title: 【Reprint】You Can Learn More About These "Imaging" Examinations Done by Hospitals
date: 2021-07-06 11:13:06
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- Skills
tags: - Reprint
- Hospital
- Imaging
- Examination
- Understanding
This article is reprinted from: You Can Learn More About These "Imaging" Examinations Done by Hospitals - Minority
With the advancement of technology, medicine is continuously developing, and the auxiliary examination methods during diagnosis and treatment are becoming increasingly advanced. The introduction of new auxiliary diagnostic medical devices and the continuous updating of various advanced imaging examination techniques have greatly improved the positive diagnosis rate and brought convenience to medical workers: after all, before the invention of the stethoscope, doctors could only listen to the patient's chest wall closely, and based on the principle of disinfecting before and after contact with patients, if there were still no stethoscopes now, doctors would not only have to wash their hands but also their faces.
However, the progress of examination methods has brought more confusion to patients. When people visit hospitals, they often rush between various examination rooms and payment windows, feeling dizzy while inevitably muttering: "How do they know to order tests for me? Is there radiation from the imaging? Is the examination so expensive because there are kickbacks?" Based on the above background, I will briefly introduce the roles and advantages and disadvantages of commonly used imaging examinations, allowing readers to have a basic understanding of the examinations they undergo.
It is important to note that this article aims to educate readers about various imaging examinations and does not recommend readers to "self-select" examinations based on this article. This article does not constitute any medical advice; for specific issues, please consult a doctor in person and follow medical advice.
Medical imaging includes imaging diagnostics and interventional radiology. Considering professionalism and practicality, this article will only briefly explain the commonly used imaging examinations in clinical imaging diagnostics. Additionally, due to the physiological and pathological particularities of populations such as children and pregnant women, this article does not cover these special groups. Of course, I am not a physics expert, and my descriptions of specific principles may not be detailed; if there are inaccuracies, professionals in the relevant fields are welcome to provide corrections.
Ultrasound Imaging: More Than Just B-Mode#
Ultrasound imaging refers to the technique of using the physical properties of ultrasound waves and the acoustic characteristics of human tissues for imaging, which is used for auxiliary diagnosis. ^1^ In simple terms, ultrasound instruments emit ultrasound waves of different frequencies through a transducer (commonly referred to as a probe), receive the sound waves that return after being reflected, scattered, refracted, and diffracted by human tissues, and then process the signals to form images.
Ultrasound imaging is divided into A-mode ultrasound, B-mode ultrasound (two-dimensional ultrasound), M-mode ultrasound, D-mode ultrasound (Doppler ultrasound), etc., based on different imaging techniques and display methods. The specific imaging principles are quite technical, so I won't go into detail; instead, I'll help you understand these types of ultrasound in a simple and straightforward way:
A-mode ultrasound imaging is one-dimensional waveform and is currently used less in clinical practice. M-mode ultrasound imaging is also a one-dimensional waveform that only looks at the state along a single sampling line, mainly used for heart examinations, allowing for the inspection of heart structure, observation of motion trajectories, and measurement of dimensions, wall thickness, and heart function. In other words, you generally won't need it unless you're looking at the heart.
The most commonly used B-mode ultrasound imaging (B ultrasound) is two-dimensional cross-sectional imaging and is widely used in clinical practice. It is also the basis for M-mode and D-mode ultrasound imaging. You can simply understand that wherever the doctor's probe pokes, a cross-section is created for viewing. Since the human body is three-dimensional, doctors need to select different cross-sections for examination—this is why ultrasound doctors are always poking around on you and asking you to turn over from time to time; it means they are using the probe to create several cross-sections for examination. You might imagine yourself as a pot of food, with the doctor's probe as the spatula; wherever the spatula goes, that's where they can see. If there's a potential issue (like the food not being cooked), they may need to poke around a bit more.
