merritt College Physical Therapy Case Report Presentations, May 2007.

The second clinical scenario involves the use of medical imaging information to guide clinical decisions during the actual occupational or physical therapy session. One of the major goals of this step is to make sure that the movement dysfunctions presented by the patient match the information obtained from the medical images. The therapist is expected to act accordingly and to demonstrate sound judgment, especially when the mismatch indicates a possible life-threatening situation. For example, if the imaging results indicate a small focal area involvement but the patient demonstrates significant impairment in movement and function, the mismatch may be indicative of a worsening and potentially life-threatening condition that must be communicated to the physician and other members of the medical team.

As mentioned earlier, the complexity of the structure and function of the nervous system makes interpretation of imaging information very challenging. There may be situations in which images may not fully explain what the patient or client is able to do in terms of function and movement (as illustrated in Case Study 37-2).

CASE STUDY 37-2    A PEDIATRIC PATIENT WITH DEVELOPMENTAL PROBLEMS AT 18 MONTHS

The patient was a full-term baby. The doctors identified through ultrasound that the child had an unusually large head and were expecting problems. At birth, they had placed the child in the neonatal intensive care unit (NICU) and obtained CT scans of his head. It had been identified that he did not have closure of his lateral and third ventricles. Their prognosis had been that the child would die within days after birth. They had encouraged the family to take him home and spend as much time with him as they had before his death.

One author of this chapter was called in to help colleagues establish realistic expectations and treatment protocols for this child. The child was 18 months of age at the time of this therapist’s first visit. The child’s motor development was very delayed owing to the large size of his head (hydrocephaly). He was also nonverbal, and the doctors had concluded that he had an extremely low level of intelligence. He had loving parents who played with him, fed him, bathed him, and interacted with him all day. They felt he had more ability than the pediatrician had stated.

This video case study illustrates a situation in which there is a mismatch between the medical diagnosis/medical imaging results and what the client presents in terms of function and movement. This case will engage and guide the therapist in a clinical decision-making process of analyzing the client’s movement and functional capabilities and relating those to the integrity of the specific areas of the nervous system that are responsible for the movement or behavior. It is recommended that the reader first look at and analyze the motor function of this child at 18 months and then 6 months later, looking specifically for increased motor function and potential. During the interim, the child did receive weekly therapy, but most of the practice was done at home with the parents interacting with the child. Analyze the movement from a motor learning and neuroplasticity perspective as you determine the potential for motor control. Then look at the MRI images that lead the doctors to diagnosis the medical problem and the prognosis of death. Then ask yourself what could be happening to cause such a mismatch between a movement and a medical diagnosis?

Note: The continuation of the case, including the video clips and imaging results, can be found on the companion website.