ELEC5622 Chap.1 Medical Imaging Foundations and Image Planes
Medical Imaging Foundations and Image Planes
The opening chapter sets up the vocabulary the rest of the unit relies on. It separates anatomical modalities such as X-ray, CT and structural MRI from functional ones such as PET and functional MRI, names the three orthogonal viewing planes, and links each pixel to the voxel of tissue it averages. It closes with the two screening measures, sensitivity and specificity, that later chapters turn into ROC analysis.
Read it as a map for the whole unit: every later modality is introduced by the signal it records and the plane in which its images are usually viewed, and every later analysis method is judged with the same screening numbers. Its worked example turns a voxel count into millilitres, the same conversion Lab 1 and Project 1 depend on.
What this chapter covers
- 01
Four jobs for medical images: diagnosing, researching, planning surgery, guiding treatment
- 02
Anatomical versus functional imaging modalities
- 03
Axial, coronal and sagittal planes through one volume
- 04
Pixels, voxels and voxel-averaged values
- 05
DICOM files and why metadata matters
- 06
Sensitivity and specificity of a screening test
Worked example · free
Lesion volume from a voxel count
- 1One voxel = 0.5 × 0.5 × 1.0 = 0.25 mm³.
- 1Tumour volume = 40,000 × 0.25 = 10,000 mm³.
- 1Since 1 mL = 1,000 mm³, the tumour occupies 10 mL.
Key terms
- Voxel
- A voxel is the 3D volume element of tissue whose averaged signal becomes one pixel value in a slice.
- Coronal plane
- The coronal plane is the vertical plane that divides the body into front and back portions.
- Functional imaging
- Functional imaging measures what tissue is doing, such as metabolism, rather than where structures lie.
- DICOM
- DICOM is the standard file format for medical images that stores pixel data together with acquisition and patient metadata.
- Sensitivity
- Sensitivity is the share of truly positive cases that a test correctly flags, TP divided by TP plus FN.
Medical Imaging Foundations and Image Planes FAQ
What is the difference between anatomical and functional imaging?
Anatomical imaging, such as X-ray, CT and structural MRI, shows where structures are and how they differ in density or water content. Functional imaging, such as PET and functional MRI, shows physiological activity like glucose metabolism, usually at coarser resolution.
Why does a thick slice blur small structures?
Each pixel reports the average value of its whole voxel. When a small structure shares a voxel with surrounding tissue, its signal is mixed with theirs, so edges inside that voxel cannot be seen and the structure appears smeared.
How do sensitivity and specificity differ?
Sensitivity is the share of diseased patients a test flags, TP divided by TP plus FN. Specificity is the share of healthy patients it clears, TN divided by TN plus FP. Raising one by moving the threshold usually lowers the other.
Why does the same MRI scanner appear in both modality families?
An MRI scanner can be run with sequences that map structure, such as T1- or T2-weighted anatomy, or with functional MRI sequences that track activity. The hardware is the same; the question the sequence asks decides whether the image is anatomical or functional.
What information does a DICOM file carry besides pixels?
It stores metadata about the patient, the acquisition and the geometry, such as voxel spacing and orientation. Without that information a volume cannot be measured in millimetres or aligned with another scan, which is why the lecture stresses metadata management.
Why does PET look blurrier than CT?
The PET lecture quotes about 1 mm spatial resolution for CT against 4 to 5 mm at best for PET. PET locates each event through a pair of detectors and averages over larger voxels, so fine anatomy is lost, which is why PET is usually read beside a CT image of the same patient.
Exam move
Draw the three planes on a sketch of a head until you can label them without hesitation, then practise describing a lesion position in each view. Make a two-column table of anatomical and functional modalities with the signal each measures. Finish by computing a voxel volume and a pair of screening rates from invented numbers, writing units on every line.
Then explain to yourself why a functional image needs an anatomical partner, using PET and CT as the example, and why a voxel count alone cannot be compared across two scans with different voxel sizes. Test yourself by naming, for each modality in the unit, whether it is anatomical or functional, what physical signal it records, and roughly how fine a detail it can resolve.
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