D-mode ultrasound imaging, or Doppler ultrasound imaging, includes spectral Doppler, tissue Doppler imaging (TFI), color Doppler energy imaging (CDE), and Doppler blood flow imaging (CDFI), among others. As the name suggests, Doppler ultrasound imaging is based on the Doppler effect. Here’s a quick refresher on high school physics: the Doppler effect refers to the change in frequency of sound waves when the source of the sound is moving relative to an observer. When an object approaches, the sound waves become "denser," and when it moves away, they become "sparser." Based on this principle, D-mode ultrasound mainly examines interfaces that can move relative to the sound source (the probe), such as moving tissues and blood flow. Therefore, although D-mode ultrasound sounds advanced, it is only valuable where there is "movement"; it is not applicable to everything.
Currently, the two common ultrasound instruments in clinical practice are B-mode ultrasound machines and color Doppler ultrasound machines. It is important to note that B-mode ultrasound machines do not only have B-mode imaging capabilities; they typically also include M-mode and spectral Doppler imaging functions. Due to the widespread use of B-mode ultrasound machines, the term "B ultrasound" has become well-known among the public and is often used to refer to all ultrasound examinations—this is actually inaccurate. Of course, as a patient, this minor naming flaw is not a big deal. After all, doctors have long been trained to respond with a smile to any title, whether it be doctor, nurse, caregiver, girl, daughter, or even waiter, and to calmly respond and attempt to understand various descriptions of symptoms. As long as you can express your meaning clearly, any term is acceptable.
Similarly, color Doppler ultrasound machines can perform Doppler ultrasound imaging and also have B-mode and M-mode imaging functions; advanced models can also conduct static and dynamic three-dimensional imaging, ultrasound contrast, and acoustic quantification examinations.
Ultrasound waves are mechanical waves and do not cause radiation damage, which is what people commonly refer to as not "eating the line." Because ultrasound examinations are non-invasive and dynamic, they can be applied to a wide range of tissues and locations, are relatively easy to operate (can be performed at the bedside or during surgery), and are relatively inexpensive (around a hundred yuan for a single site), they are widely used in clinical practice, especially in thyroid, breast, lymph nodes, and abdominal solid organs (such as liver, gallbladder, spleen, and pancreas) as well as in gynecological and obstetric imaging diagnostics. However, due to the complete reflection of ultrasound waves by gas in bones, lungs, and the gastrointestinal tract—resulting in a black area on the sonogram—ultrasound examinations have limited diagnostic value for these tissues and organs. Additionally, as mentioned earlier, ultrasound is artificially selected cross-sections for imaging, so although there are fixed standards for selecting cross-sections in ultrasound examinations, due to the complexity of human tissues and the diversity of lesions, it may not be possible to capture the "problematic" cross-section during the examination, and it heavily relies on the experience and skill level of the doctor.
X-ray Imaging Technology#
X-rays are a type of radioactive special light that has penetrating and absorbent properties. The technology that uses X-rays for imaging is called X-ray imaging technology, which mainly includes X-ray imaging and computed tomography (CT). In everyday life, "X-ray" usually specifically refers to "X-ray imaging," while CT is used to refer to "computed tomography." Of course, after saying this, you might be confused; that's okay, let me put a relationship diagram here, and then I will explain it in the most ~brutal~ simple way.
X-ray Imaging—Traditional X-ray, CR, and DR#
First, let's understand the principle of X-ray imaging. X-rays are a type of electromagnetic wave with an extremely short wavelength, possessing penetrating, absorbent, fluorescent, and photosensitive effects. The basic principle of X-ray imaging (in layman's terms) is that when X-rays irradiate the human body, different tissues absorb X-rays to varying degrees due to differences in density and thickness, resulting in an image that presents different shades of black and white (commonly referred to as different densities). X-ray imaging mainly includes traditional X-rays, computed radiography (CR), and digital radiography (DR). The differences among the three mainly lie in the imaging technology used; traditional X-rays use film for imaging, while CR and DR digitize and pixelate the images. Among them, DR has a shorter imaging time, can perform fluoroscopy, and can conduct advanced functions such as subsequent image processing, making it widely used in clinical practice. The choice and application of these technologies are influenced by the patient's condition and the actual situation of the hospital and insurance, so just know that "fluoroscopy, X-ray, plain film, CR, DR" all refer to X-ray examinations.
Readers generally have some understanding of X-rays in everyday life; they may know that white areas represent bones, while gray and black areas represent other tissues.
