8.1 Postprocessing — MPR, 3D Rendering & Quantitative Analysis

Why Postprocessing Matters on the CT Exam

Once raw projection data has been reconstructed into an axial image set (see Chapter 7), that dataset is rarely the final product a radiologist reads. Postprocessing — the manipulation of an already-reconstructed volumetric dataset into new planes, 3D renderings, or measured values — is what turns a stack of axial slices into the curved coronary reformat, the CT angiography "runoff" 3D image, or the coronary calcium score that actually drives a clinical decision. ARRT's CT Content Specifications list Postprocessing as item 1.E under Image Formation, within the 52-question (31.5%-weighted) Image Production domain, and name three specific techniques: multiplanar reformation (MPR), 3D rendering (with the named examples MIP, SSD, and volume rendering), and quantitative analysis (distance, diameter, and calcium scoring). These techniques also resurface constantly inside Procedures questions — vascular CTA "runoff" studies, aortic aneurysm sizing, and trauma 3D reconstructions all lean on postprocessing vocabulary, so mastering it here pays off across the whole exam.

Multiplanar Reformation (MPR)

Multiplanar reformation (MPR) takes an axially acquired volumetric dataset and reslices it into other planes — sagittal, coronal, or oblique — without rescanning the patient. MPR is only possible because modern helical/volumetric acquisition captures a continuous block of data with thin, overlapping slices; the thinner and more overlapped the acquisition, the closer the voxels are to isotropic (equal in all three dimensions), and the smoother the reformatted image looks. If slices are too thick or non-overlapping, reformats show a "stair-step" degradation because there is not enough raw data to interpolate a smooth new plane.

A specialized MPR variant tested on CT exams is curved planar reformation (CPR): instead of a flat plane, the software follows an operator- or software-defined centerline through a tortuous structure — a coronary artery, the aorta, or the spinal canal — and "straightens" it into a single 2D image. CPR is the standard tool for measuring vessel length and stenosis along a curved course that a flat sagittal or coronal plane could never capture in one image.

3D Rendering: MIP, SSD, and Volume Rendering

ARRT's outline names three 3D rendering techniques, and the exam expects you to match each to its data-handling method and its best clinical use:

• Maximum intensity projection (MIP): for each ray cast through the volume, only the single brightest (highest-attenuation) voxel along that ray is displayed. MIP is excellent for high-density structures — contrast-filled vessels and calcified plaque — which is exactly why it is the default 3D tool for CT angiography "runoff" studies. Its key limitation: because only the brightest voxel per ray is kept, MIP can obscure luminal narrowing hidden behind dense calcification, and a static MIP loses true depth information without cine rotation.
• Shaded surface display (SSD): the oldest 3D technique on this list. SSD applies a fixed HU threshold to segment a surface (classically bone) and displays only that binary surface — everything above or below the threshold is discarded. SSD was historically the workhorse for craniofacial and orthopedic surgical-planning models; its main weakness is that an incorrectly chosen threshold can misrepresent true anatomy, since any data outside the cutoff is simply not rendered.
• Volume rendering (VR): uses the entire dataset, assigning each voxel a color and opacity value through a transfer function so that multiple tissue types (bone, vessel, soft tissue) can be displayed simultaneously with adjustable transparency. VR is the current standard for complex vascular, trauma, and surgical-planning reconstructions because it preserves far more of the original dataset than MIP or SSD.

Quantitative Analysis: Distance, Diameter, and Calcium Scoring

• Distance and diameter measurement: a caliper tool placed on any reformatted plane. The classic exam trap is measurement plane orientation — a vessel or aneurysm diameter must be measured perpendicular to the vessel's long axis (using an orthogonal or curved reformat), because measuring on a plane that crosses the vessel obliquely will make a round structure appear elongated, falsely inflating the diameter.
• Calcium scoring: performed on a cardiac-gated, non-contrast chest CT using the Agatston method, which flags pixels above a 130 HU threshold as calcified coronary plaque and weights the score by density and area. Reported categories are standardized: 0 = no identifiable calcification, 1–10 = minimal, 11–100 = mild, 101–400 = moderate, and greater than 400 = severe/extensive. The Agatston score is a cardiovascular risk-stratification tool — it is not a stenosis measurement, and it requires prospective or retrospective ECG gating (Chapter 12 covers cardiac CT gating in full).

Exam Scenario

A CT angiogram of the aorta is performed for suspected ascending aortic aneurysm. The technologist generates axial images, then builds a curved planar reformation along the aortic centerline and a volume-rendered 3D image for the surgical team. When the radiologist requests the maximum aortic diameter, the technologist should place calipers on the CPR or an orthogonal double-oblique plane perpendicular to the vessel — not on a routine axial slice, which could cut across the aneurysm at an angle and overstate its true size. In a second case, a 58-year-old undergoing a non-contrast, ECG-gated screening CT has visible coronary calcifications; the software reports an Agatston score of 250, placing the patient in the moderate category and prompting further cardiovascular risk workup.