What is a CT scan? What should individuals pay attention to when undergoing the scan?

The waiting hall of the CT room is always crowded, especially in the morning, and patients are anxious waiting in line.

The technician in the CT room explained the examination precautions to the patient, repeating the same sentence over and over again until his throat was smoking, but he still had to say it.

Doctors in the CT room are always busy writing endless examination reports and seeing endless rare cases. Sometimes they are so busy that they don’t even have time to drink a glass of water in the whole morning.

Working in the CT room is busy and tiring, and disputes between doctors and patients occasionally occur.

CT examination takes a relatively short time and can solve many clinical problems. Overall, it is cost-effective and CT is widely used in hospitals of all sizes.

How much do the public know about CT scans? Here is a brief introduction:

What is CT?

CT is the abbreviation of X-ray computed tomography. It was born in 1967 and developed by British engineer Godfley N. Hounsfield. It began to be used in clinical practice in 1972.

The CT structure mainly includes three systems:

①Data acquisition system (DAS);

② Image reconstruction system;

③Image display and storage system.

Together with the central system controller, a complete CT is formed, and its structure is shown in the figure:

The image collected and reconstructed by the CT device is composed of a certain number of pixels with different gray levels from black to white arranged in a matrix. The gray level of the pixel is the reflection of the X-ray detected by the detector after attenuation through human tissues and organs.

X-ray attenuation? X-rays are essentially electromagnetic waves, and their intensity is attenuated due to distance and matter during the propagation process. Just like light (such as sunlight), it attenuates less when propagating in the air, and cannot penetrate objects with higher density (such as walls), causing refraction and other attenuation. Human lung tissue contains gas, which is easy for X-rays to penetrate, and the image appears black. Human bones contain calcium and have a higher density, which is difficult for X-rays to penetrate, and the image appears white. The density of human fluids and soft tissues is between the two, and they appear in different grayscales. CT images are cross-sectional tomographic images, with no overlap of front and back images, and are not affected by tissues outside the layer.

Are all CT images black and white? No! No! CT cross-sectional images can be reconstructed into different clinical images through special software, such as head and neck CT artery images.

Vascular image (VR image):

For example, brain CT perfusion image (color):

CT, why do so many people do it?

1. CT examination has a wide range of indications, including the brain, head and neck, chest, abdomen, pelvis, spine, limbs and joints, heart and other parts.

2. There are various CT examination methods, including plain scanning, special scanning techniques (thin layer scanning, target scanning, high-resolution scanning), enhanced scanning, CT vascular imaging, CT perfusion imaging, CT energy imaging, CTA-guided puncture biopsy and treatment, etc., and there are various image post-processing technologies (multi-plane reconstruction, curved surface reconstruction, volume reconstruction, simulated endoscopy, etc.).

3. The scanning speed of multi-slice spiral CT (most hospitals now use multi-slice spiral CT) has been significantly improved, the image resolution has been improved, the utilization rate of X-rays has been improved, the exposure has been significantly reduced, and the radiation to the human body has been correspondingly reduced.

How to do a whole-body CT scan?

Don't worry. Although CT scans are applicable to the whole body, the tissue structure of each part of the human body is different, the scanning method is different, and the relevant preparations before scanning are also different. Therefore, CT scans are divided into different sections (examination sites) according to different parts of the human body. Sometimes special examination methods are selected according to the disease condition and the requirements of the clinician. How to perform a CT scan depends on the clinician.

Preparation before CT examination:

1. For general inspection, remove any metal objects from the corresponding parts of the body, such as hairpins and earrings on the head.

2. For abdominal examination, bowel preparation is required, no oral barium has been taken within a week, no food or water is allowed within 4-6 hours before the examination, and 300-500ml of pure water or 1.5% iodine is taken orally before the examination. For chest and abdominal examination, breath holding training is required, pelvic CT scan, urine holding is required for 1 hour before the examination, and the degree is moderate.

3. Breath-holding training is required for heart examination.

4. For those who are unable to cooperate, sedation or anesthesia is required before the examination can begin. Critically ill patients need to be accompanied by clinical staff from relevant departments to monitor and handle changes in their condition in real time.

CT examination precautions:

1. CT examination is not recommended for special groups such as pregnant women, those preparing for pregnancy, and children.

2. During the examination, the body is required to remain still to ensure the accuracy of the examination results. It is just like taking a photo with a mobile phone in real life. If the photographed part moves, the image will be blurred.

3. When using iodine contrast agents to examine patients (such as enhanced examinations, vascular imaging, etc.), first determine whether they are allergic to contrast agents. Family members should accompany and sign. Patients with hyperthyroidism, allergies to iodine-containing drugs and reagents, and renal insufficiency are not recommended to undergo iodine-containing contrast agent examinations. After the examination, observe for half an hour and confirm that there are no discomfort symptoms before leaving the CT room.

4. Generally, family members are not required to accompany the patient in the examination room during CT examination. However, in the case of children, the elderly, or enhanced scanning, family members are still required to accompany the patient in the examination room to avoid accidents. The accompanying person in the room can wear the spare lead protective clothing in the machine room and stay away from the machine as much as possible.

Is the CT room waiting hall safe?

The waiting hall of the CT room is safe and has no radiation. The CT room is built in accordance with the "Basic Principles of Radiation Health Protection". The main protection (wall) of the room has a thickness of 2mm lead equivalent. Generally, a 24cm thick solid brick wall can reach 2mm lead equivalent as long as the mortar is full and there are no gaps. The door of the room is a special lead door, and the professional testing department regularly conducts protection effect testing.

Are there films for CT scans?

With traditional physical films, you need to wait for the film to be picked up after the examination and then show it to the doctor. Electronic film (cloud film) is a way to store and access traditional physical films by converting them into electronic films. After the examination, there is no need to wait for the film to be picked up. The doctor can view the image on the computer after the examination, saving waiting time for seeing a doctor.

Author: Nie Qing, Chief Technician, Department of Imaging, Funan County People's Hospital

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