But this looks quite powerful. Different tissues and areas present different colors, which is directly related to tissue density. The higher the tissue density, the more X-rays are absorbed, resulting in a brighter image. Therefore, in X-ray imaging, the white areas are referred to as high density, while gray and black areas are referred to as low density. If a lesion causes a change in tissue density, after reaching a certain extent, changes in black, white, and gray levels will appear in the imaging.
X-ray imaging is a superimposed image, meaning that it is a combination of all layered images from front to back of the irradiated area, similar to a "fluoroscopy" perspective. Therefore, X-ray imaging is also referred to as fluoroscopy; for example, the "chest fluoroscopy" project in school health checks and civil service or public institution employment health checks refers to chest X-ray imaging from the back to the front, but it does not present the image on film, imaging plates, or flat detectors; instead, it is directly displayed on a fluorescent screen for real-time observation. The image below is a chest X-ray from the back to the front, showing that all the tissue structures in the chest are superimposed in the image.
Using the natural thickness and density of tissues for imaging is called natural imaging. When natural imaging is "unclear," contrast agents (also known as contrast media) can be used to artificially enhance contrast, such as the oral barium meal often seen in high school chemistry problems for gastrointestinal X-ray photography, and the rapidly developing interventional procedures currently used in clinical practice for vascular imaging.
X-ray photography is convenient, cost-effective, with a single plain film costing around a hundred yuan, and the imaging display range is relatively large (as shown above, a single film can show the entire chest). The downside is that as a superimposed image, the overlapping of multiple layers of tissue may affect the judgment of lesions, making it difficult to detect subtle lesions. Additionally, due to the characteristics of tissues and the principles of X-ray imaging, the application range of X-rays is limited: for example, clinical X-ray plain films are rarely used in the abdomen, generally only for initial screening of acute abdominal conditions and stones; with the development of endoscopic technology, the use of barium meal examinations has also decreased.
Computed Tomography—CT#
CT is short for computed tomography using X-rays, and as the full name suggests, CT is also a type of X-ray imaging technology. The biggest difference between CT and ordinary X-rays lies in the word "tomography." To explain it in a straightforward way, X-rays are a combination of countless layers to produce a single image, while CT produces true cross-sectional images, meaning that multiple layers of the scanned area are imaged separately—imagine you are a baguette (or a frozen meat strip from a hot pot restaurant), hard and long, and we want to see what’s inside you, so we bring in a machine to slice you into thin pieces from head to toe, allowing us to see what each thin slice looks like—at this point, the images of each layer are not influenced by other layers, allowing for a clear observation of the tissue structure and lesions of a single layer.
This is essentially the significance of medical imaging examinations: it allows us to see areas that would normally require surgical incision without actually cutting you open, directly "displaying" issues that could only be indirectly inferred through physical examinations.
CT imaging is a digital reproduction of images, with a higher resolution than X-rays (the specific pixel differences depend on different devices and processing technologies), and the density values in CT imaging can be quantified. Images can be described using high, medium, and low densities, and CT values can directly describe density. CT typically uses axial plane cross-sectional imaging (the "waist-cut" layer), so the CT images you usually see are always oval-shaped (as shown in the image on the right). However, during CT examinations of the head and face or brain, a coronal plane scan (a large plane that divides the front and back) may sometimes be added, meaning lying down and scanning horizontally, allowing for a clear view of every direction that needs to be cut in the head.
CT imaging that directly uses the differences in tissue thickness and density is called CT plain scan, which can usually be completed within a few minutes. CT imaging that uses contrast agents to assist imaging is called CT enhancement, which takes slightly longer, depending on the examination site and enhancement technology, and may take several minutes to half an hour. With the development of CT technology, the thickness of "slices" has become thinner. After the application of spiral CT, imaging can be seen as continuous and uniform scanning. Patients can obtain printed films of selected cross-sections after the examination, while in hospitals that use the latest imaging systems, doctors can directly observe each layer of the patient's CT scan on the imaging system on the computer. If further image processing is needed, certain "technical processing" can be performed on the original scanned images without requiring the patient to undergo a second examination.
