Part 2 Pixel perfect

Quality Assurance and the digital image – are you getting your 256 shades of grey?

The second part of this article explores the issues that need to be considered in producing high-quality digital images in dental practice. The first part, written by Barbara Lamb, looked at the clinical considerations, while this second part deals with the technology considerations – both hardware and software.

The world of IT is an ever-changing place. We are surrounded by all sorts of gadgets with ever-increasing processing power and enhanced operating systems.

TVs are continually changing to bigger and better displays, curved screens and HD images. One thing is for sure, technology isn’t standing still and we are spoiled by the choices we have to take advantage of.

This is good news when for digital imaging in dental practices. Better technology leads to better sensors, X-ray machines, CT and 3D scanners and – combined with better displays – this maximises the image we can see once it is taken.

However, various types of PCs, graphics dislays and monitors, all with a wide range of prices, can lead to confusion on which digital image system should be installed to suit your needs.

Let’s look at the main parts of the IT system and explain the differences. This will allow you to make informed decisions on what systems to purchase for the best results.

PCs

There are two main processor manufacturers, Intel and AMD. It is always best to check the minimum requirements of the imaging system, as most prefer the Intel processor. With this in mind, we will concentrate on the Intel type (http://www.intel.com).

The processor, or CPU (central processing unit), is the main engine of the computer. This determines how quickly it will load and process what you are doing. Just like in a car, the bigger the engine, the faster it will go. Processors all have models – slowest to fastest is: Dual Core, i3, i5, i7 and Xeon.

In most cases, an i3 or i5 will suffice, but when using an OPG or CBCT, an i7 or Xeon may be required. Lower speed processors are available – Celeron and Pentium. These provide lower-priced PCs, but are suited mainly to ‘office’ work and wouldn’t be recommended for imaging work.

RAM (random access memory) is important and a minimum of 4GB should be installed. Where a faster processor is installed, the RAM should be increased to a minimum 8GB. RAM will work alongside the processor to load programs and data quickly – vital when you are working with large images, especially 3D or CBCT.

With Windows XP no longer available, most business PCs will come installed with Microsoft Windows 7 Professional (http://www.microsoft.com). Most are now a 64-bit operating system that supports up to 192GB of RAM.

Be careful, however, as some PCs can come with 32-bit Windows – this has a limit of just over 3GB of RAM.

Graphics

Modern PCs come with integrated graphics. This provides the connection to a monitor, and powers most basic onscreen graphics (such as icons and windows). Integrated graphics are useful for most applications run on a PC, but better performance and display can be achieved by installing a graphics card (see Figure 1, overleaf).

Graphics cards are an additional expansion card that can be installed onto the PC motherboard.

The differences between integrated and an expansion card are as follows:

  • Integrated graphics require the use of the PC processor and RAM to perform the output to the monitor. This reduces the amount of processing power and RAM available to running programs.
  • Graphics cards come equipped with their own processor, a GPU (graphics processing unit), and their own memory, called VRAM (video RAM). This means all graphics and imaging work is performed in one place and not via the main processor and RAM. It also means it will be performed more efficiently, an important factor if X-rays are being rendered on the PC.
  • Graphics cards can also output at much higher resolutions than integrated graphics. VRAM will enable your computer to load more and higher resolution textures or 3D images onto the GPU, as well as render images at higher resolutions.
  • GPU requires enough VRAM to operate smoothly. If not, it will load its resources onto the system RAM instead. However, due to the system RAM’s distance from the GPU, it is a lot slower than VRAM. This is partly the reason why integrated graphics are much slower than dedicated GPUs, since they have no VRAM and thus have to rely on the slower system RAM.

There are many different types of graphics cards and again, checking the minimum requirements of any imaging system will guide you to the best option. There are two main types used – nVidia (http://www.nvidia.com) and AMD Radeon (http://www.amd.com).

Graphics cards come with different output connectors to suit all types of monitors.

Monitors

A good monitor will provide the best definition and detail of images. It is vital to realise the importance of a monitor display. Buy a cheap one and the detail, colour mix and ability to display true black are pretty poor. This is a vital part of being able to distinguish and display the 256 shades of grey. A cheap monitor results in a low-quality display of images on the screen.

There are different types of monitors available:

TN The cheapest monitors are based on twisted nematic (TN). While having a fast response time, they have limited colour reproduction, poor black levels and narrow viewing angles. They are fine for word processing, but not for serious work.

LCDscreens use cold cathode fluorescent lamps (CCFLs) to provide backlighting. These fluorescent tubes must light the entire screen evenly. There is no way to vary the back-lighting intensity in different parts of the screen. Even if you want to show a single white pixel on an all-black screen, the light in the back needs to be blazing away at full brightness.

LEDs are LCDs that use LEDs to backlight. This produces better colour. Rather than being on at full brightness all the time, they can be dimmed or turned off entirely. This makes for much better black levels and contrast.

IPS panel technology guarantees consistent colour reproduction with wide viewing angles and high contrast and therefore it is especially recommended for graphics design and other applications that require colours to be displayed accurately. The response time of the panel allows for smooth playback while watching films and playing games, making the IPS the best all-round technology suitable for both business and home use.

Considerations

Contrast ratio

The difference in light intensity between white and black on an LCD display is called contrast ratio. The higher the contrast ratio, the easier it is to see details and differences in the shades of grey.

Luminance

Also known as brightness, it is the level of light emitted by an LCD display. Luminance is measured in nits or candelas per square meter (cd/m2). One nit is equal to one cd/m2.

Response time

The speed at which the monitor’s pixels can change colours is called response time. It is measured in milliseconds (ms).

Summary

In conclusion, the investment in a good PC system will provide better results and perform at a higher level. Details in imagery will display in greater detail and let you see far more.

Get the best processor within your budget and consider the amount of RAM in the system. Faster is better when it comes to processors and more RAM is always better in terms of performance.

Adding a graphics card will provide better display results along with better rendering of images. The extra processing power will help with performance, both in the system and in image processing.

A good monitor will give the best output of all the work done on the PC system. The better the monitor, the greater the detail that will be output and seen on the screen. This is important when viewing images and seeing greater detail. Consider an IPS type for best results.

Example of a medical grade monitor

  • Panel Technology – IPS TFT with W-LED backlight
  • Screen Size [inch/cm] – 61.1 / 24.1
  • Screen Aspect Ratio – 16:10
  • Pixel Pitch [mm] – 0.270 x 0.270
  • Brightness (typ.) [cd/m2] – 170, (350 max)
  • Contrast Ratio (typ.) – 1000:1
  • Optimum Resolution – 1920 x 1200 at 60 Hz

Example of a higher-quality PC monitor

  • Panel – IPS LED
  • Resolution – 1920 x 1080 Full HD 1080p
  • Aspect Ratio –16:9
  • Brightness – 250 cd/m2
  • Static Contrast – 1000:1
  • Advanced Contrast – 5,000,000:1
  • Response Time – 5ms
  • Viewing angle – Horizontal/vertical: 178°/ 178°; right/left: 89°/ 89°; up/down: 89°/ 89°
  • Display Colours – 16.7M
  • Pixel Pitch – 0.265 x 0.265

Figures

Fig 1 An nVidia GeForce graphics Card

Fig 2 Three types of monitor connections: VGA – Analogue output (top), HDMI – Digital HD Output (middle) and DVI – Digital Output (bottom)