Why the Brain Is Difficult to Study

1. The brain is enclosed by the skull, so it cannot be seen or reached without surgery.
2. Brain tissue is extremely delicate, and any attempt to reach it risks permanent damage to healthy regions.
3. The brain contains billions of neurones with enormous numbers of connections, so its structure is very complex.
4. Regions do not work in isolation, so removing or damaging one region to see what it does is neither ethical nor reliable.
5. Studying patients with brain damage gives useful evidence but is slow and opportunistic, because researchers must wait for a suitable case.

Scanning Solves the Access Problem

1. A scan produces an image of the brain from outside the skull, so no surgery is needed.
2. The patient is conscious and unharmed, so the same person can be scanned repeatedly over time.
3. The two techniques named in the specification are the CT scan and the PET scan.
4. They answer different questions, because one shows what the brain looks like and the other shows what it is doing.

CT Scanning

1. A CT scan uses a beam of X-rays taken from many different angles around the head.
2. Dense tissue absorbs more X-rays than soft tissue, so the amount reaching the detector varies with what the beam passed through.
3. A computer combines the readings into a detailed cross-sectional image of the brain.
4. The image shows structure, so a tumour, a bleed, a blood clot or a region killed by a stroke can be located precisely.
5. Brain function is investigated by linking the damaged region to the ability the patient has lost, which shows what that region normally does.

PET Scanning

1. A PET scan requires the patient to be injected with a radioactive tracer, usually a form of glucose.
2. The tracer travels in the blood and collects in the most active regions, because active neurones respire faster and take up more glucose.
3. The tracer emits radiation as it decays, and detectors around the head record where it is coming from.
4. A computer builds a colour coded image in which the most active regions stand out clearly.
5. A PET scan therefore shows function in real time, and the patient can be scanned while reading, listening or moving so the active region can be matched to the task.
6. Comparing a patient's scan with a healthy scan can reveal a region of unusually low activity, which is used in diagnosing conditions such as Alzheimer's disease.

Comparing the Two Techniques

1. What they show. A CT scan shows structure, while a PET scan shows activity.
2. How they work. A CT scan uses X-rays passed through the head, while a PET scan detects radiation emitted by a tracer inside the body.
3. Detail. A CT scan gives sharper structural detail, while a PET scan gives poorer resolution but real time information.
4. Speed and cost. A CT scan is quicker and cheaper, while a PET scan needs a tracer made in a nearby facility because it decays rapidly.
5. The two are often used together, so the activity shown by the PET scan can be placed accurately on the structure shown by the CT scan.

Limitations and Risks of Scanning

1. Both techniques expose the patient to ionising radiation, which slightly increases the long term risk of cancer.
2. Repeated scans increase the total dose, so scans are only carried out when they are clinically justified.
3. The equipment is expensive and requires trained staff, so scanners are concentrated in large hospitals.
4. Patients must stay completely still, which is difficult for young children and for people in pain.
5. A scan shows a correlation between a region and a task, and does not by itself prove that the region causes the ability.