CT imaging has multiple layers, high resolution, and is relatively expensive; its price usually depends on the scanned area. For example, in 2021, the price for a high-resolution spiral CT scan of the head at a provincial-level top-tier hospital was 198 yuan, while a high-resolution spiral CT scan of the chest was 378 yuan, which is relatively higher than ordinary X-rays. When doctors choose between X-rays and CT scans, they weigh the patient's condition, the location of the lesions, and economic factors. Therefore, in cases where X-ray examinations may be "unclear," patients may be advised to undergo CT scans to avoid repeated X-ray exposure (i.e., avoid "eating the line" again) and to prevent delays in diagnosis and treatment due to repeated examinations.
As mentioned earlier, X-rays are rarely used for diagnosing abdominal diseases; in clinical practice, ultrasound examinations are the first choice for initial screening of abdominal diseases, while CT examinations are often selected for further investigation. @刀客特 Leslie also mentioned in the article on health check items that lung CT scans are recommended during health checks. This is because some smaller lesions are difficult to detect in X-ray imaging but can be found in CT scans, allowing for early diagnosis and treatment, and facilitating follow-up comparisons during subsequent treatments.
Another concern for everyone is the safety of X-ray imaging examinations (including ordinary X-rays and CT scans). X-ray imaging examinations do carry a certain degree of radiation damage, and the frequency of examinations within a certain period should not be too high. For special populations such as pregnant women and children, the application of X-ray imaging needs to be more cautious. Generally, unless absolutely necessary, women planning to become pregnant within six months and pregnant women should avoid such examinations. For children, non-medical necessity should also minimize such examinations. Of course, for the average adult, undergoing X-ray imaging examinations within non-excessive doses and frequencies is generally harmless; the absorbed radiation will be metabolized within a few days.
In general, when undergoing examinations or accompanying someone for an examination, following the staff's instructions throughout the process should not pose any issues. However, in the spirit of taking responsibility for oneself, everyone should know: unless it is necessary for the examination or accompanying someone (such as needing family assistance for a restless patient), do not enter the examination room. If you must enter, follow the medical staff's instructions and wear protective lead clothing. Additionally, there are usually triage and inquiry desks or relevant staff near the examination room; if you have any questions, consult the staff outside first and do not enter the examination room at will. If you must enter the examination room, please wait until the examination is completed. Those who have undergone examinations with contrast agents can drink more water after the examination to promote the metabolism of the contrast agent.
Magnetic Resonance Imaging (MRI)#
Magnetic resonance imaging is what we commonly refer to as MRI, and it is one of the more advanced (and expensive) imaging examinations that the public can generally access. The principles of magnetic resonance imaging are relatively complex; interested friends can look up relevant materials for study. Textbooks on imaging divide the principles of magnetic resonance imaging into three steps: first, the hydrogen nuclei (1H) in the human body generate longitudinal magnetization vectors and precession in a strong external magnetic field; second, specific frequency pulses (RF pulses) are emitted to induce magnetic resonance phenomena; third, after stopping the RF pulses, the hydrogen nuclei return to their original state and generate MR signals. ^2^
Can't understand? Let's remove the technical terms for a straightforward explanation: the MRI machine creates a strong external magnetic field and places the subject within this magnetic field; at the same time, the machine intermittently emits specific frequency radiofrequency pulses, causing the hydrogen nuclei in the body to undergo specific "movements" (producing magnetic resonance phenomena). After the pulses stop, the nuclei return to their original state. The machine records the time it takes for the nuclei to return to their original state (called relaxation time), which is then converted into images through a series of complex high-tech processes. What you really need to know is just one thing: it does not use radiation; it uses a magnetic field.
The black, white, and gray levels in magnetic resonance imaging represent different relaxation times between tissues. Since the hydrogen nuclei produce MR signals representing different relaxation times during the relaxation process, different shades in magnetic resonance images are referred to as high and low signals. Different tissues have different relaxation times, and magnetic resonance imaging uses this principle for disease diagnosis. The two different relaxation times produce signals known as T1 and T2 signals; the specific meanings of high and low signals are quite complex, but for the average reader, it is sufficient to know that there are these two types of signals.
Magnetic resonance imaging is also cross-sectional imaging, but unlike CT, MRI can perform multi-plane imaging: while CT generally provides axial cross-sections (the "waist-cut"), MRI can perform cross-sectional imaging from various angles (any cut). MRI can conduct multi-parameter and multi-sequence imaging, providing high resolution for soft tissues, and is more commonly used in cranial and brain applications. Additionally, MRI can directly use water and blood in the body for imaging without the need for injected contrast agents, avoiding the side effects of iodine-based contrast agents. Since magnetic resonance imaging uses external magnetic fields and RF pulses to induce magnetic resonance phenomena in the body's nuclei, it is generally considered a safe examination with no side effects. Although MRI has many advantages, it is not without limitations. Due to its imaging principles and multi-sequence scanning, the examination takes a relatively long time (varying slightly depending on the examination, around 1-2 hours), and patients who cannot "lie still" due to seizures, restlessness, confusion, or claustrophobia may find it difficult to undergo this examination. Furthermore, its lengthy examination time is not suitable for emergencies.
Next, let's discuss the safety issues of MRI—
From a theoretical perspective, MRI does not use radiation, so there is no concern about radiation exposure, making it a "safe" examination. Unfortunately, in clinical practice, magnetic resonance imaging often sees a high incidence of accidents. This is not due to any inherent dangers of the examination itself, but rather because safety measures prior to the examination are not properly implemented. During MRI, there is a powerful external magnetic field that has a strong attraction to metals, so it is essential to remove all metal items (including but not limited to keys, jewelry, glasses, pens, coins, and removable dental appliances) before the examination. Patients with pacemakers, magnetic metal dental materials and joints, steel nails, plates, screws, stents, or intrauterine devices cannot undergo this examination.
Additionally, let's specifically mention the titanium implants that have become widely used in recent years: due to differences in the size, position, and composition (pure titanium/titanium alloy; some titanium alloy implants have proprietary alloy compositions and ratios) of the implants, their effects on MRI can vary. Generally, titanium implants are considered non-magnetic and do not affect the safety of MRI; however, they may produce artifacts that affect the examination's effectiveness. In other words, while titanium alloys can generally undergo MRI, the area around the implant may be unclear due to this. Whether or not a patient can undergo MRI should be determined by the attending physician, the physician responsible for the implant, and the imaging physician. Do not conceal your medical history for convenience or other reasons! There have been instances where a wheelchair with a person was directly sucked into an MRI machine, resulting in the machine being rendered inoperable, costing millions (it really is that expensive). I do not know why that patient entered the examination room while sitting in a wheelchair, nor how the situation was handled afterward, but being sucked into the machine must have caused personal injury to the patient, and other patients also experienced delays in their examinations due to the machine's malfunction. Therefore, when undergoing MRI or accompanying someone into the MRI examination room, please be sure to listen! Do not bring metals! No metals at all!
Conclusion#
The usage range of different imaging examinations varies, and the selection of imaging examinations is extremely complex. Even for the same disease, the examinations required may differ at different stages of the lesion; not to mention the complexity and diversity of conditions, individual differences (the patient's general condition, whether there are other diseases, whether there are contraindications for other examinations), and other complex real-world conditions (the patient's gender, age, culture, beliefs, personal and family wishes, economic conditions, insurance situations, and the hospital's equipment and technical levels, etc.).
Perhaps after reading this, you still feel confused about the specific principles of these examinations; this is completely normal. A doctor who has undergone at least five years of undergraduate study, obtained a medical license, completed residency training, and has practical clinical experience still needs to consult with colleagues when treating diseases outside their specialty, so it is even more challenging for non-professionals. If you don't believe it, ask a friend in physics to read the principles I wrote above and see if they want to hit me. Therefore, professional matters should be left to professionals; readers need to understand "why examinations are performed" and "how to cooperate and protect themselves during examinations," which is sufficient. Additionally, I would like to remind you to keep all medical records (including but not limited to medical history, outpatient records, laboratory reports, etc.) safe, and please store imaging examination films flat, do not roll them up, fold them, or throw them away.
Finally, I wish you good health.